1886.] Straining of Ships caused by Rolling. 23 



actions which, the rolling of a ship brings into play, or of the effect 

 of those straining actions npon the material of which the hull is com- 

 posed. Various writers, from Bouguer in 1746, down to Professor 

 Macquorne Rankine in 1866, and Sir E. J. Reed in 1871, have^ dis- 

 cussed the straining actions that are caused by longitudinal racking 

 and bending when a vessel is floating in statical equilibrium. Sir 

 E. J. Reed elaborately investigated the subject in a paper contained 

 in the "Philosophical Transactions of the Royal Society" for 1871, 

 and gave examples of the amounts and distribution of the stresses 

 ■caused by such straining actions in several typical ships of Her 

 Majesty's Navy. Mr. W. John supplemented this by a paper on the 

 strength of iron ships, read before the Institution of Naval Architects 

 in 1874, in which similar results were given for various classes of 

 vessels in the mercantile marine. 



The later investigations of these longitudinal straining actions 

 apply not only to the case of a ship floating in equilibrium in still 

 water, but also to cases in which she is (1) in instantaneous statical 

 equilibrium across the crest of a wave; and (2) in instantaneous 

 statical equilibrium across the hollow of a wave — the wave-length 

 being equal to the length of the ship. 



Cases frequently occur which show that the maximum stresses of 

 the material of a ship's hull are not in proportion to the results 

 obtained by the ordinary calculations ; and that certain deductions 

 that have been drawn from those results are by no means sound. 

 For instance, it is said to follow from the analogy between the longi- 

 tudinal bending action upon a ship afloat and that upon a loaded 

 girder, that there is little or no stress exerted upon that portion of a 

 ship's plating which is in the vicinity of the neutral axis for the 

 upright position ; and the inference has been drawn that, subject to 

 the consideration of the sides being occasionally brought, in some 

 degree, into the positions of flanges of a girder by large inclinations, 

 the thickness of the material may be decreased with advantage near 

 the neutral axis. Now it cannot be shown that the plating which 

 is in the vicinity of the neutral axis when the ship is upright, is ever 

 brought into such a position by the rolling of a vessel as to be much 

 affected by mere longitudinal bending. 



The reason commonly given for not decreasing considerably the 

 thickness of side plating in the vicinity of the upright neutral axis, 

 viz., that when a ship is in an inclined position, this plating may be 

 so placed as to offer the greatest resistance to longitudinal bending 

 is seen, if the matter be properly considered, to be obviously unsatis- 

 factory. 



Other propositions respecting the relative distribution of stress 

 in various parts of the structure have been deduced from consi- 

 derations and assumptions upon which the ordinary calculations of 



