OF SHIP CONSTRUCTION. 69 



ing hole, the line of fracture passing through the notches for both the tank top and 

 bottom shell longitudinals. The modulusfor this section is 303.2 inches^ for each 

 transverse floor and for the intermediate floors, or 50.5 inches" per foot of ship. 



Regarding the floor again as an encastre beam, whose span in this case is 20 

 feet (allowing a reasonable overlap with the side transverse bracket), we have — 



16,000 X 50.5 X 8 , , . ^ , . 



P = = 26,900 pounds per foot of ship. 



20 /\ 1 2 



It will be seen that the strength of the Isherwood ship falls far short of the 

 strength of the -transversely framed one as far as the collapsing of the bottom is 

 concerned. It should be noted, however, that the center-line arrangement of the pil- 

 lars in the Isherwood ship gives that ship an advantage when resting on keel 

 blocks. But this pillar arrangement is not a characteristic of Isherwood ships in 

 general, since many of them have two rows of pillars, and would then show a weak- 

 ness as compared with the transverse construction that is roughly represented by 

 the results obtained above. In either case this strength represents resistance against 

 extraordinary external forces and is not to be regarded as representing conditions 

 met with in ordinary service. The stress on the bottom of the ship, using the load 

 draught of 23 feet 6 inches, but not making any allowance for the balancing effect, 

 of the cargo, would set up a fiber stress of about 5,800 pounds per square inch in 

 the floor of the Isherwood ship, showing that for sea conditions the Isherwood con- 

 struction furnishes a large margin of strength. 



It should further be remarked that Lloyd's rules permit the omission, in trans- 

 versely framed vessels, of alternate floor plates, except forward of the three-fifths 

 length and under the engines and boilers, at which parts Isherwood vessels have ad- 

 ditional floors. In cases where alternate floor plates are omitted, only the bottom 

 frame bars and the reverse frames are fitted, and even the reverse frames may, un- 

 der certain conditions, be omitted. A vessel so constructed would show very nearly 

 the same strength of bottom as the Isherwood vessel, namely, a maximum safe load 

 of 28,500 pounds and 26,900 pounds for the two types respectively per foot length 

 of ship. 



In view of the above figures it may, then, be said that even though the Isher- 

 wood vessel does not possess the same strength of bottom, per foot length of ship, 

 as a transversely framed vessel of the same size and type, built with floors at every 

 frame, the strength developed by the Isherwood construction is ample, and the con- 

 struction is consequently more logical. 



THE SHELL PLATING. 



On the midship sections (Plates 32 and 33) the shell plating is seen to be lighter 

 for the Isherwood vessel. The fact is, however, that the thickness of the shell 

 plating of the Isherwood vessel is in accordance with Lloyd's rules, regardless of 



