70 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



the change in the system of construction, while the shell plating of the transversely 

 framed vessel was increased in thickness to compensate for the omission of the side 

 stringers. Hence no comparison of the strength of the shells need be made, except 

 in so far as they enter into the longitudinal strength of the ships. 



COLLAPSING OF THE SIDE FRAMING. 



In connection with the analysis of the resistance of the two vessels to diagonal 

 distortion, it was shown that the strength of the Isherwood transverses at longitu- 

 dinal No. 14 is represented by 40.17 per foot of length of vessel, as compared to 

 23.55 for the strength of the side frame of the transversely framed vessel at the 

 top of the bilge bracket. The Isherwood transverse at the point considered is about 

 one inch deeper than it is at the longitudinal next above, so that the strength ratio 

 of 70.6 per cent, in favor of the Isherwood vessel, does not quite apply to the 

 strength of the side framing. But owing to the great excess in strength of the 

 Isherwood transverse, we need have no hesitancy in saying that there is a consider- 

 able margin of strength of side framing in favor of the longitudinally framed ship — 

 a margin of, perhaps, about 68 per cent. 



STRENGTH OF DECKS. 



As in the case of the strength of the floors, the difference in the pillar arrange- 

 ment introduces a considerable discrepancy in the deck strength of the transversely 

 and longitudinally framed vessels. In the transversely framed vessel the strength 

 of the decks is obviously dependent on the strength of the deck beams and on the 

 maximum span, which is again the distance between the pillars, or 21 feet. In- 

 cluding, as before, a strip of the deck plating as contributing to the strength of the 

 beam, the section moduli of the deck beams are found to be 12.61 inches^ for the 

 upper, and 15.2 inches' for the second deck beam, or, per foot of ship, 5.81 inches' 

 and 7.01 inches', respectively. Using a fiber stress of 16,000 pounds per square 

 inch and applying the formula for uniformly loaded encastre beams, the upper deck 

 beams are found capable of supporting 8,860 pounds per foot length of ship, or 423 

 pounds per square foot of deck area (a rather high value, since it corresponds to a 

 6.75-foot head of water — more than twice the height of the bulwarks — or a 10- 

 foot deck load of lumber) ; while the safe load for the second deck is 10,700 pounds 

 per foot length of ship, or about 510 pounds per square foot of deck area (again a 

 rather excessive value, since it would allow the carrying of a homogeneous cargo 

 weighing 64.5 pounds per cubic foot, a figure considerably above what the buoy- 

 ancy of the vessel could support). 



It will be noted that the weakest part of the decks of the transversely framed 

 vessel is between the pillars, that is, between the hatches. In the Isherwood ship the 

 load on the decks between the hatches is supported by the deck longitudinals, from 

 which the stresses are transmitted to the bulkhead at the middle of the longitudinal 

 span between the hatches and to the hatch end transverses at the ends of that span. 



