OF SHIP CONSTRUCTION. 61 



been made for rivet heads or liners. The last two items should be very nearly the 

 same for both types of ships, so that our comparison will not suffer from the omis- 

 sion of them. 



The saving in net steel amounts, then, to 142 tons, or slightly over 7 per cent, 

 certainly a saving well worth while provided it is not accompanied by a reduction 

 in strength. This, however, does not represent the increase in cargo-carrying ca- 

 pacity, for the reduction in weight on the basis of the completed ship, including all 

 machinery, equipment, etc., equals only about 4.65 per cent, and the increase in the 

 deadweight carrying capacity is about 1.9 per cent. To the shipowner this last 

 figure is the only one of any great significance, for since it may be assumed that 

 the custom-house measurements — that is to say, the taxes levied upon the vessel — 

 are not affected by the change in the system of construction, the increase in the 

 deadweight capacity equals the increase in the earning capacity in all cases where 

 the weight, rather than the volume, is the important factor, unless, of course, the 

 cost of operating the Isherwood vessel should rise because of the need of more fre- 

 quent or more costly repairs. 



The increase in the earning capacity is, in fact, the principal claim for favor 

 made by the supporters of the Isherwood system, coupled, as it is, with the claim 

 that the grain and bale capacities are also increased. The former contention is un- 

 doubtedly true, the improvement being due partly to the fact that the saving in 

 steel means that a smaller volume is occupied by the framing, but mainly to the 

 greater facility with which the inner bottom ceiling may be run out horizontally 

 to the shell plating of the Isherwood ship, instead of being run up along the faces 

 of the margin brackets, as is commonly done in transversely framed vessels. The 

 bale capacity (i. e., the actual, "as loaded" bale capacity, not the calculated), on 

 the other hand, is naturally somewhat impaired because the transverses break into 

 the cargo holds at intervals that are but rarely a multiple of any of the three di- 

 mensions of the bales. In case of ordinary bale goods this may not be a serious 

 matter, but in case an Isherwood ship were to carry lumber it would be necessary 

 to load "short stuff" (that is, lath, shingles, or bundled flooring or ceiling), which 

 means a mixed cargo where a straight cargo might be more easily obtainable, not 

 to mention the greater stevedoring charges. 



In this connection attention is called to the fact that practically all oil tankers 

 are now being built to the Isherwood system, the principal reason being that this 

 system lends itself more readily to the peculiar bulkhead and stiffening work neces- 

 sary in that type of vessel, and only secondarily to the saving in steel weight. The 

 increase in the cubic capacity has, of course, no influence on the choice of system of 

 framing for a tanker, since the deadweight capacity, not the cubic capacity, is the 

 significant factor. 



LONGITUDINAL BENDING STRENGTH. 



As a criterion of the longitudinal bending strength, we might simply find and 

 compare the section moduli of the two vessels. The matter was, however, gone 



