132 



NATURE 



{Dec. i6, 1875 



to the two causes I have just spoken of, namely surface-friction 

 and mutual friction of the particles. The stream-line motions 

 being the same in character as those we have been considering, 

 we shall still have at each end an excess of pressure which will 

 tend to force up the sheet of ice, and along the side we shall have 

 defect of pressure tending to suck down the sheet of ice. If, 

 now, we remove the ice, the water will obviously rise in level at 

 each end, so that excess of hydrostatic head may afford the 

 necessary reaction against the excess of pressure ; and the water 

 will sink by the sides, so that defect of hydrostatic head may 

 afford reaction against the defect of pressure. 



The hills and valleys thus formed in the water are, in a sense, 

 waves ; and, though originated in the stream-line forces of the 

 body, yet when originated they come under the dominion of 

 the ordinary laws of wave-motion, and, to a large extent, behave 

 as independent waves. 



The consequences which result from this necessity are most 

 ir.t icate ; but the final upshot of all the different actions which 

 take place is plainly this — that the ship in its passage along the 

 surface of the water has to be continually supplying the waste of 

 an attendant system of waves, which, from the nature of their 

 constitution as independent waves, are continually diffusing and 

 transmitting themselves into the surrounding water, or, where 

 they form what is called broken water, crumbling away into froth. 

 Now, waves represent energy, or work done ; and therefore all 

 the energy represented by the waves wasted from the system 

 attending the ship, is so much work done by the propellers or 

 tow-ropes which are urging the ship. So much wave-energy 

 wasted per mile of travel is so much work done per mile ; and 

 so much work done per mile is so much resistance. 



The actions involved in this cause of resistance, which is sonie- 

 times termed " Wave-Genesis," are so complicated that no ex- 

 tensive theoretical treatment of the subject can be usefully at- 

 tempted. All that can be known about the subject must, for the 

 present, I believe, be sought by direct experiment. 



Having thus briefly defcribed the several elements of a ship's 

 resistance, I will proceed to draw your attention more particu- 

 larly to certain resulting considerations of practical importance. 

 Do not, however, suppose that I shall venture on dictating to 

 shipbuilders what sort of ships they ought to build : I have so 

 little experience of the practical requirements of shipowners, that 

 it would be presumptuous in me to do so ; and I could not ven- 

 ture to condemn any feature in a ship as a mistake, when, for all 

 I know, it may be justified by some practical object of which I 

 am ignorant. For these reasons, if I imply that some particular 

 element of form is better than some other, it will be with the 

 simple object of illustrating the application of principles, by 

 following which it would be possible to design a ship of given 

 displacement, to go at given speed, with minimum resistance, in 

 smooth water — in fact, to make the best performance in a 

 " measured mile " trial. 



I have pointed out that the causes of resistance to the motion 

 of a ship through the water are : first, surface-friction ; secondly, 

 mutual friction of the particles of water (and this is only practi- 

 cally felt when there are features sufficiently abrupt to cause 

 eddies) ; and thirdly, wave-genesis. I have also shown that 

 these are the only causes of resistance. I have showTi that a 

 submerged body, such as a fish, or torpedo, travelling in a per- 

 fect fluid, would experience no resistance at all ; that in water 

 it experiences practically no resistance but that due to surface- 

 friction and the action of eddies ; and that a ship at the surface 

 experiences no resistance in addition to that due to these two 

 causes, except that due to the waves she makes. I have done 

 my best to make this clear : but there is an idea that there exists 

 a kind of resistance, a something expressed by the term "direct 

 head-resistance," which is independent of the above-mentioned 

 causes. This idea is so largely prevalent, of such long standing, 

 and at first sight so plausible, that I am anxious not to leave any 

 misunderstanding on the point. 



Lest, then, I should not have made my meaning sufficiently 

 clear, I say distinctly, that the notion of head-resistance, in any 

 ordinary sense of the word, or the notion of any opposing force 

 due to the inertia of the wat^r on the area of the ship's way, a 

 force acting upon and measured by the area of midship section, 

 is, from beginning to end, an entire delusion. No such force 

 acts at all, or can act ; as throughout the greater part of this 

 address I have been endeavouring to explain. No doubt, if two 

 ships are of precisely similar design, the area of midship section 

 may be used as a measure of the resistance, because it is a 

 measure of the size of the ship ; and if the ships were similar in 



every respect, so also would the length of the bowsprit, or the 

 height of the mast, be a measure of resistance, and for just the 

 same reason. But it is an utter mistake to suppose that any 

 part of a ship's resistance is a direct effect of the inertia of the 

 water which has to be displaced from the area of the ship's way. 

 Indirectly the inertia causes resistance to a ship at the surface, 

 because the pressures due'to it make waves. But to a submerged 

 body, or to the submerged portion of a ship travelling beneath 

 rigid ice, no resistance whatever will, be caused by the inertia of 

 the water which is pushed aside. And this means that, if we 

 compare two such submerged bodies, or two such submerged 

 portions of ships travelling beneath the ice, as long as they are 

 both of sufficiently easy shape not to cause eddies, the one which 

 will make the least resistance is the one which has the least skin 

 surface, though it have twice or thrice the area of midship 

 section of the other. 



The resistance of a ship, then, practically consists of three 

 items — namely, surface-friction, eddy-resistance, and wave-resist- 

 ance. 



Of these the first-named is, at least in the case of large ships, 

 much the largest item. In the Greyhound^ a bluff ship of 1,100 

 tons, only 170 feet long, and having a thick stem and sternposts, 

 thus making considerable eddy-resistance, and at ten knots 

 visibly making large waves, the surface-friction was 58 per cent, 

 of the whole resistance at that speed ; and there can be no doubt 

 that with the long iron ships now built, it must be a far greater 

 proportion than that. M 01 cover the Gf-ey hound was a coppered 

 ship ; and most of the work of our iron ships has to be done 

 when they are rather foul, which necessarily increases the 

 relative importance of the surface-friction item. 



The second item of resistance, namely the formation of eddies, 

 is, I believe, imperceptible in ships as finely formed as most 

 modern iron steamships. Thick square-shaped stems and stem- 

 posts, more especially the latter, are the most fruitful source of 

 this kind of resistance. 



The third item is wave-resistance. To this alone of the three 

 is the stream-line theory directly relevant, and here, as we have 

 seen, it rather suggests tendencies, than supplies quantitative 

 results, because, though it indicates the nature of the forces in 

 which the waves originate, the laws of such wave-combinations 

 are so very intricate, that they do not enable us to predict what 

 waves will actually be formed under any given conditions. 



There are, however, some rules, I will not call them prin- 

 ciples, which have to some extent been confirmed by experiment. 

 At a speed dependent on her length and form, a ship makes a 

 very large wave- resistance. At a speed not much lower than 

 this, the wave-resistance is considerably less, and at low speeds 

 it is insignificant. Lengthening the entrance and run of a ship 

 tends to decrease the wave -resistance ; and it is better to have 

 no parallel middle body, but to devote the entire length of the 

 ship to the entrance and ran, though in this case it be necessary 

 to increase the midship section in order to get the same displace- 

 ment in a given length. 



With a ship thus formed, with fair water-lines from end to 

 end, the speed at which wave-resistance is accumulating most 

 rapidly, is the speed of an ocean-wave the length of which, from 

 crest to crest, is about that of the ship from end to end. 



I have said we may practically dismiss the item of eddy-resist- 

 ance. The problem, then, to be solved in designing a ship of 

 any given size, to go at a given speed with the least resistance, 

 is to so form and proportion the ship that at the given speed the 

 two main causes of resistance, namely surface-friction and wave- 

 resistance, when added together, may be a minimum. 



In order to reduce wave-resistance we should make the ship 

 very long. On the other hand, to reduce the surface-friction we 

 should make her comparatively short, so as to diminish the 

 area of wetted skin. Thus, as commonly happens in such 

 problems, we are endeavouring to reconcile conflicting methods 

 of improvement ; and to work out the problem in any given case, 

 we require to know actual quantities. We have sufficient general 

 data froin which the skin resistance can be determined by simple 

 calculation ;but the data for determining wave-resistance must be 

 obtained by direct experiments upon different forms to ascertain 

 its value for each form. Such experiments should be directed to 

 determine the wave-resistance of all varieties of water-line, cross 

 section, and proportion of length, breadth, and depth, so as to 

 give the comparative results of different /orms as well as the 

 absolute result for each. 



An exhaustive series of such experiments could not be tried 

 with full-sized ships ; but I trust that the experiments I am now 



