64 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



Square inches. 



Gross shell plate area = 28 x .66 18.48 



Less rivet holes, which for 4^^ diameter spacing = 2/9 gross area. 4.10 



Net plate area 14-38 



Effective frame area 4.00 



Total area in shear 18-38 



This corresponds to 8.47 square inches per foot length of ship for one side, or 

 16.94 square inches for both sides of the hull. The equivalent web thickness would 

 then be 1.41 square inches. 



In the Isherwood ship the case is quite different, however. We have, as be- 

 fore, the area of the shell plate, which amounts to 5.6 square inches per foot of 

 ship, for one side, after deducting for the riveting. But to this area we cannot add 

 the strength of a transverse frame. The angle connecting the transverse web frame 

 to the shell is cut at every longitudinal, and the web plate itself is deeply notched, 

 giving sufficient elasticity in the transverse members to make them useless in longi- 

 tudinal shear. It is true that some frame rivets between the edge lap and the longi- 

 tudinal would have to be sheared. If, in our case, we take the shell lap between H 

 and G strakes as the plane of longitudinal shear, four or six rivets connecting the 

 transverse clip to the shell between that lap and longitudinal number 16 would add 

 to the longitudinal shearing strength. But with transverses spaced 12 feet apart, 

 this becomes a negligible quantity and we can use the value found above, which 

 gives an equivalent web thickness of 0.932 inch as the net steel effective in longitu- 

 dinal shearing. 



Using these values for t in the formula given above, 466 tons as the vertical or 

 transverse shear and the proper respective values for the static moments and mo- 

 ments of inertia, we find that the longitudinal shear for the transverse vessel 

 amounts to 2,570 pounds per square inch, and for the Isherwood vessel to 3,920 

 pounds per square inch, which shows an increase in fiber stress of over 50 per cent. 

 In either case the stress is small, however, and the reduction in strength should not 

 be regarded as endangering the safety of the Isherwood vessel. 



TRANSVERSE STRENGTH. 



Although not as important as the longitudinal stresses, the transverse stresses 

 a vessel is subjected to are often quite severe, and it is consequently essential that 

 a change in the system of construction does not unduly weaken the vessel trans- 

 versely any more than a weakening longitudinally would be permitted. For ex- 

 ample, if data based on long experience seem to indicate that a certain bilge con- 

 nection furnishes but a reasonable margin of strength, we cannot arbitrarily sub- 

 stitute another connection of less strength without adequately compensating for the 

 loss. By doing so we should expose the vessel to serious straining when, especially 



