they may either be lapped or butted. Laps are shown at F and G in Fig. 10. For thin plating a single row of rivets is sufficient, as F; for thicker plates a double row of rivets is necessary, as G. In the first case, the breadth of the lap, F, is three and a half times the diameter of the rivet used; in the second case, G, it is six times the diameter of rivet. This gives a clearance between the rows of 1 diameters, and rather more than a diameter clear of the edge. The edges of plates are usually lapped, but in some cases, where a flush surface is necessary, the edges are connected by an edge strip. A special form of edge connection is seen when the

edge strip takes the form of a tee bar. Here the tee bar forms an edge strip and stiffener at the same time (see n, Fig. 8). When the end connection of plates has to be flush, as is usually the case, the connection is called a butt-strap. Butt-straps are single, double, treble, or quadruple riveted, according to the importance of the connection. These are shown by A, B, C, and D, Fig. 10, the breadths being respectively 61, 111, 161, and 21 times the diameter of the rivet used. Quadruple riveted straps are used for specially important joints (see Fig. 51 for one instance). In some cases butt-straps are made double, i.e. in two halves on either side.

Angle bars, etc., are connected together by a piece of angle,

fitted as shown in Fig. 14, long enough to take two or three rivets each side as necessary.

The breadths given above for butt-straps and laps are somewhat exceeded when dealing with high tensile steel. In this case a clearance of 1 diameters from the edge is considered necessary.

Spacing of Rivets.-The spacing of rivets from centre to centre along edges, butts, etc., is termed the pitch. This pitch varies according as it is necessary to have the joint watertight or not. For the former case the joint has to be caulked, and in order to do this it is necessary to have the rivets closely spaced to draw the work tightly together. The usual pitch for watertight work is from 4 to 5 diameters. Rather closer spacing, 3 to 4 diameters, is necessary for oiltight work. For non-watertight work a pitch of

7 to 8 diameters is all that is necessary.

Caulking.-All caulking should be metal to metal, filling pieces being avoided as far as possible. For laps the caulking edge must be made square (planed for important parts, as the outer bottom plating). The edge near the joint is nicked with a sharp tool and the piece so left is driven against the adjacent plate, as in Fig. 10. For butts the edges must be planed; a split is made either side of the joint and the two edges are forced together with a hollow tool, giving the shape to the butt, as in Fig. 10. Butt caulking is not so efficient as lap caulking, because a pull on the joint, or in-and-out working of the plating, is more liable to open the caulk in the former than in the latter case.

An interesting and very efficient form of butt-strap, shown at E, Fig. 10, is being adopted in some destroyers. The strap is double; the inside portion is treble riveted, and the outside portion double riveted. The alternate rivets are omitted in the last row, so that the plate is not weakened more than at the adjacent frame. The middle row is closely spaced to allow the edge to be lap caulked. If fracture of the plate occurred through this line of rivets it would be necessary to shear also all the rivets in the last row. The rivets in the two inner rows are in double shear.

CHAPTER III.

FRAMING OF VARIOUS TYPES OF SHIPS.

THE British Navy, intended to operate in all parts of the world, is necessarily made up of many different types of vessel. It would be obviously impossible in a work of this character to exhaustively

FIG. 11.-Section of H.M.S. Royal Sovereign.

describe the construction of each type of vessel in the Navy. All that is possible, or desirable, is to take certain main types, and deal with the principles of their construction. It will be seen that

the intended service of a ship has a distinct influence on the construction.

First Class Battle-ships.-These ships are heavily armoured and armed, and possess speed which is moderate compared with that obtained by

cruisers. One distinctive feature of the construction of these ships is the provision of an inner skin up to the protective deck. Sections of recent battle-ships are shown in Figs. 12 and 13. The inner skin is 31 ft. in at the middle line, the depth being somewhat less up the bilge, and it is continued up to the protective deck in the form of a vertical bulkhead. There is, in addition, the the inner coal-bunker bulkhead, so that at the side

and bilge, at the lines AA and BB, there are three skins to pierce before a vital portion of the ship is reached; at the bottom there are only two skins to pierce. This arrangement is important, in view of the possibility of being rammed or receiving other damage under water.

The double-bottom arrangement is adopted in all ships of the Royal Navy above third class cruisers. A double bottom is valuable as providing an inner skin in the event of damage to the outer bottom, and it is always subdivided into a number of watertight compartments, so as to localize any damage that might occur. A part of the space is conveniently arranged for the stowage of fresh water, forming the boiler reserve feed. Any of the doublebottom compartments, including the wings, can be flooded if desired to correct heel or trim caused by damage (see Chapter IX.).

In the Royal Sovereign the protective deck was level, with a thick belt, 8 ft. broad, as shown in Fig. 11. In the more recent

ships, the armour belt has been much reduced in thickness and increased in area, and the protective deck has been made level at the middle line, but sloping down to the lower edge of armour at the side (see Figs. 12 and 13).

Longitudinal Framing.-The main framing of these ships is arranged on the longitudinal system, the presence of the double bottom lending itself admirably to this arrangement. The foreand-aft framing below protective deck over the length of the double

bottom (about two-thirds the length) consists of a vertical keel, 31 ft. deep, and five longitudinal girders on each side (see Fig. 12). Of these the vertical keel, second and fourth longitudinals are watertight, thus dividing the double bottom from side to side into six watertight compartments. The vertical keel and longitudinals are continuous, and the latter are allowed to taper somewhat in depth towards the ends of the double bottom. The vertical keel is 25 lbs. (3 in.) thick, with two angles along the bottom 4 in. x 41 in.,