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ER eh eR en NE April, 1913 length of tank is about 34 ft. On the other hand, the average length of tank in all oil ships classed in Lloyds Register prior to 1908 is considerably. less than that, being about 26 ft. 6 in. only, "dm a matter of this kind experience speaks with final and authoritative voice. Its ver- dict is that damage in oil ships, and particularly repeated damage, has been so intimately associated with the fitting of long tanks as to justify the conclusion that the presence of these long tanks constituted a contributory cause in the production of damage. This point was fully considered when the rules for oil vessels were approved by Lloyds Register, and in these rules the maximum length of tank permitted is 28 ft. Expansion of Oil Cargo The provision for expansion in the oil cargo, due to temperature changes, was made, in the earlier steamers, by fitting trunked hatchways, the volume of these trunks being a certain per- centage of the volume of the tank to which they gave access. The co- efficient of expansion of petroleum varies considerably with the density, but generally the volume of each trunk was from 3 to 4 per cent of the volume of each tank. The continuous expansion trunk now provided (see Figs, 4: t0).9) 4s of a ccapacity tar om excess of that required to permit of free dilation of contraction. of the liquid, and is indeed less an expansion- trunk than a means of increasing the carrying capacity of the vessel. Its capacity, however, only concerns us in relation to the necessity for limit- ing the mass of free oil in the expan- sion-trunk in order, as far as possible, to avoid structural damage. It is in the expansion-trunk only that we have a free oil surface, and in order that the impulsive forces incident upon the upper boundary surfaces of the tank may be minimized, it is desirable that the width of the trunk should not ex- ceed six-tenths the width of the vessel. When oil is carried in the wings be- tween the upper and second decks, it is still necessary to make the same kind of provision as was formerly ar- ranged for the main tanks, and it is a common practice to make the volume of the hatchways giving access to the "Summer tanks," as they are called, 3 per cent of the volume of the latter. The expansion-trunk is placed in the center of the vessel. In some re- cent vessels which it was desired to adapt for carrying oil in bulk, it was suggested that the expansion-trunk might well be fitted at the sides, in- stead of in the center, in order. to avoid the difficulty and expense con- THE MARINE REVIEW sequent upon the work of making the second deck oil-tight at the sides of the ship. To secure oil-tightness there it is necessary to cut the large bulb- angle frames, bracket them above the second deck, to fit a continuous doub- ling plate to cover up the slots in the deck, and a continuous double-riveted stringer angle. The placing of the ex- pansion-trunk at the sides avoids all this, and in more than one instance the suggestion was acceptable. If the matter be further considered, however, it does not appear to be an undesir- able change to make even in the case of a new vessel (see Fig. 3). The structural arrangements present no difficulty. The transverse strength can be efficiently maintained at the sides of the vessel by some such arrange- ment as that shown in Fig. 3, the sec- ond deck being now oil-tight in the center instead of at the sides. The middle line bulkhead would stop at the second deck, the 'tween deck por- tion being omitted--an economy of material and labor. The 'tween deck central space is well adapted for gen- eral cargo, and access can be had to it by central hatchways of sufficient size. The summer tank-spaces in the ordinary design, when not filled with oil, are difficult of ventilation, but in Fig. 3, only one such space exists, and the construction in it is not so cellular as it is in the wing summer tanks--a_ feature which facilitates the dispersal of the heavy gases which tend to accumulate there. to this matter, although the facts are as stated above, the author has learned that this suggestion was made fany "years aco by. 5." Oe Ken- dall, now a surveyor to Lloyds Reg- ister, and adopted in one or two vessels. There appears to be no suf- ficient reason, however, which would explain why it is not being adopted on a more extensive scale. Systems of Construction Figs. 4 to 9 show the midship sec- tion profile and deck plans of a typi- cal oil-steamer framed on the trans- verse system. The dimensions are: Length between perpendiculars, 370 ft.; breadth, molded, 50 ft. 9 in; and depth, molded to upper deck 29 ft. 14 in. The type of framing em- ployed is that known as the web- frame system, and with very few ex- ceptions, and these mostly small ves- sels, it has been uniformly employed in transverse ships. It is, in its de- tails, well known, and it is not nec- essary to do more than point out that the side of the vessel is. sup- ported by the web frames and side stringers (the intermediate frames In regard - 129 supporting the plating locally), the bottom is supported by floors and keelsons, and the bulkheads, trans- verse and longitudinal, by plate webs and channel girders. The webs on the side of the ship are in the same transverse plane as the webs on the longitudinal bulkhead, and both be- come efficient elements in the trans- verse strength of the ship in respect of their being joined at the level of the second deck by specially strong beams. The stringers on the ship's side are in the same plane as the horizontal girders on the bulkheads, and form a rigid horizontal section extending round the walls of the oil compartments. The keelsons come in line with the webs on the trans- verse bulkheads, and the construction in other respects is clearly shown in the drawings reproduced. Rigidity of Girders It has just been said that the side of the vessel is supported by web- -frames and_ side-stringers, and the bottom by floors and Keelsons, and this type of ship is the only one of which so unqualified a statement can be made. Of an ordinary cargo ves- sel one would require to say that the side was supported by the web- frames only, and the bottom by floors only. The difference is due, of course, to the positions of the bulkheads in the two cases. In a transverse oil-ship the close spacing of the bulkheads converts the side- : stringers and keelsons on the sides and bottom into supports almost as efficient as the web-frames and floors against external loads. Thus, assum- .ing that the same scantlings of the vessel shown in. Figs. 4 to 6 are worked into an ordinary cargo ship of the same size, a simple investiga- tion will show that the strength of the side in the former case will be twice that in the latter, while the rigidity also is twice as great. The efficiency of these longitudinal gird- 'ers, in this connection, depends on their end connections and on the spacing of the water-tight bulkheads. If the bulkheads are widely spaced, then for the same _ scantlings the strength and rigidity of the sides and bottom are materially reduced. It has been seen that long tanks lead to increased impulsive forces on their boundary surfaces, but it is no less true that for the same material they also imply diminished capacity to re- sist them. The author would here point out another distinguishing feature of this system--that is to say, the very great rigidity it is possible to obtain in the