OF SHIP CONSTRUCTION. 77 



system of framing. On the other hand, it seems obvious that proof should be submitted of 

 the efficiency of bracketed connections between longitudinal frames and bulkheads before 

 full value can be allowed for the scantlings of longitudinal frames. In this connection it 

 should be noted that a very slight yielding of the bracket connection (about .0025 inch) would 

 result in a transmission of stress from the frame to the shell, and it is especially to be noted 

 that this transference of stress would occur long before the metal reached its elastic limit. 

 In fact, unless it can be proven that no infinitesimal slipping occurs in bracket riveting, and, 

 in addition, that the modulus of elasticity of the longitudinal through the connection is at least 

 equal to the modulus of the iminterrupted metal, the assumption of similar premises fails. 



This statement of facts seems perfectly obvious, yet, in so far as I know, adherents of 

 the Isherwood system of framing have not publicly submitted data bearing on this impor- 

 tant feature of that systeiTii of design. In laying out shell and deck plating, we are accus- 

 tomed to break joints, but under the Isherwood system all longitudinal frames are inter- 

 rupted at the same girth line, and that, too, coincident with the line of closely spaced rivet 

 holes incident to bulkhead installation. Apparently here is a line of weakness where com- 

 pensation must be provided, and, as heretofore stated, the sufficiency of such compensation 

 should be demonstrated. 



An an indication of the relative importance of the points here raised, the following 

 is submitted. The section moduli of the ships under discussion have been given as 



Hogging Sagging 



condition. condition. 



Transversely framed vessel 161,900 178,100 



Isherwood vessel 170,300 189,700 



Information was not given as to the exact material assumed in strength. Indepen- 

 dent calculations, however, give results in very close agreement with the foregoing if the 

 longitudinal frames of the Isherwood ship are included in strength and if for both ships 

 the decks all athwartship are considered effective, and if one-seventh of the tension ma- 

 terial is deducted for rivet holes. For the transversely framed ship intercostal longitudi- 

 nals are ignored. 



Computations have also been made deducing the section moduli for the Isherwood ship, 

 allowing 90, 75, 50 and per cent effectiveness for the longitudinal framing, on the tension 

 side, the above percentages being applied after the one-seventh deduction for rivet holes. 



Following this and acting upon the assumption that the longitudinal frames might 

 properly be given 100 per cent effectiveness in compression, section moduli were computed, 

 allowing full weight for compression frames and 90, 75, 50 and per cent effectiveness for 

 the longitudinal frames in tension, with rivet-hole deduction as before. 



In all of the foregoing computations, deck plating and longitudinals underneath the 

 deck were included all athwartship. The section computed is located just abaft the break 

 of the bridge, and inspection of the deck plans reveal that only a very short space intervenes 

 between the section computed and a deck opening,» 21 feet in width. Obviously neither the 

 deck plating nor the longitudinals are able to transmit stress through the open hatch, nor 

 does the slight increase of .06 inch in thickness of the passing strake compensate for this 

 loss. 



A second set of computations was therefore made, eliminating entirely a 21-foot breadth 



