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. 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,' and the space inside is filled with fir. The rudders of the battle-ships

1 Phosphor bronze in a sheathed ship.

of King Edward VII. class have a portion of the area before the axis (Fig. 76). This renders the steering of the ship easier, because the centre of pressure on the rudder is brought nearer the axis.

For cruisers rudders are now always "balanced," i.e. a portion of the area is before the axis. If we deal with a rectangular plate towed through the water at an angle of 30° to 40°, it is found that the centre of pressure is about one-third the breadth from the

leading edge. So to balance a rudder, i.e. to get the centre of pressure close to the axis, we need to make the area before the axis considerably less than one-half the total (see Figs. 71 and 73). In such a rudder the twisting moment even at high speeds is small, and much smaller power is needed in the steering arrangements than with an unbalanced rudder. This is specially desirable in cruisers, because of their high speed and the limited room available aft to house the gear owing to the fineness of these ships (see Fig. 163). The pressure, per square foot of rudder area, increases