1886.] Straining of Ships caused hy Rolling. 



27 



may be readily determined ; and a method of doing this is described at 

 length in the paper. 



The results given by the investigations described apply only to 

 ships rolling from side to side in still water, assuming that the water 

 offers no resistance to rolling motion. It is obvious, however, that 

 the twisting moments thus obtained must often be greatly exceeded 

 when a vessel is rolling and pitching while lying or moving across a 

 series of long ocean waves. In these circumstances the bow or stern 

 frequently has so little immersion that the righting moment acting 

 upon a portion of one end is momentarily very small, and almost the 

 whole of the energy of rotation is applied to the production of twisting 

 moments. The resistance of the water would also often increase the 

 twisting moments. 



It now remains to be seen what can be done in the way of deter- 

 mining the stresses upon the hull which are caused by the twisting 

 moments. We can learn something of the nature and distribution of 

 those stresses ; but, at present, their amounts cannot be calculated 

 with any reliable approach to accuracy. Experiments are required 

 upon the torsion of thin shells of various prismatic forms in order to 

 furnish the requisite data for dealing with so complicated a case a.s 

 that of a ship's hull. The difficulty of obtaining such data is very 

 great ; but pending the time when it is to be hoped this want will be 

 supplied, it may be useful to draw attention to some of the general 

 considerations which affect the twisting moments and the distribution 

 of the twisting strains and stresses over a ship's hull; and to the 

 bearing which these have upon the important practical problems that 

 relate to the structural strength of ships. 



The best data available for guidance in judging of the distribution 

 of strain and stress due to twisting over the structure of a ship are 

 to be found in M. de St. Tenant's investigations of the torsion of 

 prisms.* These investigations assist us to form a general idea of the 

 manner in which a ship's structure may be strained by twisting ; and 

 they also indicate the nature of the experiments that are necessary to 

 furnish data for more exact investigations. The mean amount of the 

 twisting moment upon a ship's hull at any transverse plane of division, 

 and also the maximum twisting moment, may be obtained by the 

 method described in the present paper. The torsional strength of the 

 hull at that section will depend (1) upon the thin iron or steel shell 

 of which the structure consists, being stiffened internally so as to 

 effectually resist change of form ; and (2) upon the ratio which the 

 strength of a section of such form, w r hen so stiffened, bears to that of a 



* " Memoires presentes par divers Savants a l'Aeadeinie des Sciences de 

 l'lnstitut Imperial de France," tome quatorzieme, 1856. " Memoire snr la Torsion 

 de Prismes, &c." Par M. de Saint-Venant," pp. 233-560. Also Thomson and 

 Tait's " Natural Philosophy," vol. i, Part II, sees. 699-710. 



