68 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



FAILURE OF BOTTOM OF SHIP THROUGH COLLAPSING. 



In the case of a failure of the type shown by Fig. 3, Plate 37, which might 

 occur when the weight of the vessel is resting on keel blocks along the center line, or 

 on the ground, as in the case of stranding, a considerable discrepancy exists be- 

 tween the transversely and longitudinally framed vessels. In the former we have 

 two rows of pillars, giving a transverse span of 21 feet between the pillars, while 

 in the latter case there is but a single row of pillars along the center line of the 

 vessel. In the former vessel the stressing of the bottom to failure through collap- 

 sing would, then, mean the failure of the floors at or near the center line, while in 

 the latter the most dangerous point of loading would be at some point about half- 

 way between the center line and the support at the margin, and the failure would 

 then occur near the point of loading. 



For the transversely framed vessel we find the section modulus to be 243.8 

 inches' for the floor connection at the center line (the line of fracture is supposed 

 to pass through the riveting of the floor clips to the center vertical keel, past the top 

 and bottom horizontal keel angles, and through the flat keel and rider plates) and 

 283.3 inches^ for the floor through the first lightening hole from the center line of 

 the ship. In each case a strip of the shell plating equal to two and one-half times 

 the horizontal flange of the connecting angle is included, and also a strip of the 

 tank top plating of similar width less one riA^et diameter, the tank top plating being 

 in tension, while the shell plating is in compression. 



The least section modulus is, then, 112.2 inches' per foot length of the ship. 

 Regarding the floor as an encastre beam,* and taking the allowable fiber stress as 

 16,000 pounds per square inch, we may find the safe concentrated load by the for- 

 mula — 



M= P' 



8 ' 



where M is the bending moment in inch-pounds, P is the load in pounds, and / is 

 the length of the span in inches. 



Equating this to the resisting moment we have — 



„ 16,000 X 1 12.2 X 8 J r .L X u- 



P = — = 57,000 pounds per foot of ship. 



The area of a floor section through a lightening hole near the pillar is 22.69 

 square inches, or about 10.5 square inches per foot of ship, giving a maximum shear- 

 ing stress of ^ X 57,000/10.5, or about 3,000 pounds per square inch. 



In the Isherwood ship, the failure would undoubtedly occur through a lighten- 



*It would be more reasonable to consider the floor a continuous beam, the assumption here made not being 

 strictly correct, but since this error of conception will be used for the Isherwood ship also, the final result 

 will be a very close approximation to the true relation in the strength of the two vessels. 



