62 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



into somewhat more deeply; stress diagrams for both the hogging and sagging 

 conditions were prepared. These diagrams were worked out to represent stresses 

 that would actually be likely to exist, rather than to conform to the so-called stand- 

 ard conditions. The result was that the stresses, especially for the sagging condi- 

 tion, are somewhat lower than those developed under standard conditions for vessels 

 of this size, the bending moments being 41,200 foot-tons for hogging and 9,800 

 foot-tons for sagging. 



The bending moment is maximum at about the middle of length for both the 

 hogging and sagging conditions. But at this point the ship is strengthened by the 

 long bridge, so that the point of most unfavorable combination of bending and re- 

 sisting moments falls either just forward of the forward end of the bridge, or just 

 abaft the after end of the bridge, at which points the section moduli may be taken 

 as being equal since the scantlings are the same at both points, only the sheer heights 

 changing slightly. The section moduli were, in fact, calculated for the lowest point 

 of sheer, and they may consequently be regarded as being somewhat conservative. 

 On the other hand, in working out these section moduli an important departure from 

 standard practice was made : the main rail and bulwark plating were included as 

 strength members. The reason for this was that in the Isherwood ship the main 

 rail and one of the bridge-space shell longitudinals form a practically continuous 

 member. Since the shell longitudinal certainly is a strength member, there is good 

 reason for including the main rail, and consequently also the bulwark plating, in 

 the section modulus of the Isherwood ship, especially when these members are in 

 tension. And if we decide in favor of the inclusion in the case of the Isherwood 

 ship, the corresponding members must, of course, also be included in the section 

 moduli of the transversely framed vessel, for the sake of retaining a fair basis of 

 comparison. 



The values of the section moduli, in inches to the third power, are: — 



Hogging condition. Sagging condition. 



Transversely framed vessel 161,900 178,100 



Isherwood vessel 170,300 189,700 



In each case a deduction for seven diameter rivet spacing was made from the 

 sectional area of the tension part of the section. 



By applying the flexure formula, the unit stresses, in pounds per square inch, 

 are now found to be : — 



Hogging condition. Sagging condition. 



Transversely framed vessel 6,840 T-yA77 



Isherwood vessel 6,500 Ij392 



It will be seen that the fiber stresses are, roughly, 5 per cent less in the Isher- 

 wood vessel than in the transversely framed vessel. 



This comparison holds good only, of course, when the vessel is in the upright 



