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

24. FEET.

FIG. 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

the same frame space than two passing strakes. With plates as

now worked, 20 ft. in length and over, there is no difficulty in considerably exceeding this condition. 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. (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. 4 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 -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. (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. (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

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. (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