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flat is formed of 15-lb. galvanized ribbed plating. In a large ship the flats of the submerged torpedo-rooms, auxiliary machinery compartment, etc., are fitted in this way.

Watertightness of Flats, etc.—In watertight platforms, etc., the connection of the plating with the ship's side must be made watertight. Fig. 41 shows two methods of doing this, one when the zed bar frame passes through, and the other when a simple angle bar passes through. In either case angle bars are smithed round to cover the holes completely; the riveting is closely spaced and the whole carefully caulked and made watertight. The pressure such a flat, 20 ft. below water, would have to stand if the compartment below were bilged amounts to about 60 tons per

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100 square ft., so that substantial work is seen to be very necessary. There is no difficulty in securing a watertight connection with the side at the main or middle decks of an armoured ship, because the transverse framing is not continuous through the deck, and a foreand-aft angle can be run along and caulked (Fig. 34).

Wood Decks.—The use of wood decks has been very much reduced in recent years in vessels of the Royal Navy, on account of the probability of the wood catching fire in action. Wood is now only used for weather decks, in the Admiral's apartments and the ward room, in casemates and ammunition passages, and for magazine flats. In the Admiral's apartments and ward room the flat is of Dantzic fir, 2 in. thick. In the other cases the flat is of teak, on account of the suitability of this timber to stand heavy wear and tear. For the weather decks the thickness is generally 3 in., with thicker planks called waterways round the edges of the decks, forward and aft, and round barbettes, etc. Thicker planks are worked in way of the rub of chain cables. In casemates the teak is 2 in., and in ammunition passages

14 in.

When a steel deck is laid, the wood deck is fastened by

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Fio. 42a.

through bolts to the plating between the beams. To decks which have to stand severe compressive strains, the connection of the wood deck to the steel deck is made a very efficient one, so that the wood and the steel may act together in resisting the buckling (see Fig. 42a).

When, however, as is sometimes the case, the wood deck has

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to be laid direct on to the beams, the bolts must be taken through the upper flange of the beam. For this reason, when a wood deck is thus worked, it is desirable to make the beam of tee bulb, so that the bolts may be placed zigzag, as in Fig. 42b, and not in one continuous line, as would be necessary with an angle bulb beam. When thus laid direct on the beam the bolts of the butt of deck plank would be too close if taken through the flange of the beam, and at each butt, therefore, a short piece of plate, having the width of the plank, is fitted to take the bolts (Figs. 42b and 43).

Planking is fastened to the steel deck or the beams by means of galvanized iron bolts with round heads, as Fig. 44. The heads are let in well below the surface of the deck, and the cavity is filled in with a wood plug called a dowel, well steeped in white lead. A hempen grummet is placed under the head, and the nut underneath has a plate washer and grummet (Fig. 43). All this is necessary to make the hole in the deck properly tight. It will be noticed that the neck of the bolt is square to prevent the bolt turning when the nut is being screwed up.

The edges and butts of deck planking are caulked to make

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Fig. 44. the deck watertight. The edges and butts are left with a slight opening at the surface, as Fig. 43, and into this oakum is forced, filling the opening right down to the bottom of the seam. The top is then payed with pitch. When carefully done this caulking should make the deck properly tight.

Corticine.—In the living spaces of the ship other than those mentioned above, no wood decks are fitted, but corticine, a thick linoleum, is laid direct on the surface of the steel deck. The steel deck has to be laid flush for this purpose, with edge strips on the under side worked between the beams. The upper deck of torpedoboat destroyers is also laid with corticine.

The corticine is secured to the deck by means of a solution of orange shellac and methylated spirit, and when secured the edges are stopped with putty. Round scuttles and exposed edges the corticine is secured in addition to the deck by galvanized iron strips screwed into the plating. For thin decks, as in destroyers, these strips are secured to the plating by bolts with a nut below having a washer and grummet. Where heavy wear takes place on a steel deck covered with corticine, as at the foot of ladders, the corticine is covered with plates of ribbed iron.

Watertight Hatches.—The hatches to ordinary Hats and decks are fitted with a raised coaming and a steel cover with an indiarubber joint. This indiarubber should be periodically examined, and where perished should be renewed, as a defective cover may destroy the watertightness of a complete deck. Scuttles and armour gratings to thick decks are dealt with in Chapter XIII. CHAPTER V.

PLATING OF THE OUTER AND INNER BOTTOMS.

Outer Bottom Plating.—The weight of this plating forms a good proportion of the total weight of the hull structure. It is a most important portion of the structure, because it not only contributes largely to the structural strength, but it keeps the ship watertight. One advantage of forming this plating of mild, steel has already been referred to, viz. the advantage of the ductility of steel as compared with iron. Steel ships have frequently grounded without making water, under circumstances in which an iron ship would have been in a serious condition owing to the rupture of the plating.

Shift of Butts.—An important point in connection with the longitudinal structure of any ship is the arrangement of a good

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Fiq. 45.-Shift of butts.

shift of butts. A butt must be a place of relative weakness, and the butts of the various portions of the fore-and-aft structure are arranged well clear of each other. Thus, for the outer bottom plating, it is laid down that butts are not to be closer together in

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