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Design of Oil Steamers Concluded from April Issue. F this paper had been written five years ago, my remarks on this divis- ion of the subject would have ended with the close of the previous sen- tence. In that time, however, Mr. lsherwood had produced his longi- tudinal designs, and these, particular- ly in regard to oil vessels, have been widely adopted, as will be evident from the fact that not less than 76 per cent of the number of oil ves- sels now building are on the longi- tudinal system. Amongst these are the largest ships that have yet been built for this trade, and Figs. 10 to 22, are interesting as showing not only the general principles of Mr. Isherwood's designs, but the detailed scantlings of a steamer 525 ft. in length, 66 ft. in breadth molded, and =34 ft. in depth molded. It is of the shelter- deck type, and contains 12 oil tanks situated below the upper deck, 10 of these being 22 ft. 6 in. in length, and two 28 ft. Oil fuel is carried: in a tank in'the boiler room (the ma- chinery is placed aft) and in a deep tank forward. The distinguishing fea- tures of the systems are the same here as in ordinary longitudinal cargo ships, and as they are now widely known, I need not go into them in detail. Deep transverses are fitted between the bulkheads, supporting the deck, sides and bottom, while the shell and deck plating is locally stif- fened by the longitudinal girders. These latter are cut at the bulkheads, and are worked in association with the horizontal stiffeners on the bulk- heads, to which they are attached 'bs brackets. One transverse is fitted ain each of the 22-ft. 6-in. tanks, and two in those 28 ft. in length. Comparison of Systems Without venturing on a detailed comparison between the transverse and longitudinal systems as applied to oil vessels, 'I may be permitted to make one or two observations on the latter. The first is that the two characteristics which have been held as essential in the hull construction of an oil ship are secured here also. The girders which actually support the loads on the decks, sides and bottom are de- _"Paper read before the Institution of En- ore and Ship Builders in Scotland on an, : ployed more advantageously signed of a very deep, stiff nature. These girders also and the intermedi- ate bulb angle stiffeners are continu- ous, making possible rigid attach- ments at their ends. Again, it may be pointed out that the theoretical objection which one frequently heard urged against longitudinal vessels when they were first introduced, to the eftect that they did not offer sufficient resistance to racking stress- es, certainly does not apply here. The transverse strength of the ves- sels shown in the drawings repro- duced, made up as it is by bulkheads closely spaced, and webs, is, at least, equal to that obtained in the trans- verse system. Purther, it may be held that the longitudinal system, as a system of construction, can be em- in oil vessels than in the majority of other types of cargo steamers. Compression Forces I think it is not generally realized that, owing to the distribution of weight in a fully-laden tanker, with machinery aft, the maximum _ bend- ing moment occurs: in the sagging condition, in opposition to that found in the case of an ordinary cargo ves- sel with machinery amidships, where the 'maximum is generally found in the hogging condition. The maxi- mum structural stresses, although they may be the same quantitatively, are more severe because reversed in direction, and they represent in oil ships a state of compression in the topside materials. It is well known how difficult it is to provide against compressive forces in the deck plating, and that the only effective provision is the fitting of intercostal girders, which stiffen the deck plating locally be- tween the beams. I need hardly in- sist on the effectiveness of the ghelt- er deck plating to resist compression in the vessels shown in Figs. 10 to 22, in respect of the number and dimensions of its longitudinal sup- ports. I may draw attention to an infer- ence of practical value derived from a consideration of the above. When oil of a high specific gravity is car- ried, it is sometimes unavoidable that one tank should be empty, and since A Review of the Structural Features of Tank Vessels Engaged in Oversea Trade By James Montgomery the maximum stresses are in the sag- ging condition, the empty tank should be situated amidships, and not at the ends. The scantlings of the transverses, longitudinals, etc., are determined, for the most part, on local considerations, A certain condition is assumed, and the same stress (or as near to that as pessible) is worked to in each case for each set: of girders. The condi tion generally assumed is a load rep- resented by a head of water to the upper deck, and under this and other assumed conditions the resultant stresses in the various parts are as shown in Table I, the girder being taken as fixed at the ends, and the effective length being taken to the middle of each bracket. Table I. Item--Load. Stress.* Bottom transverses, Head of water to dpmer deck = 27 ee oe ae 4.25 Side transverses, Head of water to MP PCh GEO ee ee oe ck. cae sees 3.46 Upper-deck transverses, 10 ft. head of VCS se a are webs oss wei os Heuston 3.40 Shelter-deck transverses; As in ordin- aly: "CareO ships. io. kes seek 4.80 Webs on transverse bulkheads; Head tO; upper declke seis ee 5.00 Webs on longitudinal bulkheads; Head HOP Upper. CeCe So. cs sos cs 4.80 Bottom longitudinals; Head to upper OC ee Se ie ears ok ies 2.80 Side longitudinals below second deck; dead sto "upper adeck 6.06.0 2.45 ss 6.00 Upper deck longitudinal; 10 ft. head Gi Wotet eet oo. is a ae 5.40 Longitudinals on transverse bulkheads; edd to. upper deck... fa eee 4.20 *The stress units are inches and _ tons. In connection with the foregoing, seme theoretical objection may be raised to the practice of determining the scantlings of structural parts on local considerations, but it does not appear that such objections are of much weight if the method be ju- diciously applied. It is, in .fact, the only practical method at the present stage. If a collection, of scantlings were at one's disposal, derived from _ the results of experience over a suffi- ciently long period of time, and from a sufficiently large and varied num- ber of cases, it would be easy to frame rules which would take account of all the principal factors. Take, for example, the case of the bottom longitudinals. If one knew the scant- lings which had been proved to be suitable from a number of represent- ative cases, these could be expressed graphically by a curve, the ordinates of which might be the longitudinal number of the vessel and the ab-