the collision bulkhead. It would be advisable to pack the space between these bulkheads, like an ordinary cofferdam, before ramming, to limit the flow of water aft, in case the collision bulkhead was damaged; access is obtained through the various decks for this purpose. This bulkhead has not been fitted in recent vessels. The phosphor bronze stem of a sheathed second class cruiser is shown in Fig. 68. In this case the stem is cast in two pieces, and the plating and planking have both to be recessed into the casting. The wood keel also has to be recessed as shown.

In the stems of recent ships the casting has been stopped at the main deck, the strength to the upper deck or forecastle being provided for by a bent plate.

Sternposts. The remarks already made as to the necessity of phosphor bronze stems in sheathed ships apply in this case also.

The sternpost of a single screw ship has to be formed to receive the propeller as well as to form a support on which to hang the rudder. Very nearly all the vessels now in the Royal Navy are, however, twin screw, so that the main function of the sternpost is to receive the rudder.

The shape of the sterns of ships varies considerably in different classes, but in all war-ships (except the smallest) an essential

G

condition to be fulfilled is, that the stern shall be so formed that the rudder and steering gear are well below water and under protection. In order to do this the stern is carried well abaft the rudder-head, as seen in Figs. 71 to 76, to house the rudderhead and steering gear.

Taking first the case of battle-ships, the stern in vessels up to the Canopus class was of simple construction. The sternpost was a casting shaped as shown in Fig. 69, with projections on which

the rudder could be supported.

FIG. 70.

In more recent ships the flat portion of the stern, called the "deadwood," has been cut away, as in Fig. 70, and this makes the shape of the sternpost casting rather more complicated. The object of this "cut away" was to maintain good turning qualities; this will be referred to again in Chapter XXI.

Fig. 70 shows in some detail the sternpost of a recent battleship, the lower figure giving on a smaller scale the shape of the

stern. The keel is cut up about 50 ft. from the stern, but is brought down again to allow about 7 ft. to take the blocks when docking. A 2-in. plate is worked horizontally from the sternpost, extending well forward and securely fastened to the ship's structure. This thick plate forms a substantial bed, to which the lower palms of the shaft brackets and the forward end of the sternpost casting can be secured. It also forms an excellent stiffening to the ship, to take the side bending due to putting the rudder over. The sternpost has projections to take the weight of the rudder, and is swelled out at the upper part to receive the rudder-head. This has to be made watertight by means of a stuffing gland, as in Fig. 69. The casting ends a short distance from the rudder-head, and the shape of the stern is maintained to the upper deck by means of a thick steel plate.

Coming now to the sterns of cruisers, we notice that the stern has been shaped in two ways, in both of which a "balanced" rudder is obtained

1. As Fig. 71, as adopted in large cruisers up to and including the Diadem, and in second and third class cruisers up to the present time.

2. As Fig. 73, as adopted in large cruisers since the Diadem. In this type of stern the deadwood is cut right away to facilitate turning, and the rudder is underhung with a portion of the area before the axis.

In either case the weight of the rudder is taken at the top of the sternpost casting, as in Fig. 77, which has therefore to be made specially strong on this account. A steadying pintle is provided at the lower part. Fig. 79 shows in some detail the construction of the sternpost of a large cruiser with underhung rudder.

Rudders. The shape of rudders in battle-ships up to quite recently has been, as in Fig. 75, of nearly rectangular shape, hinged at the fore side. Fig. 78 shows in some detail the construction of such a rudder. The weight is taken on the sternpost projections, and in order to make the friction as small as possible the bearings are, as shown, of hard steel. The frame of the rudder is a steel casting, lightened out as much as possible, but necessarily of massive construction at the forward end to stand the large twisting moment. The sides are covered with 15-lb. ( in.) steel plating,1 and the space inside is filled with fir. The rudders of the battle-ships 1 Phosphor bronze in a sheathed ship.