382 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[November, 



OX THE FORMS AND PROPORTIONS OF STEAM- 

 VESSELS. 



Sir — A former paper on tliis subject, which you were so good as to 

 give a place in your columns, having occasioned the observations of 

 your correspondent A. M., before proceeding further, his letter 

 demands my attention. I think Ids objections to llie method adopted 

 for ascertaining the comparative resistance of the simple forms of 

 vessels on which the calculations were founded, arises principally 

 from his having overlooked the effect of what has commonly been 

 called the "minus pressure," that is, the loss of pressiue abaft, occa- 

 sioned by the water's not closing in immediately on the vessels' after 

 body, on account of the rapidity of her motion. This effect of motion 

 is equivalent to an increase of pressure on the bow, and when the 

 velocity is such that the water will not close in on the loitttt part of 

 the immersed body, the case becomes one of simple hydraulic pressure 

 increased by motion. Besides the loss of pressure occasioned by the 

 absolute absence of water behind the moving body, it has always been 

 found that there is a great reduction in this respect before such a 

 velocity has been attained as would prevent the closing of the water 

 behind. This is occasioned by the rarefaction of the water, or its 

 being ndxed with a large portion of air, or leaving intervals of unoc- 

 cupied space in the wake of the vessel, and though these effects are of 

 course diminished, (as it W'as my intention to notice as I proceeded 

 with the subject) by the proper modification of form in the after body 

 of a vessel, I believe that in the simple forms which I have supposed, 

 moving at such velocities as are acquired by large steamers at the 

 present day, the result of these two sources of resistance would be 

 such, that the sections would rest verv nearly on the merits of their 

 respective hydraulic pressures. It is the difficulty of estimating the 

 exact influence of these two effects, which has caused all experiments 

 hitherto made, as far as I can ascertain, to prove defective in their 

 results, and apparently to contradict themselves at different velocities. 

 Sir Isaac Newton assigned twice the pressure given by M. Bomit, and 

 though lie shewed his own results to be defective, they long remained 

 the best data on which calculations on this subject could be founded. 

 The experiments of the London Society for the Improvement of Naval 

 Architecture, did not afford the means of establishing any certain law 

 for estimating these resistances, and the velocities at which their 

 trials were made being comparatively low, I do not think I am alto- 

 geflier unauthorised in adopting this mode of calculation for high ve- 

 locities and simple forms. Some experiments detailed by Dr. Hutton, 

 on the motion of bodies in air tend to support this view, as they show- 

 that resistance increases in a larger ratio than the square of the velo- 

 city even in an elastic fluid, and the deviation from this ratio becomes 

 greater and greater as the velocities are increased, a result most pro- 

 bably arising by decrease of pressure on the hinder part of the body, 

 caused by the rarefaction of the air as the speed is increased. 



After this explanation, I proceed briefly to notice the two points of 

 carriage of fuel, and action of the paddles as compared in vessels of the 

 different proportions supposed. If two such vessels required each 400 

 tons of coal for her voyage, equal to about l(i,UUO cubic feet of water, 

 the vessel 200 feet long by 40 beam, would have her draught of water 

 increased 2 feet by this additional load, wdiile the other, 200 feet by 

 30, would suffer an increase of draught of 2 ft. S in. We here see 

 that the narrower vessel has the disadvantage in several respects, she 

 has a greater depth in proportion of her body immersed, and suffers 

 consequently an increase of hydraulic pressure. lu consequence of 

 the coal occupying a greater deptli, on account of her smaller amount 

 of beam, than in the other vessel, her centre of gravity is more raised in 

 proportion, brought nearer to her axis of rotation, if it was before be- 

 low it, and raised further above it, if in the first instance it was already 

 so, as 1 believe most frequently is the case. In either case one great 

 source, though perhaps not the principal one, of her stability or power 

 of resisting a rolling motion is diminished; she is rendered less able 

 to carry sail, which may sometinips prove the only resource for safety 

 and less able to resist the stroke of the sea abeam, the latter being 

 very commonly supposed among nautical men to be the cause of the 

 President's foundering. Again, by leaving a smaller depth of the hull 

 .vacant for buoyancy above water, her power to rise over a sea is 

 diminished, and she will consequently be wetter and pitch deeper in 

 proportion to her increased load than the vrider vessel. And lastly, 

 (supposing her a vessel of the usual form) by bringing her full lines 

 more in contact with the water, her speed must sutti;r a most serious 

 diminution. Again, as regards the action of the paddle-wheels, sup- 

 possing the paddles of these two vessels to be of equal diameter, and 

 the floats having the same dip at their respective draughts of 15 and 

 20 feet. The narrow vessel again suffers a disadvantage as compared 

 with the other ; for her paddles being sunk deeper, the floats enter 



the water at a smaller angle to the horizon, and leave it at a greater, 

 and thus the effective action is diminished and the backw ater increased. 

 This defect being severely felt in narrow vessels, different means have 

 been tried to counteract it. The mean dip of the paddles has been 

 reduced by raising the wheels higher on the hull, but this method has 

 the disadvantage of making the paddles fly light when the fuel is re- 

 duced, the weather wheel frequently hardly dipping; the heavv pe- 

 destals for the paddle shaft must also be raised with it, and thus the 

 centre of gravity must also rise, and the tendency to roll be increased, 

 and if the hull he built higher out of the water in the same proportion, 

 an imnecessaiily large surface is exposed to the sidewav action of the 

 water and wind, and the rolling tendency still further augmented ; — 

 besides the engines working more rapidly increase the consumption 

 of fuel without a proportionate increase of speed. The various de- 

 scriptions of feathering paddles seem most applicable to narrow deep 

 vessels, as it is in such when deeply laden that the oblique action of 

 the floats is most felt; but even here I believe the additional power 

 requisite to overcome their friction is generally considered to counter- 

 balance the gain of power; and the floats being all constantlv vertical 

 present a much larger surface to the direct action of a head or follow- 

 ing sea, and in such case become additional imi)ediments to speed. 

 The reefing paddle seems most likely to answer the end proposed, but 

 may it not be a question whether the increased speed of the engines, 

 when the wheels are reduced in diameter, does not cause some increase 

 in the consumption of fuel ? which would certainly be avoided in a great 

 measure by building vessels of such proportions as would render any 

 such contrivances needless. Feeling the great advantage of experi- 

 mental evidence in support of expressed opinion?, it was my intention 

 to have illustrated these remarks by a few simple experiments, bear- 

 ing solely on the points they refer to, but on consideration, so many 

 difficulties appeared to oppose the probability of obtaining any data 

 sufliciently accurate by such means as I could command, that this de- 

 sign was unavoidably abandoned. It is to be hoped that the experi- 

 ments now proceeding under the auspices of the British Association, 

 of the further prosecution of which I was not aware when I entered on 

 this topic, will tend to clear away the difficulties which have hitherto 

 attended the application of theory to this interesting subject, and ad- 

 vance naval architecture to a station somewhat more on a parallel with 

 the present state of the other practical sciences. 



I am, Sir, vour's, kS;c. 



H. P. H. 



COUNT DE PAMBOUR IN REPLY TO MR. PARKES. 



Sir — In a paper written by me and inserted in your number for 

 September last : On the Momtnliim proposed by J\Ir. Josiah Parkes, as 

 a measure of the mechanical effect of locumolive engines, the following 

 passage occurs: "The author tells us that he is more accustomed to 

 handle the hammer than the pen." I have since perceived that I had, 

 there, by mistake, attributed to the paper of Mr. Parkes, on boilers 

 and steam engines, a sentence which I had read in the very useful 

 work of Mr. Armstrong, on the boilers of steam engines, preface, page 

 xi, Weale, 1S39. The two works having come to me at the same 

 time, and being precisely on the same subject, I had made the error 

 of ascribing to the one, what in reality belongs to the other. This 

 point is however without the least importance, liaving no reference to 

 the arguments presented in my paper, and I correct it only for the 

 sake of accuracy. 



Since the publication of the paper above alluded to, Mr. Parkes has 

 printed in several periodicals, a letter in which he accuses me of 

 having misrepresented his sentiments, in my refutation of his strictures 

 against me. I had thought, first, that if my paper itself were put 

 under the eyes of the persons who had read Mr. Parkes's letter, it 

 would, by the full references contained in it, show sufficiently that I 

 had not misrepresented the sentiments of Mr. Parkes; and 1 had, iii 

 consequence, only asked of the editors of the periodicals in wliich Mr. 

 Parkes's letter had been published, to insert that paper as an answer. 

 But this request having been refused by the Literary Gazette, and a 

 mutilated part only of the letter which I l-.ad sent with the paper, 

 having appeared in the Mining Journal, with the omission of what I 

 considered the most important passages, it becomes necessary for me 

 to make a different answer. Therefore, I beg you to insert in your 

 next publication, the following paper, as a reply to Mr. Parkes's alle- 

 gations. 



I remain. Sir, 



Your very obedient servant, 



G. De Pamboub. 



October 18, 1841. 



