138 , PERIOD OF VIBRATION OF STEAM VESSELS. 



Mr. Gatewood : — The basis of the paper is that, by changing the lading of the vessel 

 as a beam, the period of vibration is automatically changed. Now, in the case of the vessel 

 with which I had experience we were unable to change the loading of the vessel. So what 

 we did was to change the revolution of the engine. The trouble was with the synchronism 

 of the period of vibration of the vessel to the revolution of the engine, and it would seem 

 very natural with these lake vessels, very long and comparatively shallow, that their condi- 

 tion of lading should at times bring them into synchronism with the revolutions of the 

 engine. But if propeller blades were broken off, so that the engine would run faster, that 

 would also, it seems to me, account for the fact that the vibration would be thereby elim- 

 inated and you would get out of touch with the natural period of the vessel. 



Of course the paper does not pretend to touch upon a great many things in connec- 

 tion with vibration. It treats just one phase of the subject, that is, the vibration in the ver- 

 tical plane of the whole vessel. It does not pretend to take into consideration local vibrations. 

 I have not attempted to touch that part of the subject and have not the experience to pass 



on it. 



As to the question of modifying the vibration of a battleship by putting in water ballast, 

 the effect of adding water ballast could probably be figured along the lines of the paper for 

 the vibration of the whole vessel, but if the vibration is merely local, such as certainly occurs 

 on a number of vessels (that is, you find it in one spot and you find it nowhere else in the 

 vessel), nothing in the paper, it seems to me, would have any bearing on that phase of the 

 subject. 



The measurement for determining the vibration on the vessel which the paper illustrates 

 was very crude, because it was for the vibration of the whole vessel, but I believe it was 

 fairly accurate. 



Those who have read the paper will notice that I adopted, for measuring the vibra- 

 tion, the scheme of measuring the extension and compression occurring in the stringer plate 

 on the shelter deck of the vessel. That is to say, the vessel was treated as a beam, and as the 

 beam changed its load due to the vibration the top member of the beam or girder stretched 

 and shortened, and measurements were taken over a length of 90 feet so as to enable them 

 to be caught by the eye. While, as I say, the method is crude, it really is about as accurate 

 as could be obtained for the particular kind of vibration. It gave a very good measure of 

 the change of stresses in the ship, and I was able to count the number of vibrations by hand, 

 just as you can count the revolutions of an engine, the vibrations being in the neighborhood 

 of seventy-six. 



The vibration that is due to torsional vibration and irregularities of pitch of the pro- 

 peller, it seems to me, would not cause the kind of vibration which is discussed in the paper. 

 That is another phase of the subject which can very well be considered, if there is anyone 

 here who has the means of presenting further data on the subject. 



I remarked in opening that the introduction of the turbine was expected to eliminate 

 vibrations, and that there was some disappointment because such was not the case. I think, 

 however, that the type of vibration which is caused by the turbine is not the type of vibration 

 which had been caused by the reciprocating engine; that is, it is not the vibration of the 

 whole vessel, but of a part of the vessel — local and not general. 



The Chairman : — We will now proceed to the next paper, No. 9, entitled "Data on 

 Hog and Sag of Merchant Vessels," by Mr, T, M, Cornbrooks, Member. 



