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—This plating is arranged on the same principles as for a battle-ship, the thickness generally being somewhat less. Special attention, however, is paid to the structure at the keel and at the upper deck. Fig. 3 shows the structure at the keel of a cruiser whose ratio of length to depth is 12 "4. This is shown in comparison with the structure at the keel of a battle-ship of greater displacement whose ratio of length to depth is only 9*4. It will be noticed that the
vertical keel is deeper, and the outer flat keel is 45 lbs. (nearly 1 Jp in.), as against 30 lbs. (J in.) in the battle-ship. The middle strake of the inner bottom is 25 lbs. as against 20 lbs. in the battleship. Fig. 51 shows the sheer strake of this cruiser, i.e. the plate of side next the upper deck. It is of 25-lb. high tensile steel, and the butt-strap is 30 lbs., quadruple riveted.
The inner bottom plating of a large cruiser presents no special features, and the remarks made above apply in this case also. It has already been noticed that the wing bulkhead has been dispensed with in recent ships to give a greater coal capacity. Figs. 52 and 53 show the whole of the watertight subdivision of a large cruiser, in which it will be noticed that an inner skin is obtained well towards the ends by means of the flats, etc., to the magazines and store-rooms.
Plating of a Second Class Cruiser.—The outer bottom plating of the cruiser shown in Fig. 25 is generally 17£ lbs. (/jj in.). The flat keel is 25 lbs. (f in.), and the sheer strake 25 lbs. The inner bottom is 12£ lbs. (^ in.).
Plating of a Third Class Cruiser.—The bottom plating of the
cruiser shown in Fig. 28 is generally 121 lbs. (fg in.). In way of the engine-room, however, it is \1\ lbs. (j"e in.), to stiffen the ship in way of the fast-running machinery. The flat keel and sheer strake, next the upper deck, are both 20 lbs. (J in.). These are both reduced to 17^ lbs. beyond the half length.
Plating of Sloop.—The bottom plating of the sloop shown in Fig. 30 is generally of 10 lbs. (\ in.), with flat keel and sheer strake of 15 lbs. (jj in.).
Plating of Destroyer.—For this type of vessel high tensile steel is used for the outer bottom plating in the more recent ships. The flat keel in Fig. 32 is 36 in. x 10 lbs. (} in.), sheer strake 30 in. x 8J lbs., the remainder of the plating being 6\ lbs.
WATERTIGHT BULKHEADS, DOORS, ETC.
There are four main methods of watertight subdivision, viz. by means of
(i.) A watertight inner bottom with watertight vertical keel, longitudinals, and frames;
(ii.) Watertight decks and flats;(iii.) Transverse bulkheads; and (iv.) Longitudinal bulkheads.
We have already dealt with the first two of these. The valuable feature of a double bottom has to be dispensed with in small vessels on account of the space thus occupied. In all ships, however, we get watertight subdivision from the last three of the above (see Figs. 52 and 54).
We now deal with the bulkheads. These are not only useful, in dividing the ship into a number of watertight compartments, but they form a most valuable addition to the ship's structural strength.
Transverse Bulkheads.—These are watertight partitions which go transversely across the ship. Fig. 52 shows the large number of such bulkheads fitted in a large cruiser; Fig. 54 is for a small cruiser. The one nearest the stem, extending to the upper deck, is the collision bulkhead, and many instances have occurred, especially in merchant vessels, in which, after collision, this bulkhead has remained intact and saved the ship from possible foundering. On account of its importance it is well stiffened, and in recent ships no openings of any kind are allowed in it. Any access required to the forward side must be by means of scuttles through the decks, and if the forward space does require draining, it must be pumped out by means of a hose.
In some recent battle-ships an additional bulkhead is fitted 3 ft. abaft the collision bulkhead. This is termed the " cofferdam"