78 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



of deck, both the upper and second decks being affected. The one-seventh deduction from 

 tension material was allowed throughout this series in exactly the same manner as before. 



The hatch deduction reduced the section moduli for the transversely framed vessel to 

 129,400 for the hogging section and 143,600 for sagging. The comparative figures for the 

 Isherwood vessel, full effectiveness being allowed for framing, were 128,500 and 142,400, 

 slightly under those for the transverse ship. As before, percentage allowances were then 

 made for the framing, and a series of section moduli deduced. The results of both series 

 of calculations I have correlated in a table, and have also plotted a set of curves of section 

 moduli on a percentage base, the percentage referring to the proportion of longitudinal 

 frames included in strength. The table referred to will be found on page 79 and the 

 curves on Plate 38. 



There are four section moduli plotted for the transversely framed ship. These appear 

 as straight lines parallel to the base. In order, they are No. I, the sagging modulus for sec- 

 tion with no deductions made for deck openings ; No. II, the hogging modulus with no de- 

 ductions; No. Ill, the sagging modulus computed with hatches deducted; and No. IV, the 

 hogging modulus with hatches deducted. 



For the Isherwood ship there are eight curves. These, too, appear to be practically 

 straight lines but with varying slopes. Curve V is for the sagging modulus, no deductions 

 being made for hatch openings and percentage deductions being made for framing in tension 

 only. This curve lies wholly above curve I, indicating that if the assumptions upon which it 

 is based were valid, the Isherwood ship would be better able to resist sagging than the trans- 

 versely framed vessel, regardless of the efficiency of frame brackets of members in tension. 

 Curve VI is for the Isherwood ship sagging, the percentage allowance being applied to all 

 frames whether in tension or compression. It crosses curve I at about 71,^/2 per cent, indi- 

 cating that if hatch openings are suitably compensated for and if all longitudinal framing is 

 better than 71 J/2 per cent efficient, then the Isherwood ship would be, under these circum- 

 stances, the better able to resist sagging. 



Curves VII and VIII are for the hogging moduli under similar assumptions. They cross 

 curve II, the corresponding curve for the transverse ship, at 65 per cent and 71J4 per cent, 

 respectively. 



Coming now to curves IX to XII, for which the hatch openings have been deducted, 

 they are found to lie wholly below the corresponding curves III and IV for the transversely 

 framed ship. This would indicate the Isherwood ship as built to be inferior in longitudinal 

 strength to her sister framed on the transverse system, provided, however, that if the longi- 

 tudinal framing is actually 100 per cent efficient in both tension and compression, the mod- 

 uli resulting approach so nearly to those of the transverse ship that corrections of slight 

 inaccuracies in the curves might easily make the longitudinally framed ship equal or slightly 

 stronger than the other. 



In any event, the curves cast doiibt upon the claims of the Isherwood ship to superior 

 longitudinal strength, and if it were found necessary to add material to make it equally strong, 

 the margin in weight saving would be lessened and might be caused to disappear. 



Mr. J. W. Stewart, Member (Communicated) : — I have read with interest the paper 

 given by Mr. John Flodin, namely, "Analysis of the Isherwood System of Ship Construction." 



This paper is of interest to me, particularly so as Mr. Flodin has developed it entirely 

 from the merits of the case and has not been in collusion with me on his most clear and in- 

 teresting treatise of the various features of comparison between the two systems. 



