April 14, 1892] 



NATURE 



57i 



ship will be sure that Mr. Tower did not exaggerate the 

 ingenuity of its inventor ; and we hope some day to have the 

 details made public, more especially as the Director of Naval 

 Construction — who was another of the favoured few who had 

 seen the apparatus at work — stated that he felt sure it would 

 have a wide application for other purposes than that for which 

 it was originally devised. As a result, in a heavy sea in the 

 Channel, the apparatus reduced a roll of i8° each way to one of 

 9° each way. 



Mr. W. H. White's paper on recent experiences with H.M.'s 

 ships had of course been looked forward to with considerable 

 expectation and interest. The Director <>f Naval Construction 

 has the art by which he can render interesting almost any sub- 

 ject upon which he m rites. Probably, however, the paper was 

 disappointing to many engineers, who were misled by the title 

 into the belief that the great forced-draught and leaky-tube 

 question was going to be fought out. As a matter of fact, 

 Mr. White only referred to the boiler problem in order to 

 inform his audience that he was going to say nothing about 

 it ; and the chief point to which he turned his attention was the 

 influence of shallow water upon the trial trip speeds of modern 

 vessels. In past times when spt-eds were more moderate, the 

 Stokes Bay measured mile did well enough, but now that vessels 

 have to be tried to 20 knots and over, the depth of water is 

 wholly insufficient. Mosi of the measured miles on which ships 

 are tried have an insufficient depth of water, the notable excep- 

 tion being the Skelmorlie course on the Clyde, which appears to 

 be everythin^r that can be desired, the water beinsj both sheltered 

 and deep. The Ilumber course is apparertly the worst, as it is 

 shallow in depth and very much exposed. In connection with this 

 matter, an amusing incident a-ose during the discussion on Mr. 

 White's paper. A patriotic Scots shipbuilder had been exult- 

 ing in the superiority of the Scottish mile over all those on the 

 English coast. He was followed by a Hull constructor, who ac- 

 knowledged the excellence of 'he Skelmorlie trial ground, 

 " which," he said, "doubtless largely accounted for the very 

 excellent results attained by Clyde-built vessels." To return, 

 however, to Mr. White's paper ; he gives an instance of two sister 

 vessels Jried on the Maplin mile at diffisrent states of the tide. 

 The variation in d^-pth of water was 9 feet on 42 feet, and with 

 the same power indicated there was a diffisrence of half a knot 

 in speed. The cruist^r Edgar, in Stokes Bay, with 12 fathoms of 

 water, required 13,260 horse-power to attain 20J knots ; whilst 

 in water 30 fathoms deep she reached 21 knots with 12,550 horse- 

 power. In running from the Nore to Portsmouth, the first-class 

 cruiser Blenhnmva-xA.& 20 knots with 1 5, 750 indicated horse-power 

 in water 9 fathoms deep. On the same trip, when the vessel got 

 into water of 22 to 36 fathon^s, the speed rose to 21^ knots with 

 practically the same horse-power. In this instance the estimated 

 loss due to the shallower water was 3000 horse-power. Other 

 examples were given by the author, and by other naval architects 

 during the discussion, the most notable perhaps being that 

 narrated by Mr. Philip Watts, the chief of the Elswick Ship 

 Yard, who gave his experience with the Italian cruiser Plmonte, 

 the results being very similar to those of the Blenheim. It is 

 evident that depth of water has a far greater influence on speed 

 trials than has hitherto been generally supposed, and it should 

 be remembered that the smaller vessels, such as torpedo-boats, 

 suffiir almost as much as the larger craft. A vessel travel- 

 ling at a given speed has to carry a wave of dimensions corre- 

 sponding to the speed, whether the vessel be large or small, so 

 that the size of the vessel does not afi"ect the depth of water re- 

 quired, except in the important detail that a big vessel's keel is 

 nearer the bottom than that of a small one. Vibration was 

 another subject upon which Mr. White treated, but this question 

 was so much more fully dealt with by Mr. Yarrow in the even- 

 ing, that we may pass on to the paper of the latter contributor ; 

 skipping the three intervening, which were of less general 

 interest. 



Mr. Yarrow's paper on balancing marine engines and the 

 vibration of vessels was undoubtedly the great feature of the 

 meeting, as was his paper last year on the construction of 

 marine boilers. Unfortunately it is impossible to give a good 

 idea of Mr. Yarrow's lecture — for it was more than a paper — with- 

 out the aid of the views by which it was illustrated. These 

 were thrown, by means of the lantern and electric light, on a large 

 screen erected for the purpo.se. As everyone knows who has 

 been present on the trial trip of a torpe lo-boat, the vibration in 

 these little craft is excessive. The enormous power exerted by 

 the engines, the rapidity of their reciprocations, and the slight 



NO. I 172, VOL. 45] 



and elastic nature of the hull construction, all combine to 

 render the deck of a torpedo-boat, travelling at her best 

 speed, a most unpleasant position ; especially when ihe spray 

 is driven in sheets across the deck, and the white-hot cokes 

 are at intervals emitted from the chimney. It is natural, 

 therefore, that the question of vibration should be attacked, 

 and to a great extent solved, by one of our two great 

 toipedo-boat builders. Mr, Yarrow, some time ago, came to 

 the conclusion that it was the reciprocating weights of the 

 engines that caused vibration. There was long a general belief 

 that the propeller was the origin of nearly all vibration in screw 

 ves-els, and the belief still largely exists ; but those best ac- 

 quainted with the subject have for some time known it to be 

 erroneous. If the blades of a screw be properly balanced, and 

 in other respects if the propeller is as well made as the screws 

 of torpedo-boats have to be, there will be little difference in the 

 amount of vibration whether the screw be in position or taken 

 off, provided the engines are running at the same speed in both 

 cases. Mr. Yarr w has proved these facts most conclusively 

 by an elaborate series of trials made with workin^i models 

 and also with an actual torpedo-boat. The principal model re- 

 presented to scale a three crank tri-compound engine. This 

 was hung in a frame by means of spiral springs. The 

 weights of their pistons and other moving parts were as 

 O'-dinarily arranged in a torpedo-boat's engines. The model 

 was cau.sed to work by means of a thin and flexible steel wire, 

 so that no motion other than a rotary movement, could be con- 

 veyed to the apparatus. When the model was caused to work, at 

 a given speed, the movement was excessive. In this way was very 

 clearly brought out the fact, already known to engineers, that 

 the vibration of a boat is at its maximum when the revolutions 

 of the engine synchronize with the natural period of vibration of 

 the boat. It will be seen that a boat or ship, like any other 

 elastic structure, say a girder or a tuning-fork, will have a period 

 of vibration natural to it. The reciprocations of the engine 

 pistons cause a certain number of impulses to be communicated 

 to the hull, and if the number of vibrations and the impulses are 

 either one a multiple of the other, then the vibration will be ex- 

 cessive. This has been thoroughly proved by experience, and 

 it has been the aim ofjbuilders of high-power vessels of light 

 scantling to fit screws so designed that at maximum speeds the 

 vibrations and revolutions of the engine will not synchronize. 

 In Mr. Yarrow's model the elasticity of the hull was repre- 

 sented by the stiff spiral springs by which the model engine was 

 suspended. Having shown the way in which the reciprocating 

 weights of the engines acted so detrimentally, Mr. Yarrow next 

 proceeded to explain the manner in which he overcame the 

 difficulty. Attached to the model were weights, which he 

 termed bob- weights. These were so placed as to balance the 

 natural reciprocating parts of the engine. They were actuated 

 by eccentrics, and could be put in and out of gear as required. 

 With the bob-weight in operation, the effect was most marked, 

 the model being perfectly steady at any rate of movement. 

 The bob weights have, of course, to be of the proper weight, 

 and must be accurately placed in the longitudinal plane of the 

 engine, otherwise the balance would be destroyed. This was 

 shown by Mr. Yarrow with the model. He had first thought 

 that a good effect might be obtained by making all three pis- 

 tons of equal weight — the low-pressure piston is naturally far 

 heavier than the others — but little benefit was obtained in this 

 way. One of the most interesting parts of the lecture was the 

 photographic pictures of torpedo-boats thrown on the screen 

 \iy the magic lantern. These pictures were among the best 

 of the kind we have ever seen. The boat was moored in 

 the West India Docks so as to get still water, and a calm day 

 was chosen. The propeller was removed so that the engines 

 ran free. The first photograph was taken with the engine in its 

 ordinary condition, no bob-weights being attached. By previous 

 experiment the number of revolutions that caused the most 

 vibration had been nscertained ; we believe 240 per minute was 

 the number, and the engines were run at that speed. The boat 

 was therefore caused to vibrate excessively, and the effect was 

 clearly shown by the waves or ripples thrown off from the side. 

 These were beautifully ma' ked in the photograph, the pattern 

 caused by the intervening wave series being very curious. Many 

 pictures were given illustrating various wave phenomena due to 

 different combinations, one of the most interesting being that in 

 which vibration was caused by one of Mr. Yarrow's assistants 

 springing on the stern 240 times a minute ; an athletic feat of 

 no mean order, and one which required considerable training. 



