1902 Encyclopedia > Shipbuilding

Shipbuilding




Within the memory of the present generation shipbuilding, like many other arts, has lost dignity by building, exterided use of machinery and by the subdivision of labour. Forty years ago it was still a "mystery " and a "craft." The well-instructed shipbuilder had a store of experience on which he based his successful practice. He gained such advantages in the form and trim and rig of his vessels by small improvements, suggested by his own observation or by the traditions of his teachers, that men endeavoured to imitate him, neither he nor they knowing the natural laws on which success depended. He had also a good eye for form, and knew how to put his materials together so as to avoid all irregularity of shape on the outer surfaces, and how to form the outlines and bounding curves of the ship so that the eye might be com-pelled to rest lovingly upon them. He was skilled also in the qualities of timber. He knew what was likely to be free from "rends" and "shakes" and "cups" which would cause leakage, and which would be liable to split when the bolts and treenails were driven through it. He knew what timber would bear the heat of tropical suns without undue shrinking, and how to improve its qualities by seasoning. He could foretell where and under what circumstances premature decay might be expected, and he could choose the material and adjust the surroundings so as to prevent it. He knew what wood was best able to endure rubbing and tearing on hard ground, and how it ought to be formed so that the ship might have a chance of getting off securely when she accidentally took the ground or got on shore. Such men were to be found on all the sea-coasts of Europe and on the shores of the Atlantic in America.
A great change came over the art when steam was introduced. The old proportions and forms so well suited for the speeds of the ships and for the forces impressed upon them were ill adapted for propulsion by the paddle, and still less so for propulsion by the screw. Experience had to be slowly gained afresh, for the lamp of science burned dimly. It needed to be fed by results, by long records of successes and failures, before it was able to direct advancing feet. The further change from wood to iron and then to steel almost displaced the shipwright. Ships for commercial purposes may be said to be built now, so far as their external hulls are concerned, by draftsmen and boilermakers. The centres of the ship-building industry have changed. The ports where oaks (Italian, English, and Dantzic), pines from America and the north of Europe, .teak from Moulmein, and elm from Canada were most accessible,—these marked the suitable places for shipbuilding. The Thames was alive with the industry from Northfleet to the Pool. It still lingers, but it is slowly dying out. Travellers along the
Mediterranean shores from Nice to Genoa mark the completeness of the change which a few years have made. The Tyne and the Clyde and the Mersey have become the principal centres of the trade. It has been drawn there because the iron and the coal are near.
But, while the art of shipbuilding has lost dignity, the The science of naval construction has increased in importance, science English art is of an eminently practical character. It is of nava* shy of experiment, as being costly in itself and likely to tion_ lead to delays and changes of system and of plant. It loves large orders and rapid production. It practises great subdivision of the details in order to cheapen pro-duction, and it stereotypes modes of work. There is no lack of boldness and enterprise; but the patient continuous inquiry and the slow but sure building up of theory upon research,—this is the exception. Naval construction in England has had the good fortune during the last quarter of a century to have not only a thriving industry but a home for research. Twenty-five years ago, when the high-pressure condensing engine was in its infancy, when shipbuilding steel was not, and armour-plated ships had not yet displaced the wooden line-of-battle ship, this home was founded. The Institution of Naval Architects may be fairly called the home for research in naval construction. It owes its establishment mainly to four well-known men— John Scott Russell, Dr Joseph Woolley, Lord Hampton, for many years its honoured president, and Sir Edward Reed, its first secretary. It has published every year a volume of Transactions recording the experience of all the shipbuilders and marine engineers in England. These Transactions contain also valuable contributions irom French, Italian, German, and other eminent constructors and engineers.
Shortly after the foundation of the Institution one of its mem- Ad-bers, Mr William Froude, set up an experimental establishment miralty at Torquay, under the auspices and with the assistance of the experi-Admiralty. The object was to submit to experiment various ments or proportions and forms of ships in model in order to compare the fluid re-relative resistances in the same model at various speeds, and in sistance. different forms and proportions at equal speeds. There was some &e. reason to doubt the possibility of inferring from a model on a scale of f of an inch to a foot what would happen in a ship of corresponding form and proportions. In order to establish satis-factorily the relations between the real and the model ship a series of experiments was desirable upon a real ship in which the resistances could be measured by a dynamometer at various speeds and compared with those indicated by the model. Up to the date of this trial the " scale of comparison " which had been employed by Mr Froude was based upon prima facie theoretical truth, and it had some experimental justification. It may be stated as follows, as given by Mr Froude in the volume for 1874 of the Transactions of the Institution of Naval Architects :—
If a ship be D times the "dimension," as it is termed, of the model, and, if at the speeds Vlt V2, V3 . . . the measured resistances of the model are Tt^ R%, E3 . . . ., then for speeds DIV^ DiV^ D*V3, - - . of the ship, the resistances will be DZRV D3Rv
XXI. 102

V3B3... To the speeds of model and ship thus related it is conven-ient to apply the term " corresponding speeds." For example, sup-pose two similar ships, the length, breadth, depth, &c.,of which were double one of the other. Then, if at a given speed (say 10 knots) the resistance of the smaller ship were ascertained, we may infer that at a speed of \J2 x 10 = 14"14 knots in the larger ship there would be a resistance 8 times as great as in the smaller vessel.
This law is in accordance with the old rule that the resistance varies as the square of the velocity, and also as the area of the surface exposed to resistance. It takes into account both the resistance due to surface friction (subject to some correction) and the formation of deadwater eddies. The passage of the ship through the water creates waves which are dependent for their character upon the proportions and form of the ship. These con-stitute also an element of resistance. They are due to differences of hydrodynamic pressure inherent in the system of stream-lines which the passage of the ship creates. These wave-configurations should be precisely similar when the originating forms are similar and are travelling at speeds proportional to the square roots of their respective dimensions, because the resulting forces will be in that case as the square of the speeds. For example, if the surface of the water surrounding a ship 160 feet long, travelling at 10 knots an hour, were modelled together with the ship, on any scale, the model would equally represent, on half that scale, the water surface surrounding a ship of similar form 320 feet long, travelling at 14 "14 knots an hour ; or again, on 16 times that scale, the water surface surrounding a model of the ship 10 feet long, travelling at 2J knots. Experiment has abundantly con-firmed this proportion as to the similarity of waves caused by similar forms travelling at corresponding speeds. The resistance caused to these forms respectively by the development of the waves would therefore also be proportionate to the cubes of the dimensions of the forms and would follow the law of comparison stated above. It is necessary, however, to observe that, in dealing with surfaces having so great a disparity in length and speed as those of a model and of a ship, a very tangible correction is necessary in regard to surface friction.
The vessel tried by Mr Froude for confirming the law of com-parison was H.M.S. " Greyhound," of 1157 tons. She was towed ty H.M.S. " Active," of 3078 tons, from the end of a boom 45 feet long, so as to avoid interferences of " wake." It was found to be possible to tow up to a speed of nearly 13 knots. The actual amount of towing strain for the "Greyhound" was approximately as follows:—at 4 knots, 0"6 ton ; at 6, l-4 tons ; at 8, 2"5 tons; at 10, 4 "7 tons; and at 12, 9'0 tons.
Comparing the indicated horse-power of the " Greyhound " when on her steam trials and the resistance of the ship as determined by the dynamometer, it appears that, making allowance for the slip of the screw, which is a legitimate expenditure of power, only about 45 per cent, of the power exerted by the steam is nsefully employed in propelling the ship, and that the remainder is wasted in friction of engines and screw and in the detrimental reaction of the propeller on the stream lines of the water closing in around the stern of the vessel.
"We may describe in Mr Froude's own words the system of ex-periment now regularly carried out for the Admiralty, a system which has been successfully copied in other countries and also by a private shipbuilding firm, Messrs Denny of Dumbarton:—
"That system of experiments involves the construction of models of various forms (they are really fair-sized boats of from 10 to 25 feet in length), and the testing by a dynamometer of the resistances they experienced when running at various assigned appropriate speeds. The system may be described as that of determining the scale of resistance of a model of any given form, and from that the resistance of a ship of any given form, rather than as that of searching for the best form, and this method was preferred as the more general, and because the form which is best adapted to any given circumstances comes out incidentally from a comparison of the various results. We drive each model through the water at the successive assigned appropriate speeds by an extremely sensitive dynamometrical apparatus, which gives us in every case an accurate automatic record of the model's resistance, .as well as a record of the speed. We thus obtain for each model i a series of speeds and the corresponding resistances ; and, to render these results as intelligible as possible, we represent them graphic-ally in each case in a form which we call the ' curve of the resistance' for the particular model. On a straight base line which represents speed to scale we mark off the series of points denoting the several speeds employed in the experiments, and at each of these points we plant an ordinate which represents to scale the corresponding resistance. Through the points defined by these ordinates we draw a fair curved line, and this curve con-stitutes what I have called the curve of resistance. This curve, Whatever be its features, expresses for the model of that particular form what is in fact and apart from all theory the law of its resistance in terms of its speed; and what we have to do is if opossible to find a rational interpretation of the law. Now we can
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at once carry the interpretation a considerable way ; for we know that the model has so many square feet of skin in its surface, and we know by independent experiments how much force it takes to draw a square foot of such skin through the water at each indi-vidual speed. The law is very nearly—and for present convenience we may speak as if it were exactly—that skin resistance is as the area simply, and as the square of the speed. Now, we have so many square feet of immersed skin in the model, and the total skin resistance is a certain known multiple of the product of that number of square feet and of the square of the speed. Now, when we lay off on the curve of resistance a second curve which, represents that essential and primary portion of the resistance, then we find this to be the result: the curve of skin resistance when drawn is found to be almost identical with the curve of total resistance at the lower speeds ; but as the speed is increased the curve of total resistance is found to ascend more or less, and in some cases to ascend very much above the curve of skin resistance. The identity of the two curves at the lower speeds is the practical representation of a proposition which the highest mathematicians have long been aware of, and which I have lately endeavoured to draw the public attention to, and to render popularly intelligible, namely, that when a ship of tolerably fine lines is moving at a moderate speed the whole resistance consists of surface friction. The old idea that the resistance of a ship consists essentially of the force employed in driving the water out of her way, and closing it up behind her, or, as it has sometimes been expressed, in excavating a channel through the track of water which she traverses,—this old idea has ceased to be tenable as a real proposi-tion, though prima fade we know that it was an extremely natural one. We now know that, at small speeds, practically the whole resistance consists of surface friction, and some derivative effects of surface friction, namely, the formation of frictional eddies, which is due to the thickness of the stem and of the sternpost; but this collateral form of frictional action is insignificant in its amount unless the features of the ship in which it originates are so abruptly shaped as to constitute a departure from that necessary fineness of lines which I have described ; and we do not attempt to take an exact separate account of it. Thus we divide the forces represented by the curve of resistance into two elements,—one 'skin resistance,' the other which only comes into existence as the speed is increased, and which we may term ' residuary resist-ance.' And we have next to seek for the cause and governing laws of this latter element. Now when the passage of the model along the surface of the water is carefully studied, we observe that the special additional circumstance which becomes apparent as the speed is increased is the train of waves which she puts in motion ; and indeed it has long been known that this circumstance has important bearings on the growth of resistance. It is in fact certain that the constant formation of a given series involves the operation of a constant force, and the expenditure of a definite amount of power, depending on the magnitude of those waves and the speed of the model; and, as we thus naturally conclude that the excess of resistance beyond that due to the surface friction consists of the force employed in wave-making, we in a rough way call that residuary resistance ' wave-making resistance.'
"Perhaps I had better say a few words more about the nature and character of these waves. The inevitably widening form of the ship at her ' entrance' throws off on each side a local oblique wave of greater or less size according to the speed and to the obtuse-ness of the wedge, and these waves form themselves into a series of diverging crests, such as we are all familiar with. These waves have peculiar properties. They retain their identical size for a very great distance with but little reduction in magnitude. But the main point is that they become at once dissociated from the model, and after becoming fully formed at the bow, they pass clear away into the distant water and produce no further effect on her resistance. But, besides those diverging waves, there is pro-duced by the motion of the model another notable series of waves which carry their crests transversely to her line of motion. Those waves, when carefully observed, prove to have the form shown in detail in fig. 1. In the figure there is shown the form of a model which has a long parallel middle body accompanied by the series of these transverse waves as they appear at some one particular speed with the profile of the series defined against the side of the model; only I should mention that for the sake of distinctness the vertical scale of the waves has been made double the horizontal scale, so that they appear relatively to the model about twice as high as they really are. The profile is drawn from exact and careful measurements of the actual wave features as seen against the side of the model. It is seen that the wave is largest where its crest first appears at the bow, and it reappears again and again as we proceed sternwards along the straight side of the model, but with successively reduced dimensions at each reappearance. That reduction arises thus :—in proportion as each individual wave has been longer in existence, its outer end has spread itself farther into the undisturbed water on either side, and, as the total energies of the wave remain the same, the local energy is less and less, and


the wave-crest, as viewed against the side of the ship, is constantly diminishing. We see the wave-crest is almost at right angles to the ship, but the outer end is slightly deflected sternward from the













Fig. l.
circumstance that when a wave is entering undisturbed water its progress is a little retarded, and it has to deflect itself into an oblique position, so that its oblique progress shall enable it exactly to keep pace with the ship. The whole wave-making resistance is the resistance expended in generating first the diverging bow waves, which, as we have seen, cease to act on the ship when once they have rolled clear of the bow ; secondly, these transverse waves, the crests of which remain in contact with the ship's side ; and thirdly, the terminal wave, which appears independently at the stern of the ship. This latter wave arises from causes similar to those which ocreate the bow wave, namely, the pressure of the streams which, forced into divergence then, here converge under the run of the vessel, and re-establish an excess of pressure at their meeting. The term ' wave-making resistance' represents, then, the excess of resistance beyond that due to surface friction, and that excess we know to be chiefly due to this formation of waves by the ship."
Pursuing these experiments it was found that not only was there a certain lengtli of form necessary in a ship designed to attain a certain speed economically,—a fact which Mr Scott Russell did much to establish,—but that there was also a considerable increase in wave-making resistance dependent upon the position of the after-body or run of the ship with reference to the wave-system left by the bow. Stating this again in Mr Froude's words :—
"The waves generated by the ship in passing through the water originate in the local differences of pressure caused in the -surrounding water by the vessel passing through it; let us suppose, then, that the features of a particular form are such that these differences of pressure tend to produce a variation in the water level shaped just like a natural wave, or like portions of a natural wave of a certain length.
" Now an ocean wave of a certain length has a certain appropriate speed at which only it naturally travels, just as a pendulum of a certain length has a certain appropriate, period of swing natural to it. And, just as a small force recurring at intervals corresponding to the natural period of swing of a pendulum will sustain a very large oscillation, so, when a ship is travelling at the speed naturally appropriate to the waves which its features tend to form, the stream line forces will sustain a very large wave. The result of this phenomenon is, that as a ship approaches this speed the waves become of exaggerated size, and run away with a proportionately -exaggerated amount of power, causing corresponding resistance. This is the cause of that very disproportionate increase of resistance experienced with a small increase of speed when once a certain ; speed is reached.
'' We thus see that the speed at which the rapid growth of resistance will commence is a speed somewhat less than that appropriate to the length of the wave which the ship tends to form. Now, the greater the length of a wave is the higher is the speed appropriate to it; therefore the greater the length of the waves which the ship tends to form the higher will be the speed at which the wave-making resistance begins to become formidable. We may therefore accept it as an approximate principle that the longer are the features of a ship which tend to make waves the higher will be the speed she will be able to go before she begins to experience great wave-making resistance, and the less will be her wave-making resistance at any given speed. This principle is the explanation of the extreme importance of having at least a certain length of form in a ship intended to attain a certain speed ; for it is necessary, in order to avoid great wave-making resistance, that the 'wave features,' as we may term them, should be long in _comparison with the length of the wave which would naturally _travel at the speed intended for the ship.
" This view of the matter, then, recognizes the tendency of a ship, when the speed bears a certain relation to the length of her wave-making features, to make large waves and to incur correspond-ing wave-making resistance. But it does not take account of the possibility of the waves made by one feature of the form so placing themselves with reference to other features as, by the differences of pressure essential to their existence, either to cause an additional resistance, or on the other hand to cause a forward force which partly counterbalances the resistance originally due to their creation. The way in which this may occur we have seen strikingly exhibited in the results of the experiments I have been describing. We see that in the very long parallel-sided form the sternmost of the train of waves left by the bow has become so small that its effect on the stern is almost insensible ; and here we find, consequently, the united resistance due simply to the generation of a separate wave-system by each end of the ship. As we gradually reduce the length of middle-body, the stern is brought within the reach of waves large enough to produce a sensible effect, and according as it is brought into conjunction with a crest or hollow, the total wave-making resistance becoming least of all (except at the very highest speed) when the middle-body is reduced to nothing."
The variations in residuary resistance due to these transverse wave-formations are variations of quasi-hydrostatic pressure against the after-body, corresponding with the changes in its position with reference to the phases of the train of waves, there being a com-parative excess of pressure (causing a forward force or diminution of resistance) when the after-body is opposite a crest, and the reverse when it is opposite a trough.
It may be proper to introduce here some remarks as to the stream lines which have been referred to in the foregoing considerations. The statement of the case as given by Mr Froudj, and derived by him mainly from the investigations of Prof. Rankine, is as follows :—
" By a ' perfect fluid' is meant one the displacements of which Stream are governed solely by the laws expressed in the equation of fluid lines, motion, the particles of which therefore are without viscosity, and are capable of gliding rectilinearly along a perfectly smooth surface or past each other without frictional interference. By an imperfect fluid is meant one in which, as in water, as well as those with which we are practically acquainted, such frictional interference is inevitable.
" Dealing first, then, with the case of steady rectilinear motion in a perfect incompressible fluid, infinitely extended in all directions, it is plain that the motion will create differences of pressure, and therefore changes of velocity, in the particles of the surrounding fluid, which thus move in what are called ' stream lines.' At the commencement of the motion of the body the particles of the fhrd undergo acceleration in their respective stream-line paths, and these accelerations imply a resistance experienced by the body ; but after the motion has become established the differences of pressure satisfy themselves by keeping up the stream-line con-figuration ; the energy which the particles receive from the body while they are being pushed aside by it along their stream-line paths is finally redelivered by them to it as they collapse around it, and come to rest after its passage, and the integrals of the + and - pressures on the body are exactly equal at every moment. The manner in which this is effected is governed by the general laws of fluid motion, as expressed by the well-known equations; and, since these equations contain no term which implies a loss of energy, the energy existing in the body, as well as in the stream-line system, remains unaltered ; so that, if the motion is steady, or without acceleration or retardation, the body passes through this theoretically perfect fluid absolutely without resistance. Nor must it be thought a paradox (for it is unquestionable) that even a plane moving steadily at right angles to itself through a perfect fluid would in the manner described experience no resistance. But if the fluid, instead of being infinite in all directions, be bounded by a definite free surface parallel to the line of motion, such as a water level, the existence of this surface cuts off the reactions of all those particles which would have existed beyond the surface had the fluid been unlimited alike in all directions, and which would have given back in the manner described the energy imparted to them. By the absence of these reactions the stream-line motions which would have existed in the infinite fluid are modified, and the differences of pressure involve corresponding local eleva-tions of the surface of the water in the vicinity of the moving body. And since, in consequence of the action of gravitation (the force which controls the surface), a water protuberance seeks immediately to disperse itself into the surrounding fluid in accordance with the laws of wave motion, the local elevation partly discharges itself along the surface by waves which carry with them the amount of energy embodied in their production. This energy is, in fact, part of the aggregate energy which was imparted to the particles of fluid while they were being pushed aside, and which, in the infinitely extended fluid, would have been wholly restored to the body during their collapse after its passage, but is now, in fact.

dissipated. The exact equality between the + aud - pressures no longer exists, and the body experiences a definite resistance which it would not do if the fluid wore infinite in all directions.
"It is clear, moreover, that the nearer the moving body approaches the surface the greater are the differences of pressure to be satisfied, the greater will be the waves formed, and the greater the dissipation of energy. Thus, for example, a fish will experi-ence an increase of resistance as its path lies nearer to the surface, the train of waves it creates becoming then a visible accompani-ment of its progress. A fortiori, when the body moves along the surface as a ship does on water, those differences of pressure which would exist during the motion if the fluid were infinite in all directions satisfy themselves in still larger waves, which, in fact, are the waves which accompany the body in its motion. The waves which thus visibly accompany a vessel in transitu form a marked phenomenon in river steaming. Thus we see how, although in a perfect fluid extended infinitely in all directions, a body, when once put in motion, would move absolutely without resistance, yet, when the fluid is bounded by a gravitating surface at or near the line of motion, the body will experience resistance by the formation of waves, notwithstanding that the fluid is a perfect one.
"If the fluid is again supposed to be infinite in all directions, but imperfect, the phenomena previously described undergo appropriate modifications, and the moving body will also suffer a specific resistance,—in the first place by its having to overcome the friction and viscosity of those particles of the fluid with which it is in contact, and next because the friction of the surrounding particles inter se destroys that orderly arrangement of the stream-line con-figuration which allows of the energy imparted to the particles being returned without loss. If the supposed imperfect fluid is bounded by a free surface, as already described, and the body moves at or near this surface, it will experience resistances depend-ing on fluid friction, almost exactly in the same manner as if the fluid were infinite in all directions. It will also experience very nearly the same resistance in virtue of the wave-making action as in the perfect fluid ; and we here see the two sources of resistance existing independently of each other, and due to totally different causes."
Stability. Important as the question is as to the effect of form upon resist-ance, that of its effect upon stability or steadiness at sea is even more so. Before the use of steam for the propulsion of ships the speed which could be attained in seagoing ships by sail power was largely a question of stability or power to carry a large spread of canvas without inclining or " heeling " too greatly. Small differences in the form of the transverse sections of the ship in the region of the load water-line and under water were influential in this respect, and naval constructors occupied themselves greatly with such ques-tions. The form of the problem completely changes when the pro-pelling power is no longer an upsetting force. The important questions in steam ships are the proportions of length, breadth, and depth ; the form of "entrance" and "run" ; the construction of propelling machinery within the ship ; and the proportions, form, and number of revolutions of the propeller. But, while this is so, the effect of the stability of the steamship upon her behaviour at sea, as a question of rolling or "labouring," remains very great. There are, moreover, a very large number of seagoing ships still dependent upon sails for their propulsion, and the question of sailing power is very important in vessels employed on our coasts for commerce and for pleasure. The latest and most complete in-vestigation of questions of stability is to be found in Sir Edward J. Eeed's recently published work, The Stability of Ships. There is a more popular exposition of the subject by Mr W. H. White, director of naval construction, in his Manual of Naval Architecture (1877, 2d ed. 1882), of which use has been made in the following pages.
Fig. 3.
When the cavity is
filledT with water its weight, called in relation to the ship the " displacement," may be supposed to be concentrated at B, fig. 3, which is the centre of gravity of the "displacement" or of the displaced water. This centre of gravity is usually known in relation to the ship as the "centre of buoyancy." The weight of this water may be supposed to be concentrated at B, and to act vertically downwards. As this water would remain in the cavity at rest, its downward pressure must be balanced by equal upward
A ship floating freely and at rest in still water displaces a volume of water exactly equal in weight to her own weight. The circum-stances of the water in which she floats are in fact the same whether the cavity made in the water by the ship is filled by the ship as in fig. 2, or by a volume of water having the same weight as the ship (fig. 3). When the ship oc-cupies the cavity the whole of her weight may be sup-posed to be con-centrated at her centre of gravity,
G, fig. 2, and to act vertically downwards.
pressures, that is by the buoyancy of the surrounding water. These upward pressures must act in the same w7ay as if there were a single pressure equal and opposite to the weight of the water, and acting through the " centre of buoyancy." In fig. 2 a ship is represented floating freely and at rest in still water. Her total weight may be supposed to act vertically downwards through the centre of gravity G, and the buoyancy vertically upwards through the centre of buoyancy. The second condition which the ship-floating freely and at rest in still water will always satisfy is there-fore said to be that her centre of gravity will lie in the same vertical line with the centre of gravity of the volume of water which she displaces. So long as the ship rests under the action of these opposing and balanced forces the line joining the centres B and G is vertical and represents the common line of action of the weight and buoyancy. There are of course horizontal fluid pres-sures'acting upon her, but these are balanced among themselves.
The ship may be floating at rest, but under constraint, and not freely. There may be the pressure of wind on the sails, or the-strain of a rope holding her in a position of rest although the-centres B and G are no longer in the same vertical line. Fig. 4 represents such a case. The vessel is at rest, but there is some ex-ternal force operating other than that of buoyancy; and the equal and opposite forces of the weight and buoyancy act in different vertical lines, and no longer balance each other. They form a mechanical "couple," tending to move the ship from the position of con-strained rest in which she is shown. If W represents the total weight of the ship (in tons), and d the per-pendicular distance between the parallel lines of action of the weight and buoyancy (in feet), then the operative moment of the "couple" is represented by the product of the two quantities W and A, mea-sured in foot-tons. If the constraint is removed, and the vessel is freed from all external forces save those of the fluid in which she floats, she will move under the operation of the "couple" towards the upright position until the consequent alteration in the form of the cavity of the displacement brings the centre of buoyancy into the same vertical with the centre of gravity of the ship. What has been-illustrated by reference to transverse inclination of the ship is equally true of oblique or longitudinal inclinations. If the position of the weights in the ship remains unaltered under such changes of inclination the centre of gravity remains unaltered. In all calcula-tions it has to be assumed that the centre of gravity is a fixed point in the ship, and that movable weights will be secured in the ship. With this assumption the position of the centre of gravity of a ship-can be correctly assigned by calculation, small disturbances caused by movements of men, &c., not being large enough to be appreciable.
The statical stability of a ship may be defined as the effort which-she makes when inclined steadily by external forces to overcome the constraint and return to the position in which she floats freely, at or near the upright. This effort, as already explained, depends upon the position of the centre of buoyancy B, or the distance from the vertical line through G which the altered form of the cavity of the displacement has caused it to assume. It may always, be measured by the product of the two quantities W (in tons) and d (in feet) (see fig. 4). This product in foot-tons is known as the "moment of statical stability" for the particular angle of inclina-tion and corresponding position of B which are assumed. A little reflexion will show that when large angles of inclination are reached the centre B ceases to recede from the vertical through the centre of gravity of the ship, but will, as the inclination increases, approach this vertical line, and eventually pass to the other side of it.
The moment of statical stability is at its maximum when the distance d is greatest. The angle which the ship has reached when the centre B has reached this point is called the " angle of maximum stability." As the centre B travels backwards from this position with the increasing inclination of the ship the dis-tance d decreases and the righting power of the ship decreases pro-portionately. When B passes the vertical line through G the-moment of stability changes its character and becomes an upset-ting force, which will continue to act until the ship reaches a new position of rest, usually bottom upwards. The angle which the ship reaches before this change takes place, i.e., when B passes to the other side of the vertical line through G, is called the " angle of vanishing stability" and it indicates the ship's "range of

ostability." The change may occur at very small angles if the ship is crank and her sides are low in the water. It may not and sometimes does not occur, on the other hand, until the ship is lying on her beam ends.
If a curve is plotted out showing these positions and indicating _also how d first increases and then decreases as the ship is inclined more and more from the upright, the curve is known as the curve of stability. A '' stiff ship " is one which opposes great resist-ance to inclination from the upright when under sail or acted upon by ex-ternal forces. A " crank ship" is one very easily inclined, the sea being sup-posed to be smooth and still. A "steady ship " is one which when exposed to the action of waves keeps nearly upright. Crank ships are usually the steadiest ships. Changes in the height of the point of intersection M (fig. 4) above the centre of gravity indicate corresponding changes in the stiffness of a ship. Speaking generally, the stiffness of the ship may be considered to vary with the height of M above G. The line BM does not cut GM in the same point at considerable inclinations as it does at a very small inclination. The point of intersection at the smallest conceivable inclination receives a definite name. It is known as the metacentre, and the distance GM is in this condition called the metacentric height. See HYDROMECHANICS.
The following table contains particulars of the metacentric heights of different kinds of vessels of war, and the corresponding time of an oscillation in still water :—

Names of Ships. Metacentric Height. Period of a Double Eoli.
H.M.S. " Sultan,"
H.M.S. "Inconstant,"
' H.M.S. "Devastation,"
American monitor (shallow draft)
'' Inflexible," when rolled in still ) Feet. 2-5 2-8 37
14-0
7-65 Seconds. 8-9 8-0 6-76 2-7
107
Generally speaking, decrease in metacentric height is accompanied by a lengthening of the period of an oscillation. The ship swings more slowly as she loses stiffness. Oscilla- There is no sensible difference in the time occupied by a ship in tions. a swing or roll from side to side, whether she rolls through only three or four degrees on either dde of the upright or twelve or fifteen degrees. For larger angles there would be small differences.
The tables which have been given show some remarkable changes in the stability conditions in ships of war within recent years. Sailing ships were formerly made with so little deviation from existing types that it was not found to be necessary to ascertain their exact measure of stability or to lay down rules for regulating it. The position of the centre of gravity was modified by ballast, and as much as nine or ten per cent, of the displacement was allowed for this. Heavy rolling and great uneasiness of ship from excessive stability had often to be endured. In other cases crank -ness or inability to carry sail had to be accepted. When armoured ships were first introduced they had about the same metacentric height (6 feet) as is to be found in the earlier sailing frigates. The "Normandie " in the French navy and the " Prince Consort" in the English navy had from 6 to 7 feet, and they were exceed-ingly uneasy and deep-rolling ships. It was soon discovered that a reduction in metacentric height would cure this evil. The later ships in both navies were accordingly designed to have a metacentric height of about 3 feet. The '' Magenta " had 3J feet and the " Hercules" 3 feet. This change altered the period during which the ship made a double oscillation, i.e., from star-board back to starboard, to 14 to 16 seconds instead of 10 to 11 seconds, as it had been in the "Normandie" and "Prince Con-sort." The effect on the behaviour of the ship in a seaway was most remarkable. These ships with small metacentric height might be put into the trough of a sea, and as the waves crossed them they steadily rose and fell, hardly inclining their masts. The _effect on gunnery practice was also valuable, but there is always a peril attending steadiness obtained by such means : vessels having small metacentric height require careful handling under sail or they may be overset and lost. There is another defect in this system, viz., that wounds in action will cause the ship to incline sooner and more considerably, and they become more dangerous than they would be in a stiffer ship. Bilge-keels and water chambers are now employed in the English navy, together with, and as opposing influences to, much greater metacentric height.





These devices were introduced into the "Inflexible" in order to counteract the influence of a metacentric height of 8 feet which was designedly given to her. They have proved very effective, but there is another feature in this vessel which has tended to prevent uneasiness and heavy rolling. The time of an oscillation, or quickness of rolling, depends not only upon the metacentric height but upon the moment of inertia about a longitudinal axis. The time of an oscillation from starboard to starboard may be
written thus :—
2T = 1-1 sjWJm,
where T is the ship's period in seconds for a single roll, m is the metacentric height, or height of metacentre above the centre of gravity in feet, and K is the radius of gyration in feet. The moment of inertia is increased by widening the ship, putting heavy armour on her sides, and placing the turrets and guns out towards the sides of the ship. It was seen that these features in the "Inflexible," which were elements in her design, would favour her and tend to counteract the great metacentric height. The event has shown that, while a metacentric height of 6 feet in the "Normandie" gave 10 seconds to 11 seconds period, 8 feet in the " Inflexible" only gives 11 seconds as a period, corresponding with a radius of gyration of 28 feet. The feeling expressed that "in order to provide against the impossible contingency of the loss of stability by complete waterlogging of the ends we had made an intolerable ship was not justified. The ship is now so stiff that when the ends are waterlogged the running in and out of all her guns on one side only inclines her 2J degrees, while in the "Monarch" when intact and. light the same operation inclines the ship 5 degrees.
The resistance offered by the water to the rolling of the ship Resist-consists of three parts :—(1) that due to the rubbing of the water ance to against the bottom of the ship as she rolls ; (2) that due to the oscilla-flat surfaces which are carried through the water, such as outside tion. keels and deadwood ; (3) the creation of waves by the rolling ship to replace those which move away from the ship. The creation of these surface waves expends energy and checks the motion of the ship which makes the creative effort.
Mr White, giving briefly the results of some of the experiments of Mr Froude made for the Admiralty, says :—
Period of Double Roll.
"Experiments have been made by Mr Froude to show how rapidly the rate of extinction may be increased by deepening bilge- Bilge-keels. A model of the ' Devastation' was used for this purpose, keels, and fitted with bilge-keels, which, on the full-sized ships, would represent the various depths given in the following table. The model was one thirty-sixth of the full size of the ship, and was weighted so as to float at the proper water-line, to have its centre of gravity in the same relative position as that of the ship, and to oscillate in a period proportional to the period of the ship. In smooth water it was heeled to an angle of 8J degrees, and was then set free, and allowed to oscillate until it came practically to rest, the number of oscillations and their period being observed. The following results were obtained :—
Model fitted with
Seconds.
1-77
1-9
1-9
1-92
1-99
314 124 8
5Î 4
Number of Double Rolls before Model was practically at rest.

No bilge-pieces
A single 21-inch bilge-keel on each side
,, 36-inch ,, ,,
Two 36-inch „ „
A single 72-inch ,, ,,
" Not'content with obtaining the aggregate value of the resistances for ships, Mr Froude has separated them into their component parts, assigning values to frictional and keel resistances as well as to surface disturbance. In doing so, he has been led to the con-clusion that surface disturbance is by far the most important part of the resistance offered to rolling, as the following figures given by him for a few ships will show:—

Ships. Frictional. Keel, Bilge-keel, and Deadwood. Total Resistance. Surface Disturbance.
Inconstant
Greyhound 354 140 96 120 5,036 4,060 2,944 700 20,000 21,500 14,100 4,700 14,610 17,300 11,060
3,880
" Frictional and bilge-keel resistances in this table have been obtained by calculation from the drawings of the ship, Mr Froude making use of data as to coefficients for friction and for head resistance which he had previously obtained by independent experiments, and which may therefore be regarded as leading to thoroughly trustworthy results. It will be noticed that in no case does the sum of the frictional and keel resistances much exceed

one-fourth of the total resistance, while it is much less than one-fourth in other cases. The consequence is that surface disturbance must be credited with the contribution of three-fourths or there-abouts of the total resistance, a result which could scarcely have been predicted. Waves are constantly being created as the vessel rolls, and as constantly moving away, and the mechanical work done in this way reacts in a reduction of the amplitude of successive oscillations. Very low waves, so low as to be almost impercept-ible, owing to their great length in proportion to their height, would suffice to account even for this large proportionate effect. For example, Mr Froude estimates that a wave 320 feet long and only 1| inches in height would fully account for all the work credited to surface disturbance in the fourth case of the preceding table.
" Another important deduction from the figures in the table is the large proportionate effect of ' keel' resistance as compared with frictional resistance, thus establishing the advantages of deep bilge-keels. Ships of the Royal Navy recently constructed have been furnished with much deeper bilge-keels than were formerly in use, but a limit to the depths that can be fitted is often reached, because of the necessity for compliance with certain con-ditions and extreme dimensions, in order that the vessels may be able to enter existing docks." Water In the Royal Navy advantage has also been taken of the power cham- of loose water within a ship to quell motion. It was first employed bers. in the " Inflexible " in a part of the ship lying above the bomb-proof deck, and at the level of the water-line. Its use resulted from a discussion, when the " Inflexible " was designed, of the probable effect of water entering this region of the ship through shot holes. The matter has since been thoroughly established by experiment, and affords a new and valuable means of preventing heavy rolling in ships having large initial stability. There is now no hesitation in giving a metacentric height of 6 feet, and obtaining all the security against upsetting which this ensures, because it is felt that the violent rolling formerly inseparable from stiffness can be prevented. The investigation into this matter has been conducted by Mr Philip Watts, Mr R. E. Froude, and Mr W. E. Smith, acting for the Admiralty.
The accompanying memorandum, prepared by the present writer in 1884, gives the general results :—
' In investigating the phenomena attending the use of water as a means of quelling the motion of ships, Mr Froude has not only taken advantage of the experiments made in the ' Edinburgh' by running men across the decks, but he has also studied similar phenomena in a model water-chamber, mounted, not on a model of a ship, but on a large pendulum weighted to the required 'period,' the relative level of the model chamber and the axis of rotation being made to correspond approximately to scale with that in the ship.
' The conclusions, stated in the form of a comparison between the quelling effects of bilge-keels and of moving water, are as follows:— (1) There is a certain depth of water in the chamber which gives the maximum effect; this is dependent upon the width of the chamber and the period of the ship. (2) With this depth of water the growth of resistance to rolling commences almost at zero of angle, whereas with either a greater or less depth there is practically no resistance at all due to the water up to a certain angle, which angle increases with increase of departure from the proper depth. (3) At larger angles of roll the disadvantage of departure from the proper depth of water is not marked. (4) The resistance of water in a chamber does not increase at all uniformly with increase of angle of roll, but increases rapidly at first and at the larger angles becomes more nearly constant for all angles. (5) The best quantity of water for the original chamber in the 'Edinburgh' was 43 tons ; the best for the chamber enlarged by removal of cork walls was 79 tons ; and the best for the chamber extending to the sides of the ship would be 100 tons. The first-named extension improved the resistance at 10° by 21 per cent., and the further extension by another 22 per cent.
" As compared with bilge-keels the matter is stated as follows :— while 2 feet addition to the breadth of the bilge-keels adds in round numbers two-thirds to the existing extinguishing power of hull and bilge-keels on the 'Edinburgh' at all angles of rolling, the fully extended water-chamber adds at 3° of roll about six times, and at 5° about three times that power ; at 12° the chamber adds no more than 2 feet of bilge-keel, while at 18° it only adds half as much. It is therefore evident that, while both are valuable, the water-chamber is for most kinds of service much the more valuable of the two.
" Explaining the cause of the phenomena, Mr Froude says :—_ " 'The extinguishing or quelling effect of water depends, of course, cmteris paribus, upon the value of the moment represented by the transference of water from side to side, i.e., with a given quantity of water, upon the distance moved hy its centre of gravity. This distance increases with increr.se of angle of roll, and consequently the extinction similarly increases up to a certain point, where we appear to have approximately reached the maximum possible transference of water, and consequently the maximum extinction of which the quantity of water is capable with the dimensions of the chamber. This point occurs generally at a moderate angle, and above this angle the extinction becomes practically constant. But the extinguish-ing effect of the water of course largely depends also upon the timing of its motion from side to side,—the extinction being greatest when that motion takes place most nearly at the time of extreme angle of ship, i.e., in such a manner as that the water may be as much as possible running downhill when it is moving across, and as much as possible upon the rising side of the ship when it is stationary. If, on the other hand, the motion of the water across were to take place when the ship is quite upriglit, the extinction would be nil. It is therefore conceivable that for the same total degree of motion or transference of water we may have a very different degree of extinction, according to the timing of that motion. In the motion of the water energy is necessarily wasted, and it is clear that, if we are dealing with a permanent condition of things, i.e., if the ship is being steadily maintained at a constant angle of roll, this waste of energy in each run of water from side to side must be exactly equal to the energy taken out of the-ship in each swing by the extinction. The motion of the water may be and generally is of a type very wasteful of energy, the water either rushing, across in a mass, and consuming its energy by breaking with great violence against the opposite side, as it does at the larger angles of rolling, or, at more moderate angles, running across in a breaking wave or bore, which consumes its energy as it goes in its own internal resistance; and under these circumstances the timing of the motion appears invariably to approacii pretty nearly to that giving the maximum extinction for the degree of motion. But the motion of the water sometimes takes the form of a mere alternating slope of surface, or tidal swing from side to side, and here there is very little waste of energy, the energy of motion of the flow of water in one direction being converted into potential energy in the shape of rise of water at the side, and then given out again to the water flowing back to the other side, and so on. The waste of energy in this form of motion being: almost nil, the timing is almost exactly that appropriate to no extinction, the water being in the middle of its passage across and the surface being level when the ship is upright.'
'' The value of the chamber of course increases as its length in the direction of the keel of the ship increases. The actual size of the chamber we adopted appears to give valuable results, although its extent was necessarily limited."
Tabular Statement of Results of the Above Experiments.

Empty Chamber. Existing Bilge-keels. Fully Extended Chamber. Empty Chamber. Two Feet additional Width of Bilge-keel. Wave Slope.
Steady rolling in co-_
Irregular rolling represented by angle accumulated from rest in five successive co- i _5
10 15 20
_5
10, 15 20 o9 3-8 10-6 16-6
1-3 4-5 10-7 16-6 3-8 7-5 11-1 14-7
41 7-7 11-2 14 8 _39 1-22
2-63 4-59
_47
1-32 2-67 4-64

In some lectures recently delivered Mr Smith, assistant con-structor of the navy, illustrated the use of water in quelling motion by models as shown below.

ra a/ r
\ ' 11


Fig. 6.
" The models represented the midship sections of the ' Admiral' class, and were both of the same weight and size. Each model was mounted onv trunnions, marked T, and both oscillated freely on these trunnions in, exactly the same time. The models were placed one behind the other, so that the parallelism of the masts was evident to the audience. The model in fig. 7 was provided with a glass tube into which varying quantities of water could be put. An amount of water representing T£5 of the total weight of the model, i.e., 100 tons in a 10,000-ton ship, was now placed in the tube, the models were started from the same angle as before, and the-model with the loose water, instead of keeping up exactly with the other, or rolling more violently, came almost instantaneously to rest.
" The tube was filled with varying quantities of water, and the effect was. always to stop the model much sooner than the model with no weights free to move. The two models were always started from the same angle, so that their relative behaviour could be easily seen. When the tube was quite full there was practically no effect. The two models rolled almost together.' The same effect resulted from the motion of a marble representing in. weight 100 tons in a ship of 10,000 tons. The same reduction must always occur in a rolling ship if we have a loose weight of any kind, whether the

weight be water or a gun. If this reduction did not take place we should have something to explain which would be quite inexplicable. For suppose we have two ships alike in all respects as regards size, shape, weight, time of oscillation, &c., and situated on precisely the same seas, but one having all her weights properly secured, and the other with a weight capable of traversing the deck every time the ship rolls. If the two vessels were to roll to exactly the same extent we should have the sea not only rolling the ship with the loose weight to the same extent as the ship with all her

weights fixed, but the sea would, in addition, be doing all the work involved
in the traversing of the heavy weight across the deck, which is quite
impossible under the circumstances of perfect similarity we have supposed.
The sea can only do the same work on both. In the one case that work
consists entirely in rolling the vessel, in the other it consists partly of
rolling the ship and partly in dashing the weight about. The rolling in the
latter must therefore inevitably be less than in the former case."
Dynami- . Dynamical stability is the "work" done or energy expended in
cal heeling the ship from the upright to any inclined position. The
stability, unit of " work " employed in measuring dynamical stability is a foot-ton. When the vessel is gradually inclined the forces inclining her must do work depending upon the amount of the statical stability at the successive instantaneous inclinations passed through, and these are given by the curve of stability already described. Dynamical stability is of value as a means of comparing the resistance of ships to upsetting under the action of suddenly applied forces, such as squalls of wind. Illustrating this Mr White says :—
"Roughly speaking, it may be said that a force of wind which, steadily and continuously applied, will heel a ship of ordinary form to a certain angle will, if it strikes her suddenly when she is upright, drive her over to about twice that inclination, or in some cases further still. A parallel case is that of a spiral spring ; if a weight be suddenly brought to bear upon it, the extension will be about twice as great as that to which the same weight hanging steadily will stretch the spring. The explanation is simple. When the whole weight is suddenly brought to bear upon the spring, the resistance which the spring can offer at each instant, up to the time when its extension supplies a force equal to the weight, is always less than the weight; and this unbalanced force stores up work which carries the weight onwards, and about doubles the extension of the spring corresponding to that weight when at rest."
Structure.
The changes which have come about in materials and modes of construction within the last 50 years have been most remarkable. The first steamer built expressly for regular voyages between Europe and America was not built until 1837. Dr Lardner stated at about this date : " We have as an extreme limit of a steamer's practicable voyage, without receiving a relay of coals, a run of about Great 2000 miles." The " Great Western," built by Patterson of Bristol Western, and engined by Maudslay of London under the superintendence of Sir I. K. Brunei, was the first such ship, and she was launched July 19, 1837. She was 212 feet long between the perpendiculars, 35 feet 4 inches broad, and had a displacement of 2300 tons. She was propelled by paddles. Iron vessels were built early in the present century for canal service, then for river service, and later for packet service on the coasts. In about the year 1838 iron vessels of small dimensions were built for ocean service. The largest iron vessel built up to 1841 was less than 200 feet long. In 1843 we get for the first time the ocean-going steamship in its present form, built Great of iron, and propelled by the screw. This was the " Great Britain. Britain," 286 feet long, projected and designed by Brunei. Time has abundantly justified these bold enterprises on the part of Brunei, which he had to carry through in the face of great opposi-tion. He entered with equal boldness on another innovation in 1850, viz., the use of very large dimensions on the ground of economy of power. It was not until 1852 that he had the oppor-
Great Eastern
tunity to put these views forward in a way to satisfy him. The different sizes of vessels discussed before the design was finally settled for the "Great Eastern" were as follows :—
No. ' Length. Breadth. Midship Section. Draught.
1
2 3 4 663 634 609 730 79-9 76-39 73-5 87 1,646 1,640 1,639 2,090 24 25 26 28
The dimensions eventually settled were—length, 680 feet; beam, 83 feet; mean draught, about 25 feet; screw engine, indicated horse-power, 4,000, and nominal horse-power, 1600; paddle, indicated horse-power, 2,600, and nominal horse-power, 1,000 ; to work with steam 15 lb to 25 lb ; speed of screw, 45 to 55 revolutions ; paddle, 10 to 12.
The "Great Eastern," produced by the joint skill of Brunei and Scott Russell, remains in advance of present practice, although she has served as a model for the best of it. Her great size rend-ered it possible to give to her an amount of security against fatal injury to her hull which cannot be attained in smaller ships. It is a mistake to suppose that large ships are less secure than small ones. The large ship can receive without inconvenience a wound which would be fatal to a small one, and the possibilities of obtaining high speed increase with the size. Had a higher speed been aimed at in the " Great Eastern," it might have altered the whole current of her history, and changed also the history of ship-building itself.
The question of bulkheads, on which Brunei insisted so much in Bulk-this ship, is one which underlies all questions of construction. If heads, the number of bulkheads in ships were increased as they ought to be, the numbers and sizes of the ribs or frames of the ship would be modified, and the system of construction generally would be changed, and become more like that of the " Great Eastern." The question is therefore one which justifies some further consideration, so that it may be popularly understood.
Iron ships are commonly made with less than half their bulk out of water. If water enters such a ship, and the amount which enters does not exceed in bulk that portion of the bulk of the ship which is out of the water, and which will, when immersed, exclude the water, then the ship, if she does not turn over, will still float. If, however, the inflow cannot be stopped, but continues, the ship soon sinks.
Let us suppose the case of a ship 50 feet long, 10 feet wide, and 10 feet deep, divided into five equal parts by four watertight par-titions, and floating in water with half its bulk immersed (fig. 8). Suppose now that a hole is made in the middle of this ship under the water, so that water can flow-freely in, then the part of the ship which is shaded
ceases to have floating power. The water in this shaded place is no longer displaced, but is admitted, and if the ship is to continue afloat, the other parts of the ship must displace water to the amount by which this shaded part has ceased to do so. As it is one-fifth of the whole immersed bulk which is lost, the remaining four compart-ments must sink, so as each to support one-fourth of the whole, instead of one-fifth, as before ; i.e., the draught of water, or im-mersion of the whole ship, will be increased, and the ship will, if she has stability enough to keep upright, finally float at rest again at this deeper immersion. The water will rise in the centre com-partment to the level of the water outside, and will then cease to flow in. The additional immersion will be only one and a quarter feet, but in an ordinary ship, divided into compartments of equal length, there would be a greater increase of immersion by the injury of a centre compartment, because the end compartments are narrow, and must sink deeper in order to bear their share of the burden imposed by the loss of the buoyancy of the centre division.
Or it may be other than a central compartment which is damaged, and in that case the ship tips, and finds a new floating line, with the end towards which the damaged division lies depressed more than the other end.
If it should happen that the divisional partitions, or bulkheads as they are called, rise only a few inches above the water level which the ship floats at when undamaged, then, on the occur-rence of a bad leak filling one compartment, the tops of the bulk-heads are brought, by the increased immersion of the ship, beneath the water-level, the water will rise through the hatches, or open-ings in the deck, in the damaged compartment, will flow over the entire deck, and the ship will be lost, either by the filling of other compartments by the water passing down into them, or by the capsizing of the ship. This latter event will generally happen, although only one compartment is full, if the sea has free access to











Recommendations of the Council of the Institution of Naval Archi-tects.
the deck from end to end of the ship, and it becomes wholly immersed.
In 1866 the president of the Institution of Naval Architects said : " The circumstances of the sad event of the loss of the 'London,' accompanied as it was by the simultaneous loss of another ship of still larger size, and of a higher reputed character " (the "Amalia,"), "was, I think, an event so remarkable that I should be very sorry indeed if this annual meeting of this Institution were to pass by without some notice being taken of the extraordinary circumstances of the loss of that ship, and without some discussion upon what we suppose to be the causes of the loss, and the faults, if any, of the construction of those ships." "The passengers who pass to and fro are not judges of the question ; they can take no precaution for their own safety; it is to the skill and science of those who build these ships that the passenger trusts, and to the care which the legislature and the Government are bound to take of their fellow-subjects."
Subsequently the council of the Institution arrived at the following conclusions and offered them as recommendations to shipbuilders and shipowners:—
" 1. No general rule can be safely laid down for regulating the proportions of length and depth to the breadth of a ship, and a great variety of proportions of length and depth to breadth may be safely adopted, and the ship made sound and seaworthy, by judicious form, construction, and lading.
" 2. The construction load-water-line of every ship, and her scale of displacement from light to load-water-line, should be appended to every design of a ship, showing the extreme draught to which she should be laden; and measures should be taken to ensure that this information be recorded on the ship's papers. It is desirable also that along with a ship's papers, in the possession of the captain, there should always be carried a scale of displacement, a sail draft, and a set of outline plans of the ship, comprising a longitudinal section, and at least four cross sections of the ship. On these plans should be marked the capacity, in tons of 40 cubic feet, of each compartment of the hold. The surplus buoyancy of each compartment up to the load-water-line, or its power to carry deadweight, should be given in tons deadweight. These papers should always accompany the ship's register, and a copy of them should be lodged in the custom house of the port from which the ship hails.
" 3. There is a minimum height of freeboard which cannot be safely reduced in sea-going ships of ordinary fitment; and it is desirable to fix this minimum height. Freeboard should be understood to be the vertical height of the upper surface of the upper deck (not spar-deck) at the side, amidships, above the load-water-line. The proportion of freeboard should increase with the length. One-eighth of the beam is a minimum freeboard for ordinary sea-going ships of not more than five breadths to the length, and 3*2 of the beam should further be added to the freeboard for each additional breadth in the length of the ship ; this would give— For a ship of 32 feet beam and 160 feet long, 4 feet freeboard ; For a length of 192 feet, 5 feet freeboard ; For a length of 224 feet, 6 feet freeboard;
For a length of 256 feet, 7 feet freeboard;—the beam remaining the same. But, as the addition of a spar-deck on long vessels may be considered an equivalent or substitute for the increased freeboard required for extra length, a complete spar-deck would leave the freeboard of these extra lengths at the original height of 4 feet.
" 4, It is not considered desirable to offer any recommendations with regard to poops and forecastles. It must depend entirely upon the professional judgment of the designer of a ship, whether, looking to her proportions, form, and purpose, the additions of poop and forecastle are expedient and safe. In general, where poops and forecastles are adopted, they should be closed and seaworthy, but their weight may be inexpedient in long fine ships ; and there are cases where a light top-gallant forecastle (i.e., an open forecastle raised above the level of the upper deck) may be useful in keeping heavy seas out of the ship. In general, spar-decks in long ships are preferable to poop and forecastle, and no diminution of freeboard should be allowed for a poop or forecastle.
" 5. It would add much to the strength and security of steamships if transverse and longitudinal bulkheads, coal bunkers, iron lower decks, and screw alley were all so connected with the hull of the ship and with each other as to form independent cellular compartments, watertight, and having all their communications with the decks and each other by water-tight doors worked from the deck. In proportioning the compartments of a ship (and especially of ships devoted to passengers) it is very desirable so to arrange them that if any two adjacent compartments be filled, or placed in free communication with the sea, the remaining compartments will float the ship. It is considered that no iron passenger ship is well constructed unless her compartments be so proportioned that she would float safely were any one of them to fill with water, or be placed in free communication with the sea. Double bottoms are to be regarded as a great element, both of safety and strength, in the structure of a large iron ship.
"6. It is very desirable that sufficient ventilation should always be pro-vided in passenger ships to admit of closing all side scuttles and battening aown, or otherwise enclosing, all hatches in bad weather.
" 7. In regard to hatchways and openings in the deck no limits can be set to their size ; but it is desirable to carry the beams of the ship across them without interruption wherever practicable ; they may also be made remov-able where required, being replaced on going to sea. All coamings over engine and boiler rooms in passenger ships should be as high as practicable, of iron, and riveted to the beams and carlings. Openings in the deck may be fitted with solid coverings, hinged in place so as to be readily closed.
" 8. It being considered that all openings in the sides or ends of vessels are subject to accidents that endanger the safety of ships, it is desirable that the side and stern windows should, in addition to the glass lights, have hinged dead-lights, with a view to their being always in place, and that all cargo ports should be strongly secured by iron cross bars.
" 9. It is believed that all openings from and communications with the sea from engine-room and pipes should be protected by conical, or Kingston, or sluice valves, and similar precautions should be taken for all openings through the bottom of the ship, where damage to pipes or ship would admit water into the holds.
" 10. It is considered that all steam vessels, if of iron, should have a brass-barrelled hand-pump to every compartment except the forward and after ones (the former to have a sluice cock), or that, as a substitute for these pumps, there should be patent pumps having independent connexions to this extent. They should also have a donkey engine and pump capable of pumping from the bilge and from the sea, of feeding the boilers, and of throwing water on deck. All vessels should have one or more bilge-pumps, worked by the large engines, with bilge injection pipes if the engines have condensers. In large vessels the donkey engines should have a separate boiler high above the water-line, and also communication with the main boilers. All vessels should have a set of bilge pipes connecting every hold and the engine compartments with these pumps. As a security against fire there should be pumps on the upper deck, fitted as force pumps, and pro-vided with a sufficient length of hose (with the necessary copper delivery jets) to reach either extremity of the vessel, and also provided with suction hose or pipes from the sea. The cocks by which the working of the pumps is regulated should be carefully arranged and marked, and great care should be taken that both cocks and pipes are accessible. A plan of the whole should accompany the ship's papers, and the crew should be periodically exercised in their use.
" 11. The stowage of a ship, whether done by contract or not, should be done under inspection of the captain of the ship, and should be conducted under his own orders only ; and he alone should be held responsible for the good stowage of his ship. Ships are often very badly stowed, the weights being sometimes too low, thus causing them to roll with such rapid and violent motions as to carry away the spars, and otherwise endanger the safety of the ship, and at other times too high, thus making the ships crank, and liable to turn over. A ship may, however, generally, whatever her form, be so stowed as to avoid both dangers. As the character of the ship in these respects varies, so does the number of oscillations she would make per minute if she were set rolling in still water, by men running across her deck, or other means, and then allowed to come to rest; that is, if the ship be crank the number of oscillations per minute will be few, and if she be too stiff they will be numerous ; but, under the same conditions of stowage, the number will always be very nearly the same, whatever the amount of the impulse to set her rolling may be. Although this peculiarity has long been known to scientific men, no such observations have been made in merchant ships as would justify any specific rule on the subject. It is, however, most desirable that information should be collected upon it, and that the attention of the owners and captains of vessels should be called to it.
" 12. It is believed that the present rules of the Board of Trade regarding boats, life-boats, and their tackle are good in principle. The responsibility for keeping all boats in constant readiness and efficiencjr obviously rests on the captain, and must fix on him the blame for all neglect and its consequences. Every open boat built of iron or steel should be fitted with sufficient watertight spaces to float her.
" 13. The system of proportioning anchors and cables by Lloyd's, and of proving under licence of the Board of Trade by Act of Parliament is so far satisfactory; but, as the proof-test alone cannot establish the excellence of the cable, the reputation of the makers must be relied upon.
" 14. In order to provide for the rapid clearance of the upper deck from water which may break over the ship, flap-boards should be fitted to the lower part of the bulwarks, sufficient in number and in area to admit of the rapid escape of the water.
" 15. Water-closets on decks below or near the water-line may be the means of gradually and imperceptibly flooding the ship, and _ endangering her safety, unless the pipes and valves are strong and are carefully fitted."
It is in the directions indicated in these recommendations that the honesty and skilfulness of the modern builder of steam and sailing ships of war come into play, and some judgment may be formed by the general public of the character of the ship by inquiring into matters upon which the council thought it neces-sary to make such recommendations. The guarantee which the public have of the fitness of passenger ships for service, as a question of proper construction and state of efficiency, is the survey and certificates of the Board of Trade. The law runs thus :—
The owner of every steam vessel constructed or intended to carry passengers Board of (except vessels which fall within the definition of foreign-going ships con- rjTra(je tained in the Mercantile Marine Act, 1850, and are employed in the conveyance of the royal public mails or despatches under contract with and under surveys, the superintendence of the lord high admiral or the commissioners for executing the office of lord high admiral) shall cause such steam vessel to be surveyed twice at least in every year, at the times hereinafter directed, by a shipwright surveyor and by an engineer surveyor appointed for the purposes of this Act by the lords of the said committee, such shipwright surveyor in the case of an iron steam vessel being a person properly qualified to survey iron steam vessels, and shall obtain a declaration of the sufficiency and good condition of the hull of such steamer, and of the boats and other equipments thereof, required by this Act; and also, if the lords of the said committee so require, a statement of the number of passengers (whether deck passengers or other passengers) which such vessel is constructed to carry, under the hand of such shipwright surveyor, and a declaration of the sufficiency and good condition of the machinery of such steamer under the hand of such engineer surveyor; and in such declarations it shall be dis-tinguished whether such vessel is in construction and equipments adapted for sea service as well as for river or lake service, or for river or lake service only; such declaration shall state the local limits within which such vessel is, in the judgment of the surveyor, adapted for plying ; and in the case of seagoing vessels the declaration of one of the surveyors shall contain a statement that he is satisfied that the compasses have been properly examined and adjusted; and such owner shall transmit such declarations to the lords of the said committee within fourteen days after the dates thereof respect-ively.
As to the fifth recommendation of the council of the Institution of Naval Architects, it must be observed that there is at present no law relating to the subdivision of steamships. There was a clause (No. 300) in the Merchant Shipping Act of 1854, which was virtually a reproduction of clause 20 of the Steam Navigation Act of 1851, and which read as follows :—
"1. Every steamship built of iron of 100 tons or upwards, the building of which commenced after the 28th day of August 1846, and every steamship built of iron of less burden than 100 tons, the building of which commenced after the 7th August 1851 (except ships used solely as steam tugs), shall be divided by substantial transverse watertight partitions, so that the fore part of the ship shall be separated from the engine-room by one of such partitions, and so that the after part of such ship shall be separated from the engine-room by another of such partitions.
"2. Every steamship built of iron, the building of which commences after the passing of this Act, shall be divided by such partitions as aforesaid into not less than three equal parts, or as nearly so as circumstances permit.
"3. In such last-mentioned ships each such partition as aforesaid shall be of equal strength with the side plates of the ship with which it is in contact.

l: 4. Every screw steamship built of iron, the building of which commences after the passing of this Act, shall, in addition to the above partitions, be fitted with a small watertight compartment inclosing the after extremity of the shaft."
The above law was repealed by the Act dated 29th July 1862, and on the 28th August 1863 the Admiralty applied to the Board of Trade to know whether the Board of Trade officers were em-powered under any circumstances to insist on iron vessels having watertight compartments when employed in conveyance of mails a,nd passengers, observing that the Admiralty were still of opinion that the regulations in force prior to the Amendment Act of 1862 in respect of contract packets should not have been relaxed. They considered such vessels should have compartments so arranged that if any one of them became filled with water the loss of buoyancy thereby occasioned should not endanger the safety of the ships, as recommended by them in their communication of the 17th December 1860. To this the Board of Trade replied (3d Sep-tember 1863) that their surveyors no longer had any power to require given watertight partitions to be fitted in passenger steam-ships—though they agreed with the Admiralty in thinking that steam vessels carrying passengers and mails should be provided with a sufficient number of watertight partitions,—and had no reason to suppose that the Admiralty would not insist on such partitions being fitted in all steamships employed in conveyance of mails. They further say that the enactments in the Act of 1854 were repealed, not because of any doubts as to the necessity of proper and sufficient watertight partitions, but because those enactments which required only two of such partitions for all sizes and classes of ships had become practically useless or mischievous. It was found that in large vessels more partitions than the Act required were necessary to secure the safety of the ship, and it was thought better to leave builders and designers unfettered in pro-viding extra strength and security to meet the various forms, sizes, and descriptions of ships than to tie them down by general statutory regulations which could not be so framed as to meet the varying wants and circumstances of the shipbuilding trade.
In a return by the Board of Trade to the House of Commons, dated 11th August 1875, setting forth the instructions issued to their surveyors under the Merchant Shipping Acts, 1854 to 1873, clause 26 reads—
" Surveyors should not refuse to grant a declaration for a vessel solely on the ground that bulkheads are not fitted, that the ordinary bulkheads are not watertight, or that the bulkheads fitted are otherwise defective, unless they are of opinion that the want of, or the defective state of, the bulkheads renders the ship unseaworthy, in which case they are fully justified in refusing to grant a declaration. They should, in all cases in which they refuse to grant a declaration for a vessel in consequence of defects relative to bulkheads, forward to the Board of Trade a full statement of their reasons for thinking that those defects render the hull of the vessel unseaworthy. Collision watertight bulkheads, at least, must be fitted in all seagoing steamers. The surveyors are also to see that an after watertight compart-ment is fitted to cover the stern-tube of the screw-shaft, both in old and in new vessels."
Ineffi- This regulation has been reissued in the latest instructions to cient Board of Trade surveyors, dated 1884. It thus comes about that subdivi- the number of bulkheads forming watertight compartments, the sion of number of doors in them, and how they are fastened, are made the iron and subject of consideration by the Board of Trade at their inspections ; steel but the fact is that the great majority of ocean-going steamers ships. are not divided into watertight compartments in any efficient manner, and many losses in collision, grounding, and swamping are due to this. Although all steamships have some bulkheads, and some have many bulkheads, they are as a rule distributed in such a way, or are so stopped below the water-level, that for flotation purposes after perforation those lying between the foremost collision bulkhead and the after bulkhead through which the screw shaft passes are practically useless.
With the exception of some four hundred ships, there are no iron steamships afloat which would continue to float were a hole made in the bottom plating anywhere abaft the collision bulkhead and outside the engine-room, or which would not founder were water admitted through breaches made by the sea in weak superstructures and deck openings. Of the four hundred ships referred to as having properly designed bulkheads two hundred are essentially cargo-carriers. They are generally built with five subdivisions, the machinery space being one. Iron sailing ships are without exception undivided into compartments. They have by law a collision bulkhead near the bow, and that is all. Between June 1881 and February 1883 there were about one hundred and twenty iron steamships lost, of speeds of nine to twelve knots, not one of which was well constructed according to the opinion of the council of the Institution of Naval Architects.
It may be said that wooden ships were not divided into water-tight compartments, but it must be remembered that in a wooden ship there is far more local resistance to a blow either in collision or by grounding, and that a wooden ship takes a much longer time to settle down in the water and sink. Also, when wood was employed for passenger and trading ships speeds were much lower and traffic and risks of collision very much less.
The shipbuilding registries prescribe rules for the government
I L D I N G 817
of the builder who desires to have their certificate, and these rules have been so carefully framed and so honestly enforced that English-built ships are as a rule well and solidly constructed. The recent (8th June 1882) rule of the London Lloyd's register as to the important subject of division into compartments is as follows, and it may be hoped that it will be effective :—
" Screw-propelled vessels, in addition to the engine-room bulkheads, to have a watertight bulkhead built at a reasonable distance from each end of the vessel. In steamers 280 feet long and above an additional bulkhead is to be fitted in the main hold, extending to the main or upper deck, about midway between the collision and engine-room bulkheads ; and in steamers of 330 feet long and above an additional bulkhead is to be fitted in the after hold, extending to the same height."
"The foremost or collision bulkhead in all cases to extend from the floor plates to the upper deck. . . . The engine-room bulkheads to extend from the floor plates to the upper deck in vessels with one, two, or three decks, and to the main deck in spar- and awning-decked vessels. The aftermost bulkhead will be required to extend to the upper deck unless the arrange-ment of bulkheads be submitted to and approved by the committee. . . . In sailing vessels the foremost or collision bulkhead only will be required."
It is not intended by the foregoing remarks, serious as they are, to blot the splendid record of shipbuilding achievement in Great Britain during the last twenty years. The shipowners, ship-builders, marine engineers, Lloyd's surveyors, and the Board of Trade have all shared in a development of shipping which, in amount and in general efficiency, is not only without parallel in the history of the world, but, as it still appears to us who have witnessed it, almost incredible. It still is to be regretted that expansion has been thought of and sought more ardently than greater security and efficiency. The men who have studied to improve their structural arrangements because of their love of true and good work, and with no prospect of recognition or reward, have been comparatively very few.
There is, perhaps, no structure exposed to a greater variety of Strains strains than a ship, and none in which greater risks of life and to which property are incurred. A thorough practical knowledge of the ships are disturbing forces in action either to injure or destroy the several subject, combinations embraced in its structure is therefore most import-ant. Some of these forces always act, whether the ship be at rest or in motion. She may be at rest floating in still water, and will be at rest if cast on shore ; and, when there, she may be resting on her keel as a continuous bearing, with a support from a portion of her side, or she may be supported in the middle only, with both ends for a greater or less length of her body left wholly unsupported, or she may be resting on the ends with the middle unsupported, or under any other modification of these circumstances ; and under all these the strains will vary in their direction and in their intensity.
If the ship be in motion the same disturbing forces may still be in action, with others in addition which are produced by a state of motion. When a ship is at rest in still water, although the upward pressure of the water upon its body is equal to the total weight of the ship, it does not necessarily follow that the weight of every portion of the vessel will be equal to the upward pressure of that portion of the water directly beneath it, and acting upon it; on the contrary, the shape of the body is such that their weights and pressures are very unequal.
If the vessel be supposed to be divided into a number of laminae of- equal thickness, and all perpendicular to the vertical longi-tudinal section, it is evident that the after laminae comprised in the overhanging stern above water, and the fore laminse comprised in the projecting head also above water, cannot be supported by any upward pressure from the fluid, but their weight must be wholly sustained by their connexion with the supported parts of the ship. The laminae towards each extremity immediately con-tiguous to these can evidently derive only a very small portion of their support from the water, whilst towards the middle of the ship's length a greater proportionate bulk is immersed, and the upward pressure of the water is increased.
A ship floating at rest under the view just taken of the relative displacement of different portions of the body, if the weights on board are not distributed so that the different laminae may be supported by the upward pressure beneath them as equally as possible, may be supposed to be in the position of a beam supported at two points in its length at some distance from the centre, and with an excess of weight at each extremity. At sea it would be exposed to the same strain ; and if supported on two waves whose crests were so far apart that they left the centre and ends com-paratively unsupported, the degree of this strain would be much increased. The more these two points of support approach each other, or if they come so near each other that the vessel may be looked upon as supported on one wave, or on one point only in the middle of her length, the greater will be the tensile strain oh the upper portion, and the crushing strain on the lower portion of the fabric of the ship. A vessel whose weights and displacements are so disposed as to render her subject to a strain of this kinct beyond what the strength of her upperworks will enable her to bear, will tend to assume a curved form.
The centre may curve upwards by the excess of the pressure beneath it, and the ends drop, producing what is called " hogging." The main remedy for these evils is in the strength of the deck and

upperworks, and their power to resist a tensile strain. There is seldom a want of sufficient strength in the lower parts of the vessel to resist the crushing or compressing force to which it is subjected. The decks of vessels should not, therefore, be too much cut up by broad hatchways ; and care should be taken to preserve entire as many strakes of the deck as possible. The tensile strength of iron can be brought to bear most beneficially in this respect.
Though these are the strains to which a ship is most likely to be exposed, it by no means follows that there are no circumstances under which strains of the directly opposite tendency, when pitch-ing, or otherwise, may be brought by recoil to act upon the parts. The weights themselves in the centre of the ship may be so great that they may have a tendency to give a hollow curvature to the form, and it is therefore equally necessary to guard against this evil. When this occurs, the vessel is technically said to be " sagged," in distinction to the contrary or opposite change of form by being hogged. The weight of machinery in a wooden steam-vessel, or the weight or undue setting up of the main-mast, will sometimes produce sagging. The introduction of additional keelsons tended to lessen this evil, by giving great additional strength to the bottom, enabling it to resist extension, to which, under such circumstances, it became liable ; and, as the strain upon the deck and upperworks becomes changed at the same time, they are then called upon to resist compression.
When the ship is on a wind, the lee-side is subjected to a series of shocks from the waves, the violence of which may be imagined from the effects they sometimes produce in destroying the bul-warks, tearing away the channels, &c. The lee-side is also sub-jected to an excess of. hydrostatic pressure over that upon the weather side, resulting from the accumulation of the waves as they rise against the obstruction offered to their free passage. These forces tend in part to produce lateral curvature. When in this inclined position, the forces which tend to produce hogging when she is upright also contribute to produce this lateral curvature.
The strain from the tension of the rigging on the weather side when the ship is much inclined is so great as frequently to cause working in the topsides, and sometimes even to break the timbers on which the channels are placed. Additional strength ought therefore to be given to the sides of the ship at this place ; and, in order to keep them apart, the beams ought to be increased in strength in comparison with the beams at other parts of the ship.
The foregoing are the principal disturbing forces to which the fabric of a ship is subjected ; and it must be borne in mind that some of these are in almost constant activity to destroy the con-nexion between the several parts. Whenever any motion or working is produced by their operation between two parts, which ought to be united in a fixed or firm manner, the evil will soon increase, because the disruption of the close connexion between these parts admits an increased momentum in their action on each other, and the destruction proceeds with an accelerated pro-gression. This is soon followed by the admission of damp, and the unavoidable accumulation of dirt, and these then generate fermentation and decay. To make a ship strong, therefore, is at the same time to make her durable, both in reference to the wear and tear of service and the decay of materials. It is evident from the foregoing remarks that the disturbing influences which cause "hogging" are in constant operation from the moment of launch-ing the ship. As this curvature can only take place by the compression of the materials composing the lower parts of the ship and the extension of those composing the upper parts, the importance of preparing these separate parts with an especial view to withstand the forces to which they are each to be subjected cannot be over-rated by the practical builder. Curves of In his Manual of Naval Architecture, Mr W. H. White gives illus-
strains.
H.M.S. "Minotaur."
trations of the still-water strains upon two ar-moured ships in the British navy the " Minotaur " and the " Devas-tation."
In these diagrams the curves B represent the distribution of the
buoyancy. The ordinates of the curve are proportionate to the
displacement of ad- „
jacent transverse sections of the ships. The curves W represent the distribution of the weight of the ships and their lading. The curves L repre-sent the excesses and defects of buoy-ancy obtained from the two curves B and W and set off from a new base line. The
H.M.S.
'' Devastation. "
excess of buoyancy above the line is exactly equal to the defect of buoyancy below it. The curves M indicate the bending moments. The ordinates of the curve lying above the base are obtained by summing all the moments, whether upwards or downwards, about the point in the length of the ship where the ordinate is taken. It may happen, as in the case of the "Devastation," that the moments will tend to cause hogging for a portion of the length and will then change their character, and at other por-tions of the length will tend to
safety
cause sagging. Where the curve M crosses the base line there is no strain of either hogging or sagging tending to bend the ship there. In the '' Minotaur " there is a hogging tendency throughout. The amount at the midship section is very great, being represented by the moment 4 '5 feet x 10 '690 tons. After Sir Edward Reed left the Admiralty he strongly expressed his fears that this strain was too considerable for " Agincourt."
Designing.
The principal plans of a ship are the "sheer" plan, giving in outline the longitudinal elevation of the ship; the " body" plan, giving the shape of the vertical transverse sections ; and the "half-breadth" plan, giving the projections of transverse longi-tudinal sections. In addition to these the builder is furnished by the designer with elevations, plans, and sections of the interior parts of the ship, and of the framing and plating or planking.
The thicknesses or weights of all the component parts are specified in a detailed specification, in order that the ship when completed may have the precise weight and position of centre of gravity con-templated by the designer. In the case of ships built for the British navy all the building materials are carefully weighed by an agent of the designer before they are put into place by the builder. As-each section of the work is completed, the weight is compared with the designer's estimate in the designing office. As soon as the incomplete hull is floated the actual displacement is measured, and compared with the weights recorded as having gone into the ship. It is also the practice in the Royal Navy to calculate the position of the centre of gravity of the incomplete hull, and its draught of water before it is floated, in order to avoid all risk of upsetting from deficiency in stability at that stage of construction. The ship is usually found to float in precise accordance with the estimate. When completed ships float at a deeper draught than was intended, or are found to be more or less stable than was wished, this is nearly always due to additions and alterations made after the com-pletion of the design. Where the designer is at liberty to complete the ship in accordance with the original intention there ought to be precise correspondence between the design and the ship.
In designing a ship of novel type the designer has to pass all the building details through his mind and assign them their just weights and proportions and positions. Every plate and angle bar and plank, every bar and rod and casting and forging, and every article of equipment has to be conceived in detail and its effect estimated.
Building.
The term " laying off" is applied to the operation of transferring Laying to the mould loft floor those designs and general proportions of a 0ff, ship which have been drawn on paper, and from which all the preliminary calculations have been made and the form decided. The lines of the ship, and exact representations of many of the parts of which it is to be composed, are to be delineated there to their full size, or the actual or real dimensions, in order that moulds or skeleton outlines may be made from them for the guidance of the workmen.
A ship is generally spoken of as divided into fore and after Fore and bodies, and these combined constitute the whole of the ship ; they after are supposed to be separated by an imaginary athwartship section bodies, at the widest part of the ship, called the midship section or dead-flat. The midship body is a term applied to an indefinite length of the middle part of a ship longitudinally, including a portion of the fore-body and of the after-body. It is not necessarily parallel or of the same form for its whole length.
Those portions of a wooden ship which are termed the square and cant bodies may be considered as subdivisions of the fore-bodies and after-bodies. There is a square fore-body and a square after-body towards the middle of the ship, and a cant fore-body and a cant after-body at the two ends. In the square body the sides of the frames are square to the line of the keel, and are athwartship








Timbers.


Keel.












Floors.
vertical planes. In the cant bodies the sides of the frames are not square to the line of the keel, but are inclined aft in the fore-body and forward in the after-body. The reason for the frames in these portions of a wooden ship being canted is that, in these parts of the ship, the timber would be too much cut away on account of the fineness of the angle formed between an athwartship plane and the outline or water-line of the ship. The timber is there-fore turned partially round till the outside face coincides nearly with the desired outline, and it is by this movement that the side of a frame in the cant fore-body is made to point aft, and in the cant after-body to point forward.
In wooden ships the term "timbers" is sometimes applied to the frames only, but more generally to all large pieces of timber used in the construction. Timbers, when combined together to form an athwartship outline of the body of a ship, are technically called frames, and sometimes ribs.
The keel, in the United Kingdom at least, is generally made of elm, on account of its toughness, and from its not being liable to split if the ship should take the ground, though pierced in all directions by the numerous fastenings passing through it. It is generally composed of as long pieces as can be obtained, united to each other by horizontal scarphs. The rabbet of the keel is an angular recess cut into the side to receive the edge of the planks on each side of it. The keel is connected forward to the stem by a scarph, sometimes called the boxing scarph, and aft to the stern-post by mortice and tenon. The apron is fayed or fitted to the after-side of the stem, and is intended to give shift to its scarphs, the lower end scarphs to the deadwood. The keelson is an internal line of timbers fayed upon the inside of the floors directly over the keel, the floors being thus confined between it and the keel. Its use is to secure the frames and to give shift to the scarphs of the keel, and thus give strength to the ship to resist extension lengthways, and to prevent her hogging or sagging. The foremost end of the keelson scarphs to the stemson, which is intended to give shift to the scarphs connecting the stem and keel. The frames or ribs are composed of the strongest and most durable timber obtainable.
The floors in the Government service were carried across the keel with a short and long arm on either side alternately, so as to break joint, and between the frames the space was filled in solid.
Longitudinal pieces of timber are worked round the interior of a Shelves, ship for the purpose of receiving the ends of the beams of the several decks ; they are called shelves, and are of the greatest importance, not only for this purpose, but also as longitudinal ties and struts.
The beams of a ship prevent the sides from collapsing, and at Beams, the same time carry the decks. The beams are spaced, and their scantling settled upon, according to the strength required to be given to the decks, and to suit the positions of the masts and hatchways, and other arrangements connected with the economy of the ship. All beams have a curve upwards towards the middle of the ship, called the round-up. This is for the purpose of strength, and for the convenience of the run of the water to the scuppers. Wooden beams are single piece, two, three, or four piece beams-according to the number of pieces of timber of which they are composed. The several pieces are scarphed together, and dowelled and bolted, the scarphs being always vertical.
The connexion of the ends of the beams to the sides of the ship has been made in various ways. The points to be considered, with reference to this connexion, are—that the beam is required to act as a shore or strut, to prevent the sides of the ship from collapsing, and also as a tie to prevent their falling apart, that the beam shall not rise from its seat, and that it shall not work in a fore-and-aft direction.
That the beam may be an effective shore, nothing more is neces- Water sary than that the abutment of the end against the ship's side may ways.-be perfect. In order that it may act as a tie between the two sides, it is generally dowelled to the upper surface of the shelf on which it rests ; and the under surface of the waterway plank which lies upon it is sometimes dowelled into it. These dowels, therefore, connect it with the shelf and the waterway, and through this means it is thus connected with the sides of the ship.
From the short outline previously given of the disturbing forces acting on a ship it will be seen that the strain on the ends of the beams to destroy their connexion with the side and loosen the fastenings must be very great when the ship is under sail, either on a wind or before it—that is, either inclined or rolling. The
























Plank-
principal action of these forces is to alter the vertical angle made by the beam and the ship's side—that is, to raise or depress the beam, and so alter the angle between it and the side of the ship above or below it. On the lee-side the weight of the weather side of the ship and all connected with it, and of the decks and everything upon them, as well as the upward pressure of the water, all tend to diminish the angle made by the beam and the ship's side below it, and consequently increase the angle made between them above it. The contrary effect is produced on the weather-side, where the tendency is to close the angle above the beam and open that below it. If the beam, when subjected to these strains, be considered as a lever, it will be evident that the fastenings to prevent its rising ought to be as far from the side as is consistent with the convenience or accommodation of the ship, and that, while the support should also be extended inwards, the fastening to keep down the beam-end should be as close to the end of the beam, and consequently to the ship's side, as it can be placed.
The plank, or skin, or sheathing of a ship, both external and internal, is of various thicknesses. A strake of planking is a range of planks abutting against each other, and generally extend-ing the whole length of the ship. A thick strake, or a combination of several thick strakes, is worked wherever it is supposed that the frame requires particular support—for instance, internally over the heads and heels of the timbers, both externally and internally in men-of-war vessels between the ranges of ports, and internally to support the connexion of the beams with the sides and at the same time form a longitudinal tie. The upper strakes of plank, or assemblages of external planks, are called the sheer-strakes. The strakes between the several ranges of ports, begin-ning from under the upper-deck ports of a three-decked ship in the British navy, were called the channel wale, the middle wale, and the main waie. The strake immediately above the main wale was called the black strake. The strakes below the main wale diminished from the thickness of the main wale to the thickness of the plank of the bottom, and were therefore called the diminishing strakes. The lowest strake of the plank of the bottom, the edge of which fits into the rabbet of the keel, is called the garboard strake.
Plank is either worked in parallel strakes, when it is called " straight-edged," or in combination of two strakes, so that alternate seams are parallel. There are two methods of working these combinations, one of which is called "anchor stock," and the other "top and butt." The difference will be best shown by fig. 13, The difference in the intention is that in the method of working two strakes anchor-stock fashion, the narrowest part of one strake always occurs opposite to the widest part of the other strake, and consequently the least possible sudden interruption of longitudinal fibre, arising from the abutment, is obtained. This description, therefore, of planking is used where strength is especially desirable. In top and butt strakes the intention is, by having a wide end and a narrow end in each plank, to approximate to the growth of the tree, and to diminish the difficulty of procuring the plank. When the planking is looked upon as a longitudinal tie, the advantage of these edges being, as it were, imbedded into each other is apparent, all elongation by one edge sliding upon the other being thus prevented. The shift of plank is the manner of arranging the butts of the several strakes. In the ships of the British navy the butts were not allowed to occur in the same vertical line, or on the same timber without the intervention of three whole strakes between them.

Of the internal planking the lowest strake, or combination of strakes, in the hold, is called the limber-strake. A limber is a passage for water, of which there is one throughout the length of the ship, on each side of the keelson, in order that any leakage may find its way to the pumps.
The whole of the plank in the hold is called the ceiling. Those strakes which come over the heads and heels of the timbers are worked thicker than the general thickness of the ceiling, and are distinguished as the thick strakes over the several heads. The strakes under the ends of the beams of the different decks in a man-of-war, and down to the ports of the deck below, if there were any ports, were called the clamps of the particular decks to the beams of which they are the support—as the gun-deck clamps, the middle-deck clamps, &c. The strakes which work up to the sills of the ports of the several decks were called the spirketting of those decks—as gun-deck spirketting, upper-deck spirketting, &c.
The fastening of the plank is either " single," by which is meant one fastening only in each strake as it passes each timber or frame ; or it may be " double," that is, with two fastenings into each frame which it crosses ; or, again, the fastenings may be "double and single," meaning that the fastenings are double and single alternately in the frames as they cross them. The fastenings of planks consist generally either of nails or treenails, excepting at the butts, which are secured by bolts. Several other bolts ought to be driven in each shift of plank as additional security. Bolts which are required to pass through the timbers as securities to the shelf, waterway, knees, &c., should be taken advantage of to supply the place of the regular fastening of the plank, not only for the sake of economy, but also for the sake of avoiding unnecessarily wounding the timbers.
The decks of a wooden ship must not be considered merely as platforms, but must be regarded as performing an important part towards the general strength of the whole fabric. They are generally laid in a longitudinal direction only, and are then use-ful as a tie to resist extension, or as a strut to resist compression. The outer strakes of decks at the sides of the ship are generally of hard wood, and of greater thickness than the deck itself ; they are called the waterway planks, and are sometimes dowelled to the upper surface of each beam. Their rigidity and strength is of great importance, and great attention should be paid to them, and care taken that their scarphs are well secured by through-bolts, and that there is a proper shift between their scarphs and the scarphs of the shelf.
"When the decks are considered as a tie, the importance of keep-ing as many strakes as possible entire for the whole length of the ship must be evident; and a continuous strake of iron or steel plates beneath the decks is of great value in this respect. The straighter the deck, or the less the sheer or upward curvature at the ends that may be given to it, the less liable will it be to any alteration of length, and the stronger will it be. The ends of the different planks forming one strake were made to butt on one beam, and, as the fastenings are driven close to the ends, they did not possess much strength to resist being torn out. The shifts of the butts, therefore, of the different strakes required great attention, because the transference of the longitudinal strength of the deck from one plank to another was thus made by means of the fastenings to the beams, the strakes not being united to each other sideways. The introduction of iron decks or partial decks under the wood has modified this.
These fastenings have also to withstand the strain during the process of caulking, which has a tendency to force the planks sideways from the seam ; and, as the edges of planks of hard wood will be less crushed or compressed than those of soft wood when acted on by the caulking-iron, the strain to open the seam between them to receive the caulking will be greater than with planks of softer wood, and will require more secure fastenings to resist it. It may also be remarked that the quantity of fastenings should increase with the thickness of the plank which is to be secured, for the set of the oakum in caulking will have the greater mechanical effect the thicker the edge.
When the planks are fastened, the seams or the intervals between the edges of the strakes are filled with oakum, and this is beaten in or caulked with such care and force that the oakum, while undisturbed, is almost as hard as the plank itself. If the openings of the seam were of equal widths throughout their depth between the planks, it would be impossible to make the caulking sufficiently compact to resist the water. At the bottom edges of the seams the planks should be in contact throughout their length, and from this contact they should gradually open upwards, so that, at the outer edge of a plank 10 inches thick, the space should be about ^ of an inch, that is, about XV of an inch open for every inch of thickness. It will hence be seen that, if the edges of the planks are so prepared that when laid they fit closely for their whole thickness, the force required to compress the outer edge by , driving the caulking-iron into the seams, to open them sufficiently, must be very great, and the fastenings of the planks must be such as to be able to resist it. Bad canlking is very injurious in every way, as leading to leakage and to the rotting of the planks them-selves at their edges.
Ships are generally built on blocks which are laid at a declivity Launch-of about f inch to a foot. This is for the facility of launching ing. them. The inclined plane or sliding plank on which they are launched has rather more inclination, or about |- inch to the foot for large ships, and a slight increase for smaller vessels. This inclination will, however, in some measure, depend upon the depth of water into which the ship is to be launched.
While a ship is in progress of being built her weight is partly supported by her keel on the blocks and partly by shores. In order to launch her the weight must be taken off these supports and transferred to a movable base ; and a platform must be erected for the movable base to slide on. This platform must not only be laid at the necessary inclination, but must be of sufficient height to enable the ship to be water-borne and to preserve her from striking the ground when she arrives at the end of the ways. For this purpose an inclined plane a, a (fig. 14), purposely left unplaned to diminish the adhesion, is laid on each side the keel, and at about one-sixth the breadth of the vessel distant from it, and firmly secured on blocks fastened in the slipway. This

.
inclined plane is called the sliding-plank. A long timber, called a bilgeway b, b, with a smooth under-surface, is laid upon this plane ; and upon this timber, as a base, a temporary frame-work of shores c, c, called " poppets," is erected to reach from the bilge-way to the ship. The upper part of this frame-work abuts against a plank d, temporarily fastened to the bottom of the ship, and firmly cleated by cleats e, e, also temporarily secured to the bottom. When it is all in place, and the sliding-plank and under side of the bilgeway finally greased with tallow, soft soap, and oil, the whole framing is set close up to the bottom, and down on the sliding plank, by wedges /, /, called slivers or slices, by which means the ship's weight is brought upon the " launch " or cradle.





When the launch is thus fitted, the ship may be said to have three keels, two of which are temporary, and are secured under her bilge. In consequence of this width of support, all the shores may be safely taken away. This being done, the blocks on which the ship was built, excepting a few, according to the size of the ship, under the foremost end of the keel, are gradually taken from under her as the tide rises, and her weight is then transferred to the two temporary keels, or the launch, the bottom of which launch is formed by the bilgeways, resting on the well-greased inclined planes. The only preventive now to the launching of the ship is a short shore, called a dog-shore on each side, with its heel firmly cleated on the immovable platform or sliding-plank, and its head abutting against a cleat secured to the bilgeway, or base of the movable part of the launch. Consequently, when this shore is removed, the ship is free to move, and her weight forces her down the inclined plane to the water. To prevent her running out of her straight course, two ribands are secured on the sliding-plank, and strongly shored. Should the ship not move when the dog-shore is knocked down, the blocks remaining under the fore part of her keel must be consecutively removed, until her weight overcomes the adhesion, or until the action of a screw against her fore-foot forces her off.
allowing the keel to take the entire weight of the vessel. The two pieces a, a, which are shown in fig. 15 as being secured to the
A different mode of launching is sometimes practised in British merchant-yards, and has been long in use in the French dockyards,

ship's bottom, are the only pieces which need be prepared according to this system for each ship, the whole of the remainder being available for every launch. A space of about half an inch is left between them and the balk timber placed beneath them, as it is not intended that the ship should bear on these balk timbers in launching, but merely be supported by them in the event of her heeling over. The ship, therefore, is launched wholly on the sliding-plank c, fitted under the keel.
If a ship is coppered before launching, so that putting her into
a dry-dock for that purpose becomes unnecessary, it is then desir-
able that she should be launched without any cleats attached to her
bottom. The two sides of the cradle are prevented from being forced
apart when the weight of the ship is brought upon them by chains
passing under the keel. Each portion of frame-work composing
the launch has two of these chains attached to it, and brought
under the keel to a bolt which passes slackly through one of the
poppets, and is secured by a long forelock, with an iron handle,
reaching above the water-line, so that when the ship is afloat it
may be drawn out of the bolt. The chain then draws the bolt, and
in falling trips the cradle from under the bottom. There should
be at least two chains on each side secured to the fore-poppets,
two on each side secured to the after-poppets, and two on each side
to the stopping-up, and this only for the launch of a small ship ;
in larger ships the number will necessarily be increased according
to the weight of the vessel and the tendency that she may have,
according to her form, to separate the bilgeways. This tendency
on the part of a sharp ship by a rising floor, or by her wedge-
shaped form in the fore and after bodies, is great, but there is not
much probability of a ship heeling over to one side or the other.
Slop The importance of the work of the designer cannot be too highly
work. estimated. Unfortunately there is, as has been said, "slop work " in designing as well as in putting the structure together. There is often an absence of any attempt at precautions where multiplied accidents have shown them to be necessary, as well as inconceivable carelessness in the details rendering provisions for security, where they exist in principle, useless in practice.
In the Report of the Royal Commission on Unseaworthy Ships, dated September 22, 1873, we read as follows :—"Competent wit-nesses state that many merchant ships are built with bad iron, that they are ill put together, and sent to sea in a defective condition. It is also said that they are frequently lengthened without addi-tional strength, and are consequently weak ships. The number of iron steamers which have been lost in the last few years, many of them having been surveyed and classed under the London or Liverpool registers, raises a question whether the regulations of these registers are sufficiently stringent to insure good shipbuilding. The directors of the Bureau Veritas have deemed it necessary to revise the rules of their register, and to increase the scantling. In the race of competition among shipbuilders it is probable that inferior materials and bad workmanship are ad-mitted into ships."
The Commissioners on Unseaworthy Ships, referring to the proposal that the Board of Trade should superintend the con-struction, the periodical inspection, the repair, and the loading of all British merchant ships, said: "We consider it to be a question worthy of serious consideration, whether, in the case of passenger ships, the certificate of the Board of Trade, so far as regards specific approval, should not be expressly confined to the number of passengers to be allowed and to the accommodation for their health, comfort, and general security,—all questions of unseaworthiness of hull, machinery, and equipment being left to the owners, subject only to a general power of interference in case of danger sufficiently apparent to justify special inter-vention. "
Where ships have to meet the stress of battle as well as that of the sea faithfulness of work is even more imperative. It is not only necessary to have perfect work, but there must also be multiplied safeguards and provisions against damage by shot, shell, ram, and torpedo as well as against the enemies which are common to all ships. In the article NAVY the peculiarities of the ship of war are described. Regarding them here simply as ships, they may be said to be distinguished neither by size nor speed. They have been far. outstripped in size, the longest English ship of war built within the last twenty years being only 325 feet in length, while there are Atlantic passenger ships 200 feet longer. They have also been outstripped in speed. The highest speed ever attained in a vessel of war is that of the " Iris " and " Mercury"; and as they are only 300 feet long it is easier in vessels of greater length to get higher speeds with less engine power, and easy also to maintain it in a seaway both as a question of form and power, and also as a matter of coal endurance. The following table gives the relative dimensions of large 14-knot ships :—

Ship's Name. Length divided by Breadth (on Water Line). I.H.P. Dispt. in Tons. (Dispt.)*
"Adriatic," (
(White Star Line) (
H.M.S. "Dreadnought,"
H.M.S. "Sultan,"
H.M.S. "Inflexible,"
H.M.S. "Neptune,'^ ) late "Independencia," \ ft. in.
—7 = 10-45
33^8- 5-42 61 6 0 iz
3300 K-RA
4-32
75 0
804 °- 5-01 60 10 0 Ui 3,600 8,000 8,600 8,000 8,500 8,250 10,886
9,286 11,500
9,063 408-3 491-2 441-8 509-5 434-7
0 per ton weight of hull. 0
The differences between the amount and complexity of fitting Cost, in the ship of war and the merchant ship are represented by the greatly increased cost per ton weight of hull. It must, however, be premised that the war ship has the weight of hull kept down to a very low standard to enable her to carry her offensive and defensive equipment,—far lower than is usual in the merchant ship. The first-class merchant ship costs £28 per ton weight of hull and about £13 per indicated horse-power for the engines. The ship of war built by the same builders under contract with' the Government costs from £60 to £65 per ton weight of hull for unarmoured ships, and from £70 to £75 or more for armoured ships. In the case of an unarmoured vessel, having a protecting deck over machinery and magazines, recently ordered, the prices were as follows :—
General average £60 10
Average of three London firms. 66 0
Accepted tender 57 6 0
8 0 per I.H.P.
' 0 „ 0 „
The engines for the same vessel were:—
General average £15
Average of three London firms 17 5
Accepted tender 11
In the case of a larger armoured ship the rates were :—
Average price per ton weight of hull £81
Accepted tender 71
Average price per I.H.P. of engines 11
Accepted tender 10

The use of heavy ordnance in recent times as the sole weapon for naval warfare brought about a marked distinction between the merchant vessel and the war ship, which had not previously existed. The revival of the ram and the adoption of the torpedo tend to abolish this distinction and to bring about an approxima-tion again.
It is difficult to say what, in the very near futnra, will be the
2 Of this the vertical armour (costing before it is worked up, £70 to £90 per
ton) is nearly 2000 tons. 3 Average of six vessels built by Elder*
distinguishing characteristics of the ship of war. They will not Charae-be speed or size or coal endurance, or the power of striking with teristics -the ram, the torpedo, or the gun. It will be quite easy to arm of war merchant ships with these weapons, and some of these ships ships.

already outstrip the war vessel in the important advantages of :size and fieetness and carrying power. It is apparently in pro-tective advantages that the essential difference will lie.
The merchant ship is badly provided against fatal damage by collision, or by a blow delivered in any manner by which water is admitted into the ship. The propelling machinery of these ships and their steering apparatus are also dangerously exposed to artillery fire. Excepting torpedo boats, the ship of war of any size has its propelling machinery either under water or under cover of armour, and in a great number of cases there is either protection for the steering apparatus or there are two propellers. Changes The approximation towards war-ship arrangements which is needed needed in the merchant ship is the adoption of more than one screw and of in mer- greater breadth of ship, so that defences round machinery may be chant created in time of war. Both these changes in merchant-ship ships. practice are demanded also by mercantile interests. The increase in breadth amidships would greatly reduce the risk of foundering in collisions and give more spacious accommodation amidships. Such increase when accompanied by fine ends is also favourable to speed.
The use of two screws is economical of power, and is a much-needed security against the evil results of an accident to an engine, a shaft, or a propeller. The time will doubtless come when a single propeller in a large passenger ship will be regarded as an unpardonable fault, and when the division into compartments now common will be held to be no better than a delusion and a snare. Armour. The protection given to the regular ships-of-war by side armour, or by a protecting deck, at or near the water-line, will probably become a definite and indispensable feature in them, and may, perhaps, be 1 their only distinguishing characteristic, apart from their outfit and equipment.
If this should prove to be the issue of events, their course will
have been very indirect. In the ships-of-war of the last century
no attempt was made to employ armour on the sides or to prevent
the passage of projectiles and water into the holds by means of a
protecting deck. There was a dock just below the water-line, but
it had no protective qualities. It served, among other things, to
furnish passage ways in action for the carpenter and his crew to
get at the inner side of the wooden walls of the ship at and near
the water-line, so that when shot entered there the holes might be
immediately plugged. When screw propulsion was introduced into
these ships, and it was found practicable to keep the engines and
boilers under water, it would have been possible to place a deck
over the machinery and beneath 'the water, which would have
greatly added to the security of the engines, boilers, and magazines.
The space above this deck might also have been so subdivided into
_ compartments as to have protected the buoyancy and stability of
the ship against the immediately fatal results of the invasion of
water. The protection of the buoyancy and stability by these
means would not have been absolute, in the sense of making the
ship safe, but it would have been of the utmost value as compared
with ships, otherwise similar, but having no such protection.
Com- Thirty years passed between the date when screw-propeller engines
mitteeon were placed beneath the water-level in ships of war and that at which designs, a committee on designs, under the presidency of Lord Dufferin, pro-posed to place such a covering deck over them, or to construct a water-line raft-body. The proposal of the main body of the com-mittee was to associate such a raft-deck for the protection of the buoyancy and stability of the ship against artillery with a central armoured citadel. That of the minority was to suppress the armour in the region of the water-line entirely, and to protect buoyancy, stability, machinery, and magazines by a raft-deck alone. In 1873 the plan as indicated by the main body of the committee was put into practice nearly simultaneously in the " Duilio" and " Dandolo" in Italy and in the " Inflexible " in England. In 1878 the system as conceived in principle by the minority of the committee of 1871, although not in the manner they recommended, was adopted in much smaller vessels in the British navy. A raft-deck was intro-duced into the "Comus" class of corvettes of 2,380 tons displace-ment, a class which was regarded as unarmoured. Since that date the raft-deck has been adopted in a more or less complete form in nearly all classes of unarmoured ships in the English navy. So it has come about that, out of some 850 unarmoured ships of war built and building in Europe, 47 have such protecting raft-decks. Of these 32 are English. There can be no doubt that all unarmoured ships of war will eventually be protected in this manner. The num-ber of so-called ironclads built and building in Europe is 270. Of these, 34 are based on the recommendation of the committee on designs ; 18 of them are English. There are six other English ships with central citadels and under-water protecting decks, built more than twenty years ago, but the raft-body principle is absent in them.
If the passage from the steam line-of-battle ship of 1840-1860 to the "Admiral" class of 1884 had been made under the guidance of the principles of the committee of 1871, European nations would not find themselves possessed of large fighting ships covered from end to end, or over large areas of their sides, with thin armour, penetrable to a very large proportion of the guns brought against them. But the sailors of 1854-1860 did not take the view that
I L D I N G
buoyancy and stability, and machinery and magazines, were the vital parts, needing defence by armour or by a raft-deck. They dreaded the effects of shell exploding between decks, setting fire to the ships, and converting the decks, crowded with men, into slaughter-houses. Their demand was, '' Keep out the shells." So it came about that iron armour-plates, thick enough to keep out the most powerful shell of the time, were worked upon the sides of the ships, and the guns were fought through ports cut in this armour. This feeling was so strong that the English Admiralty built the '' Hector " and '' Valiant" with armoured batteries overlapping by many feet at each end the armour beneath them, which protected the buoyancy, stability, machinery, and magazines. Guns in-creased in power, and the armour was gradually thickened to resist them, until from 4J inches of armour, through which broadside ports were cut, 9 inches and 10 inches were reached. But this thickening of the armour had so reduced the possible number of the . guns in a ship of moderate size, and the guns required for breaching such armour had so increased in weight, that the broadside ship had to give way to the turret or barbette ship, in which about four such guns were all that could be carried, and these had to be worked on turn-tables in or near the central line of the ship.
The point now reached in all navies is that the broadside iron-clad with ports cut through an armoured side, as invented in France by M. Dupuy de Lome, and copied by every power, is obsolete. Guns must be worked singly or in pairs. on revolving turn-tables, each turn-table being surrounded by an armoured tower, forming the loading chamber or protecting the mechanism. The side armour protecting the buoyancy, stability, machinery, and magazines, although not introduced for that purpose originally, is retained in France for very large ships, is given up in Italy in favour of a raft-body, and is retained partially in England and Germany in conjunction with a raft-body.
The use of armour has arrested the development of the shell. But it is not inconceivable that its abandonment in front of the long batteries of guns in the French and Italian ships will invite shell attack, and make existence in such batteries, if they are at all crowded, once more intolerable. It remains to be seen whether in that case exposure will be accepted, or a new demand made for armour, at least against the magazine gun and the quick-firing gun. If exposure is accepted, it will be on the ground that the number of men at the guns is now very few, that the gun positions are < numerous and the fire rapid, and that, if the guns had once more to he fought through ports in armour, the number of gun positions would be reduced, and the fragments of their own walls, when struck by heavy projectiles, would be more damaging than the projectiles of the enemy.
Internal armour for the protection of the heavy armour-breach-ing guns must be retained so long as such guns are used, and if they were abandoned an enemy could cover himself with armour invulnerable to light artillery. This the French attempted to do in inaugurating the system. They have been driven from it by the growth of the gun. Abandon the heavy gun, and complete armour-plating might again be adopted.
"We must conclude that the buoyancy, stability, machinery, and magazines must be protected as far as possible against fatal damage from a single blow of these armour-breaching guns. The tendency will be to come to the lightest form of such protection. That lightest form appears to be a protecting deck a little above the water-level throughout the greatest part of its surface, but sloping down at the sides and at the ends, so as to meet the side walls of the ship under the water-line. However the armour is arranged (apart from a complete covering with invulnerable plat-ing), —whether as a belt with its upper edge 3 feet out of the water, as in the French ships ; as a central armoured citadel and a raft-body at the ends, like the English and German ships ; or as a raft-body throughout, like the Italian ships,—shot holes in action will admit water and gradually reduce the necessary stability of the ship. In the French ships the assistance of the unarmoured upper parts is as necessary to prevent them from upsetting in anything but smooth water as is the assistance of the unarmoured rai't ends in the English and German ships. In the intact condition the English ships have far greater stability than those of France. In the English ships a reserve of stability is provided, against the con-tingency of loss by injuries in action. In the French ships no more is provided than is required for the intact condition. The French have not accepted the position taken up in England that much greater initial stability may be given to heavily-armoured broad ships than is usually given, without causing heavy rolling. Nor have they accepted the further incontrovertible truth that the free passage of water in the raft-body from side to side of the ship in rolling is rapidly effective in quelling the motion and bringing the ship to rest in the upright position.
Propulsion.
The propulsion of ships by sails differs from the drifting of Sails, bodies in the air before the wind in a most important respect. Ships may drift or sail in the direct course of the wind, and they

will then differ from air-borne bodies only in the comparative slowness imposed by the resistance of the water. Ships having the same length as breadth, or rather opposing the same form and area to side progress as to forward progress could never do other than sail before the wind. No disposition of canvas could make them deviate to the right or left of their course to leeward. But by an alteration of form giving them greater length than breadth, and greater resistance to motion sideways than to motion endwise, they came to possess the power of being able not only to sail to the right or left of the course of the wind, before the wind, but also to sail towards the wind. The wind can be made to impel them towards the point from which it is blowing by means of the lengthened form acted on by the resistance of the water.
Motion directly towards the wind cannot be maintained, but by sailing obliquely towards it first to one side and then to the other progress is made in advance, and the vessel "beats to windward." The action is like that which would be required to blow a railway car to the eastward by the action of an easterly wind. If the line of rails were due east and west, and the wind were always direct from the east, the thing could not be done. But with a wind to the south or north of east, by setting a sail in the car so that its surface lies between the course of the wind and the direction of the rails, it would then receive the impulse of the wind on its back and would drive the car forwards. There would be a large part of the force of the wind ineffective because of the obliquity of the sail; and of the part which is effective a large portion would be tending to force the car against the rails sideways, but there would be progression to windward. In the case of the ship the resistance to side motion is due to the unsuitability of the proportions and form for progress in that direction as compared with progress ahead, but still there is motion transversely to the line of keel. This motion is called leeway. As the ship moves to leeward and ahead simultaneously there is a point of balance of the forces of the fluid against the immersed body—a centre of fluid pressure. The object of the constructor is to place the masts in the ship in such positions that the centre of pressure of wind upon the sails shall fall a little behind or astern of this centre of resistance of the fluid. In that case there is a tendency in the ship to turn round under the action of these two forces, and to turn with her head towards the wind. This tendency is corrected by the action _of the rudder. If the tendency to turn were the other way, although that could also be corrected by the rudder, yet there would be danger of the wind overcoming the rudder action in squalls, and the ship would then come broadside to the wind. In that case, while she might have been quite capable of bearing the pressure of the wind blowing obliquely upon her sails, she might have her sails blown away, or her masts broken, or be her-:self capsized by the direct impulsion of the wind upon the sail and upon the hull of the ship.
Manjr examples of disposition of sails might be given. Their disposition is always made to satisfy the conditions that as much sail as possible is required, but if the vessel is small it must be capable of being instantly let go in a squall, or when the wind is gusty. Otherwise, where it cannot be readily let go, its area should be capable of reduction in squally weather, still retaining its efficiency, so that no pressure of the wind should be capable of upsetting the ship. If a sudden violent squall should strike the ship she should find relief, not by a large inclination, but by the blowing away of the sails out of the bolt-ropes, or the carrying away of the masts. One or other of these must of course happen if the area of canvas and the strength of the sails and of the spars are so proportioned at the moment the squall strikes the ship as to be less than the resistance offered by the stability of the ship to a large inclination. Ships are sometimes, when struck by a squall, blown over on to their sides, the sails being in the water. If the sails or spars are then cut .away or otherwise got rid of the ship may right herself.
In the Transactions of the Institution of Naval Architects for 1881, Mr W. H. White says :—
"Any investigation of the behaviour of sailing ships at sea must take account of the conditions belonging to the discussion of their rolling when no sail is set, and must superpose upon those conditions the other and no less difficult conditions relating to the action of the wind upon the sails, the influence of heaving motions upon the stability, and the steadying effect of sail-spread.
"It may fairly be assumed that the labours of the late Mr W. Froude have made it possible to predict, with close approximation to truth, the behaviour of a ship whose qualities are known and which has no sails set, when rolling among waves of any assumed dimensions. By a happy com-bination of experimental investigation and mathematical procedure, Mr Froude succeeded in tracing the motion from instant to instant, and checked the results thus obtained by comparison with the actual observations made in a sea-way on the behaviour of the 'Devastation.' The details of his method, and examples of its application, will be found in the Transactions for 1875, and in the appendix to the report of the 'Inflexible' committee.
" The conclusion I have reached, after a careful study of the subject, is that we need very considerable extensions of our knowledge of the laws of wind-pressure before more exact investigations will be possible so as to enable us to pronounce upon the safety or danger of a sailing ship. Nor must it be overlooked that sailing ships are not to be treated as machines worked under certain fixed conditions. Their safety depends at least as much upon seamanship and skilful management as upon the qualities with which they are endowed by their designers. Moreover, it is idle to pretend that, in determining what sail-spread can be safely given to a ship, the naval architect proceeds in accordance with exact or purely scientific methods. He is largely influenced by the results of experience with other ships, and thus proceeds by comparison rather than by direct investigation from first principles. Certain scientific methods are employed, of course, in making these comparisons. For example, the righting moment at different angles of inclination is usually compared with the corresponding 'sail-moment'; but even here certain assumptions have to be made as to the amount of sail to be reckoned in the calculation, and as to the effective wind-pressure per unit of sail-area. Between ship and ship these assumptions are unobjectionable, but they are not therefore to be regarded as strictly true.
"The calculations of curves of stability and the determination of the ranges of stability for ships form important extensions of earlier practice. But, even when possessed of this additional information, the naval architect must resort to experience in order to appreciate fairly the influence of sea-manship and the relative manageability of ships and sails of different sizes. There can be no question but that a good range and large area of a curve of stability denote conditions very favourable to the safety of a ship against capsizing. But, in practice, it frequently happens that such favourable conditions can scarcely be secured in association with other important qualities, and a comparatively moderate range and area of the curve of stability have to be considered when the designer attempts to decide whether sufficient stability has been provided. Under these circumstances experience is of the greatest value; a priori reasoning cannot take the place of experi-ence, because (as remarked above) the worst combination of circumstances cannot be fixed, and because some important conditions in the problem are yet unsettled. Certain arbitrary standards may be set up, and ships may be pronounced safe or unsafe; but this is no solution of the problem. There are classes of ships in existence which have been navigated in all weathers, under sail, and in all parts of the world, which might be pronounced unsafe if tested by some of the standards that have been proposed; but the fact that not a single vessel of that class has been capsized or lost at sea during many years willprobablybe accepted, inmost quarters, as sufficient evidence of the seaworthiness of these classes, and as an indication of the doubtful authority of the proposed standards."
For the different kinds of sails, and for sailmaking, see SAIL. The "Comet" was the first steam-vessel built in Europe that Steam plied with success in any river or open sea. She was built in Scotland in 1811-12 for Mr Henry Bell, of Helensburgh, having been designed as well as built by Mr John "Wood, at Port-Glasgow. The little vessel was 42 feet long and 11 feet wide. Her engine was of about four horse-power, with a single vertical cylinder. She made her first voyage in January 1812, and plied regularly between Glasgow and Greenock at about 5 miles an hour. There had been an earlier commercial success than this with a steam vessel in the United States, for a steamer called the " Clermont" was built in 1807, and plied successfully on the Hud-son River. This boat, built for Fulton, was engined by the English firm of Boulton & "Watt. The reason for this choice of engineers by Fulton appears to have been that Eulton had seen a still earlier steamboat for towing in canals, also built in Scotland, in 1801, for Lord Dundas, and having an engine on "Watt's double-acting principle, working by means of a connecting rod and crank and single stern wheel. This vessel, the " Charlotte Dundas," was successful so far as propulsion was concerned, but was not regularly employed because of the destructive effects of the propeller upon the banks of the canals. The engine of the canal boat was made by Mr William Symington, and he had previously made a marine engine for Mr Patrick Miller, of Dalswinton, Dumfriesshire. This last-named engine, made in Edinburgh in 1788, marks, it is said, the first really satisfactory attempt at steam navigation in the world. It was employed to drive two central paddle-wheels in a twin pleasure-boat (a sort of "Castalia") on Dalswinton Loch. The cylinders were only 4 inches in diameter, but a speed of
5 miles an hour was attained in a boat 25 feet long and 7 feet broad. The first steam vessel built in a royal dockyard was also called the " Comet." She appears to have been built about the year 1822, and was engined by Boulton & Watt. This ship had two engines of forty horse-power each, to be worked in pairs on the plan understood to have been introduced by the same firm in 1814. In 1838 the " Sirius " and " Great Western " commenced the regular Atlantic passage under steam. The latter vessel, proposed by I. K. Brunei, and engined by Maudslay Sons
6 Field, made the passage at about 8 or 9 knots per hour. One year earlier (1837) Captain Ericsson, a scientific veteran who Screw is still among us (1886), towed the Admiralty barge with their pro-lordships on board from Somerset House to Blackwall and back pellers. at the rate of 10 miles an hour in a small steam vessel driven by
a screw.
The screw did not come rapidly into favour with the Admiralty, and it was not until 1842 that they first became possessed of a screw vessel. This vessel, first called the '' Mermaid" and afterwards the "Dwarf," was designed and built by the late Mr Ditchburn, and engined by Messrs Rennie. In 1841-3 the "Rattler," the first ship-of-war propelled by a screw, was built for and by the Admiralty under the general superintendence of Brunei, who was also superintending at the same time the construction of the " Great Britain," built of iron. The engines of the "Rattler," of 200 nominal horse-power, were made by Messrs Maudslay. They were constructed, like the paddle-wheel engines of that day, with vertical cylinders and overhead crank-shaft, with wheel gearing to give the required speed to the screw. The next screw engines made for the Boyal Navy were those of the "Amphion," 300 nominal horse-power, made in 1844 by Miller and Ravenhill. In these the cylinders took the horizontal

position, and they became the type of screw engines in general use. This ship had a screw-well and hoisting gear for the screw. In 1845 the importance of the screw propeller for ships of war became fully recognized, and designs and tenders were invited from all the principal marine engineers in the kingdom. The Government of that day then took the bold step of ordering at once nineteen sets of screw engines. Six of these had wheel gearing; in all the rest the engines were direct-acting. The steam pressure in the boilers was from 5 to 10 lb only above the atmo-sphere, and if the engines indicated twice the nominal power it was considered a good performance. The most successful engines were those of the "Arrogant " and " Encounter " of Messrs Penn. They had a higher speed of piston than the others, and the air-pumps were worked direct from the pistons, and had the same length of stroke. These engines developed more power for a given amount of weight than other engines of their day, and were the forerunners of the many excellent engines on the double-trunk plan made by this firm for the navy. The engines with wheel-gearing for the screws were heavier, occupied more space, and were not so successful as the others, and no more of that description were ordered for the British navy. Econo- Up to 1860 neither surface-condensers nor superheaters were mical used in the navy. The consumption of fuel was about 4J lb per engines, one horse-power per hour. In that year (1860) three ships, the "Arethusa," " Octavia," and "'Constance," were fitted respectively by Messrs Penn, Messrs Maudslay, and Messrs Elder, with engines of large cylinder capacity to admit of great expansion, with sur-face-condensers and superheaters to the boilers. Those of the " Arethusa " were double-trunk, with two cylinders ; those of the " Octavia " were three-cylinder engines ; and those of the " Con-stance " were compound engines with six cylinders ; the first two were worked with steam of 25 H> pressure per square inch, and the last with steam of 32 lb pressure. All these engines gave good results as to economy of fuel, but those of the " Constance " were the best, giving one indicated horse-power with 2J lb of fuel. But the engines of the '' Constance " were excessively complicated and heavy. They weighed, including water in boilers and fittings, about 5J cwts. per maximum indicated horse-power, whereas ordi-nary engines varied between 3| and 4| cwts.
For the next ten years engines with low-pressure steam, surface-condensers, and large cylinder capacity were employed almost exclusively in the ships of the Royal Navy. A few compound engines, with steam of 30 lb pressure, were used in this period with good results as to economy, but they gave trouble in some of the working parts. Compound engines, with high-pressure steam (55 lb), were first used in the Royal Navy in 1867, on Messrs Maudslay's plan, in the '' Sirius." These have been very successful. In the Royal Navy as well as in the mercantile marine, the compound engine is now generally adopted. They have been made rather heavier than the engines which immediately preceded them, but they are about 25 per cent, more economical in fuel, and, taking a total weight of machinery and fuel together, there is from 15 to 20 per cent, gain in the distance run with a given weight.
Rednc- Wrought-iron is largely used in the framing in the place of cast-
tion in iron, and hollow propeller shafts made of Whitworth steel. By
weight these means the weight is being reduced, and it is to be hoped
of that a still further reduction may yet be made by the use of high-
engines, class materials in the engines and steel in the boilers.
. Mr Thornycroft, of Chiswick, and others, by means of high rate of
revolution, forced combustion, and the judicious use of steel, have
obtained as much as 455 indicated horse-power with a total weight
of machinery of 1 If tons, including water in boilers. The ordinary
weight of a seagoing marine engine of large size, with economical
consumption of fuel, excepting a few of very recent construction,
would be six or seven times as great. By closing in the stoke-
holes and employing fans to create a pressure of air in them
capable of sustaining from one to two inches of water in the gauges
the consumption of coal per square foot of fire-grate per hour may
be raised to 130 lb and upwards. The indicated horse-power
which can be obtained in ordinary cases with the steam-blast in
the chimney to quicken consumption does not exceed ten. But
by the forced draft above described it can be raised with ordinary
boilers to 17 to 18 indicated horse-power per square foot of fire-
grate. In torpedo boats with locomotive boilers over ,'28 horse-
power per foot of fire-grate is attainable.
Effi- The following observations on efficiency are taken from the work
cieucy. of Mr Sennett on The Marine Steam Engine :—
" In every machine there are always certain causes acting that produce waste of work, so that the whole work done by the machine is not usefully employed, some of it being exerted in overcoming the friction of the mechanism, and some wasted in various other ways. The fraction representing the ratio that the useful work done bears to the total power expended by the machine is called the efficiency of the machine ; or—
. Useful work done.
Efficiency — srvi ;—r
Total power expended.
In the marine steam engine, in which the useful work is measured by its propelling effect on the ship, there are four successive stages, in each of which a portion of the initial energy is wasted, and these four causes all tend to decrease the efficiency of the engine as a whole.
" In the first place, only a portion of the heat yielded by the combustion of the coal in the furnaces is communicated to the water in the boiler, the remainder being wasted in various ways. The fraction of the total heat evolved by the combustion of the coal, that is, transmitted to the water in the boiler, is in ordinary cases not more than from ^ to This fraction is called the efficiency of the boiler.
" Secondly, the steam, after leaving the boiler, has to perform mechanical work on the piston of the engine ; but this work, in consequence of the narrow limits of temperature between which the engine is worked, is only a small fraction of the total heat contained in the steam—say from \ to according to the kind of engine and rate of expansion employed. This fraction, repre-senting the ratio of the mechanical work done by the steam to the total amount of heat contained in it, is called the efficiency of the steam.
" Thirdly, in the engine itself a part of the work actually per-formed by the steam on the pistons is wasted in overcoming the friction of the working parts of the machinery and in working the pumps, &c. The remainder is turned into useful work in driving the propeller. The fraction representing the ratio that this useful work bears to the total power exerted by the pistons is called the efficiency of the mechanism.
" Fourthly, the propeller, in addition to driving the ship ahead, expends some of the power transmitted to it in agitating and churning the water in which it acts, and the work thus performed is wasted,—the only useful work being that employed in overcoming the resistance of the ship and driving her ahead. The ratio of this useful work to the total power expended by the propeller is called the efficiency of the propeller.
" The resultant efficiency of the marine steam engine is made up of the four efficiencies just stated, and is given by the product of the four factors representing respectively the efficiencies of the boiler, the steam, the mechanism, and the propeller. Any improvement in the efficiency of the marine steam engine, and, consequently, in the economy of its performance, is therefore due to an increase in one or more of these elements."
Under STEAM ENGINE will be found a discussion of the first three of the efficiencies enumerated above. Propulsion and pro-pellers have to be considered here.
"The principle upon which nearly all marine propellers work," Pro-says Mr Sydney Barnaby, '' is the projection of a mass of water in pellers, a direction opposite to that of the required motion of the vessel. When a vessel is in motion at a regular speed the reaction of the mass of water projected backwards by the propeller is exactly equal to the resistance experienced by the vessel. When it is clearly understood that propulsion is obtained by the reaction of a mass of water projected sternwards with a velocity relative to smooth water, the absurdity is at once seen of attempting to get a pro-peller to work without slip. If there is no slip there is no resultant propelling reaction except in the limiting case where the mass of water acted upon is infinite. The whole problem therefore resolves itself into this—What is the best proportion between the mass of water thrown astern and the velocity with which it is projected, that is, if the screw propeller is under consideration, the ratio between its diameter and its pitch ?"
" There are four different kinds of propellers apart from sails— the oar, the paddle-wheel, the screw, and the water jet.
"The first and oldest of them—the oar—maybe used in two ways. The action may be intermittent, as in rowing, when water is driven astern during half the stroke and the instrument brought back above the water ; or its action may be continuous, as in sculling. When used as in rowing it is exactly analogous to a paddle-wheel, while the action of the scull closely resembles that of the screw. It is supposed that in the ancient galleys, which were propelled by a large number of oars in several tiers or banks, the oars hung vertically and worked inwards and outwards with a sculling action. They were not removed from the water, but served as props when the vessel was aground. The oars were always propelling the vessel, in both parts of the stroke. The rowers generally sat with their faces outwards and forwards. There was great overhang of the sides to allow of several tiers of rowers one above another. The oar as used for rowing is a very efficient instrument. To obtain the maximum efficiency out of it a con-stant pressure should be maintained upon the oar, so that the water is started gradually from rest, and the acceleration uniformly in-creased throughout the whole of the stroke. A glance at a univer-sity crew will show that the stroke is kept up with a uniform pressure and without any jerk."
Speaking of the screw propeller, MrS. Barnaby says:—"The speed with which water can follow up the blades of a screw depends upon the head of water over it, but when the immersion is suffi-

cient to exclude air a head of water equivalent to 30 feet is sup-plied by the atmosphere, as has been pointed out by Prof. Osborne Reynolds. Experiments on the model of the Thornycroft screw have shown that the efficiency, which is as much as 70 per cent, when properly immersed, falls to about 50 per cent, when breaking the surface of the water. As a result of a change from a diameter of 5 feet 10 inches to 4 feet 6 inches the speed of the first-class torpedo boat was raised from 18 to 20 knots, other conditions re-maining the same.
" There is no doubt that the stern is the best position for the screw. As a vessel passes through the water the friction imparts motion to the layer of water rubbing against the side. This layer increases in thickness towards the stern, so that, after the vessel has passed through, a considerable quantity of water is left with a motion in the same direction as the vessel. If the screw works in this water it is able to recover some of the energy which has been expended by the ship in giving it motion. The speed of this water, which Kankine estimates may be as much as one-tenth of the speed of the vessel, does not depend upon the form but upon the nature and extent of the surface. As it is a necessity that there should be such a wake, it is a distinct advantage to place the propeller in it and allow it to utilize as much as possible of the energy it finds there. It is important not to confound this water, which has had motion given to it by the side and bottom of the ship, with the wave of replacement, that is, the water filling in behind the ship. It should be the aim to interfere as little as possible with this motion, as such interference augments the resist-ance of the ship very considerably, even in well-formed ships. The propeller should therefore be kept as far away from the stern as possible.
In the small high-speed stern launches the propeller has been kept outside the rudder, with advantage to the speed. What is required is that before reaching the screw the water shall have given out upon the stern of the ship the energy put into it by the bow. If a screw propeller is placed behind a bluff stern so that its supply of water is imperfect it will draw in water at the centre of the driving face, and throw it off round the tips of the blades, like a centrifugal pump, thus producing a loss of pressure upon the stern of the vessel. For very high speed vessels several propellers would enable the weight of the machinery to be kept down. The weight of an engine of a given type per indicated horse-power varies inversely as the number of revolutions per minute ; that is, the greater the number of revolutions the less the weight per Indicated horse-power.
" There is a certain quantity of work which must he lost with any propeller, and it is equal to the actual energy of the discharged water moving astern of the propeller with a velocity relative to still water. As this energy varies as the weight multiplied by the square of the velocity, if we double the quantity of water acted upon we double the loss from this cause, but if we double the velocity with which the water is discharged we increase the loss fourfold. This shows the advantage of acting upon a large column of water, and leaving it with as small a speed as possible relative to still water. For this reason the screw is a more efficient instrument than a paddle-wheel, and the jet propeller, with its small area of jet, is so much inferior to the screw. From the above con-siderations it would appear that the larger the diameter of a screw and the smaller the slip the greater the efficiency would be. There is, however, another element of loss which has to be con-sidered, which imposes a limit to the size of a screw in order to obtain the best efficiency. This element is the friction of the screw blades. How large the effect of this element may be is shown by the case of H. M. S. ' Iris.' This ship was originally fitted with two four-bladed propellers, 18 feet in diameter, and with 18 feet pitch or velocity of advance per revolution. She obtained a speed with these propellers of 15J knots with an ex-penditure of 6369 horse-power. Two blades were then taken from each propeller, reducing the total number from eight to four. The indicated horse-power then required for the same speed was 4369, or two thousand less horse-power. This amount had been lost in driving the four additional blades." Causes '' The causes of loss of work incidental to propellers of different ofineffi- kinds may be summed up as follows:—(1) Suddenness of change ciency. from velocity of feed to velocity of discharge. Propellers which suffer from this cause are the radial paddle-wheel and the common uniform pitch screw; while those which in varying degree avoid it are the gaining pitch screw, the feathering paddle-wheel, Ruthven's form of centrifugal pump, and the oar. (2) Transverse motion impressed on the water. Propellers which lose in efficiency from this cause are ordinary screw-propellers, which impart rotary motion, radial wheels, which give both downward and upward motion on entering and leaving the water, and oars, which impart outward and inward motion at the commencement and end of the stroke respectively. This loss is greatly reduced in the guide-propeller, as the guides take the rotary motion out of the water and utilize it in so doing. (3) Waste of energy of the feed water. This is experienced in the jet propeller as generally applied."
The present condition of the case of screw steamship propulsion
appears, according to Mr Froude's estimate, to be that, calling the
effective horse-power (that is, the power due to the net resistance)
100, then at the highest speeds the horse-power required to over-
come the induced negative pressure under the stern consequent on
the thrust of the screw is 40 more; the friction of the screw in the
water is 10 more ; the friction in the machinery 67 more ; and air
pump resistance perhaps 18 more ; add to this 23 for slip of screw,
and we find that, in addition to the power required to overcome the
net resistance = 100, we need 40 + 10 + 67 + 18 + 23, making in all
258; i.e., at maximum speeds the indicated power of the engines
needs to be more than two-and-a-half times that which is directly
effective in propulsion. (N. B.)
Boatbuilding.
The foregoing article may be supplemented by a brief account of boatbuilding. The distinction between this and shipbuilding is not of a marked character and cannot be sharply defined. But for all practical purposes the builder of a vessel without a deck, or but partially decked, and propelled partly by sails and partly by oars, or wholly by oars, may be defined as a boatbuilder.
The boats in general use at present may be classified as racing boats, pleasure boats, or boats used for commercial purposes. Racing boats (compare ROWING) are generally built of mahogany, and are the most perfect specimens of the boatbuilder's art. The out-rigger sculling boat measures from 30 to 35 feet long, 12 to 14 inches in breadth, and 9 inches in depth, weighing only from 35 to 45 lb, and the eight-oared outrigger, being from 55 to 65 feet long by 2 feet 2 inches to 2 feet 5 inches in breadth, weighs about 300 lb. Pleasure boats vary in form and dimensions, from the 15-feet row-ing boat used on thesea-coast to the gondola type found principally on the canals of Venice and used occasionally on the Thames, &c, for ceremonial pageants. Boats used for commercial purposes embrace fishing, canal, and ships' boats. Fishing boats (compare FISHERIES) are gradually passing from the sphere of the boat-builder to that of the shipbuilder,—the open boats of former years being in many cases replaced by large, strong, decked craft more able to withstand the gales of the British coasts. Canal boats are gene-rallyTong, narrow, and shallow, from 50 to 70 feet long by 8 to 10 feet in breadth, and from 4 to 5 feet in depth. All sea-going vessels are required by statute to be provided with boats fully equipped for use, not few-er in number nor less in their cubical contents than what is specified for the class to which the ship belongs. The boats vary considerably in form and dimensions as well as in material and construction, according to the service intended. The number of boats a passenger steamer of 1000 tons and upwards is required to carry is six or seven, according to the dimensions of the boats. In either case two of the largest boats must be fitted as lifeboats. If the smaller number is carried, the set will consist of two lifeboats, one launch, two cutters or pinnaces, and one gig.
Lifeboats are built both ends alike, having a sheer or rise from midships towards stem and stern of J inch to § inch per foot of length. They have air-cases of copper or yellow metal fitted in the ends and along the sides of the boat, of sufficient capacity to give each person carried in the boat one and a half cubic feet of strong enclosed air-space (compare vol. xiv. p. 570). Cutters are similar in form but of smaller dimensions than lifeboats ; pinnaces are about the same dimensions as cutters, but have square sterns. Gigs are of lighter construction and finer form than pinnaces. A service boat called a dingy is also carried, for the conveyance of light stores between the shore and the vessel. Boats, when carried so close to the funnel of a steamer as to be injuriously affected by the heat therefrom, have of late years been built of zinc, iron, or steel. Those built of steel have plates -jV inch thick and galvan-ized, the keel, stem, stern, and deadwood knees being of wood, to which the plating is attached.
The following is an outline of the method of construction. The designer lays down on paper the lines and body-plan of the craft, which are afterwards traced full size on the floor of the drawing-loft. From these full-sized sections moulds are made. The stem and stern posts, having been cut out to the shape designed, are tenoned into mortices in the keel. Two knees overlap, and bind the stem and stern posts to the keel, and are bolted with through bolts and clenched outside over a ring or washer. A stout batten of wood is then nailed between the stem and sternpost heads to connect them together, and a line is then stretched from stem to sternpost to represent the water-line. The keel, stem, and stern posts being in position on the stocks, the stem and stern posts are then plumbed and secured by stays of wood. The rabbets in the keel, stem, and stern posts are then cut out with a chisel, after which the moulds are put into their proper places, plumbed with the water-line, and kept in position by stays. The planking is then proceeded with, strake after strake, and when the boat is planked up to the top strake the floors and timbers are put in. The floor extends across the keel and up to the turn of the bilge. They are fastened through the keel with copper or yellow metal bolts and to the planking with copper nails.

The timbers generally are about 1 inch by } inch, and are sawn out of a clean piece of American elm, then planed and rounded. After being steamed they are fitted into the boat, and as soon as each is in position, and before it cools, it is nailed fast with copper nails. The gunwale is next fitted, a piece of American elm about 2 inches square ; a breast-hook is fitted forward, binding the gunwale, top strake, stern, and apron together ; and aft the gunwale and top strake are secured to the transom by either a wooden or iron knee. A waring or stringer, about 3 inches by § inch, of American elm, is then fitted on both sides of the boat, about 8 to 9 inches below the gunwale, on the top of which the thwarts or seats rest. The thwarts are secured by knees, which are fastened with clench bolts

through the gunwale and top strake and also through the thwart and knee. The boat generally receives three coats of paint and i3 then ready for service.
The following are the dimensions of boats in the British merchant service:—


== TABLES ==


Footnotes

2 Of this the vertical armour (costing before it is worked up, £70 to £90 per
ton) is nearly 2000 tons. 3 Average of six vessels built by Elder*



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