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the same frame space than two passing strakes. With plates as now worked, 20 ft in length and over, considerably exceeding this condition.

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Fig. 46a.—Special form of bulkhead liner.

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there is no difficulty in Thus, in Fig. 45, 24-ft. plates are worked and five passing strakes are obtained. The same principle has to be borne in mind when arranging the butts of the vertical keel and longitudinals, inner bottom, etc., so as to avoid any weakness of the structure in any one frame space (see Fig. 46).

Plating of a Battleship.—The outer bottom plating of a battle-ship is made 25 lbs. (| in.) over the length of the double bottom, and 20 lbs. (i in.) at the ends. The fore-and-aft rows of plating, called strakes, are riveted to one another by double riveted laps. The plating is worked on the raised and sunken system as shown in Fig. 47. At every frame in way of each outside strake a filling piece or liner is necessary to get good riveting between the frame and the plate. These liners are often lightened, as shown, by oval holes between the rivets. At the middle line an additional outside plate, 30 lbs. (|-in.), is worked, called the outer flat keel, the inner plate being the inner flat keel (see Fig. 3).

Riveting in Outer Bottom.—Fig. 46 shows in some detail the arrangement of the riveting, etc., in the outer bottom plating near the middle line. The lower angles of the vertical keel are connected to the flat keels by 1-in. rivets, and as the vertical keel has to be watertight, these rivets on one side are closely spaced, viz. 4A to 5 diameters. On the other side a wider spacing, viz. 8 diameters, is all that is necessary. Close spacing is necessary for the watertight longitudinal, No. 2, and wide spacing for the non-watertight longitudinal, No. 1. The butt-straps of the inner and outer flat keel are treble riveted with 1-in. rivets, and extend from the keel angles to the edges of the plate concerned. The edge of the inner keel is connected to the outer keel by a single row of rivets, to get a good connection and to allow the edge to be caulked. The edge riveting of the outer flat keel is double riveted with 1-in. rivets. The remainder of the riveting is \ in. diameter, as shown. The butt-straps are double riveted, and for the outside strakes extend from the longitudinals to the edge of the inside strakes. For the inside strake the strap extends the whole width of the plate.

The ordinary frames are connected to the outer bottom by J-in. rivets spaced 7 to 8 diameters. For the watertight frames it is necessary to have the rivets closely spaced, viz. 4^ to 5 diameters. This close riveting cuts away a lot of material from the outer bottom plating in one transverse section of the ship, and causes this section to be a distinct line of weakness. In order to compensate for this, a wide liner called a bulkhead liner is fitted to each outside strake instead of the ordinary liner. This wide liner forms a sort of strap over the weak place, and in this way the strength at the watertight frame can be brought up to an equality with that at an ordinary frame. In the case of the second strake from the keel in Fig. 46, it will be noticed that the watertight frame comes next to a butt-strap, and the strap is made wide enough to act as a bulkhead liner and butt-strap combined.

Fig. 46a shows a form of bulkhead liner adopted in some ships to economize weight. The outside strake is only pierced at every other rivet, so that the reduction of strength is not so great at the section, and the liner can be smaller than would otherwise be necessary.

The side plating above the protective deck is recessed back from the side of the ship to make room for the armour and backing (see Fig. 18). This plating is in two thicknesses, each of 20 lbs. (J in.), for 9-in. armour. No butt-straps or edge strips are necessary for this plating, as each thickness acts as security to the edges and butts of the other thickness. Above the armour the plating is 20 lbs. (J in.) to the upper deck, except where an armoured battery is worked between the main and upper decks, as Fig. 13, in which case the double thickness of plating is carried right up.

The side plating is doubled in way of any protective plating to form a flush surface. Thicker or doubling plates are also worked in way of the stem and sternpost, in way of hawse-pipes, and where necessary to take the chafe of anchors and cables, and in other places where special local strength is required, as in wake of torpedo-tubes. Covering plates are worked at the lower and upper edges of the side armour, as Figs. 18 and 34, connected to the armour by tap rivets. The edges of these plates are caulked so as to make the joint at the armour watertight.

In some battle-ships and cruisers the topside above the upper deck is worked to the boat deck, forming a closed-in superstructure (see Fig. 22). This plating is not intended to take any structural strains, and the sides are severed by the gangway ports. The

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plating of the boat deck is also severed, and a sliding joint is made as shown in Fig. 5.

Inner Bottom.—The inner bottom of a battle-ship is generally 15 lbs. (f in.), with the middle line strake 20 lbs. (\ in.). This plating extends to the fourth longitudinal, and the inner skin is continued to the protective deck by means of the wing bulkhead, which is 15 lbs. It is most important that convenient access should be provided to all the watertight compartments of the double bottom, in order that the condition of the plating, etc., may be periodically examined. The best arrangement would be to have the two manholes at opposite corners of the compartment, so that the space might be well ventilated before entering. Owing, however, to the obstructions caused by the boiler bearers, etc., this is not generally possible; an actual arrangement adopted in one ship is shown in Fig. 49. The manholes should be on raised coamings, with a hinged cover, secured by a number of butterfly nuts as shown in Fig. 48. In some cases a hinged cover cannot be obtained, owing to some obstruction, and in these cases the cover is connected to the coaming by a chain. A plug is fitted to the manhole cover to enable the compartment to be sounded when necessary. This plug is hollow, as shown in Fig. 48, with a small vent-hole at the side. When the plug is slightly unscrewed, this hole allows the air to escape when flooding, and will also show when the compartment is full. The plug can then readily be screwed up tight. It is very necessary that the air should be allowed to escape so that the tank may be completely filled. At

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Fig. 49.—Manholes in inner bottom. Fio. 50.—Watertight manhole on

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other places, as on watertight flats or bulkheads, where occasional access through is necessary for inspection, the edge of the hole is stiffened by a strip, and a portable plate is secured by studs as shown in Fig. 50.

We have already noticed that although the inner bottom proper ends about one-sixth the length from each end, yet a virtual inner bottom is obtained well to the ends by the watertight flats, etc.

These double bottom spaces are carefully tested for watertightness by filling with water, the pipe conveying the water giving a head of about 5 ft. above the L.W.L. If the test is not satisfactory, the defects have to be made good and the compartment retested.

Outer and Inner Bottom Plating of a First Class Cruiser.

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