bending moment, will determine the necessary size for the rudder-head, unless the ahead conditions require a larger diameter.

In any rudder, the head being under water, it is necessary that the hole in the sternpost should be made watertight. The hole is lined with gunmetal, and the rudder-head is cased with a gunmetal sleeve, as shown in Figs. 69 and 77. A stuffing gland is fitted at the top to make the hole watertight.

Projections are cast on the rudder on each side to bring up against the sternpost when the rudder is hard over; these, however, are being omitted in some recent ships.

It was formerly the practice to supply each ship with a mould giving the actual shape of the rudder. It is the present practice to supply a sketch of the

rudder on a large scale, giving complete figured dimensions.

Shaft Brackets.-In twin screw vessels a considerable length of the propeller shafting is outside the ship, and brackets are fitted on either side, just forward of the propellers, to take the weight of the after end of the shafting, etc. For steel ships the brackets are of cast steel, for sheathed ships of phosphor bronze. These brackets do not have to take any fore-and-aft thrust (this

being taken in the engine-room by the thrust block), but they have to bear the very considerable weight of the propeller, etc. Because of this the attachment to the structure of the ship has to be very

secure.

Fig. 82 shows the arrangement in a recent battle-ship. The arms are flattened out at the top and bottom. The upper palm passes inside the ship, where it is riveted to a thick fore-and-aft plate. The lower palm is shaped with a scarph to fit a corresponding scarph on the other bracket. The two brackets are then

securely fastened to the 2-in. fore-and-aft plate, and to the forward end of the sternpost casting (see also Fig. 70). The arms of the brackets are of pear-shaped section, with the blunt end foremost. This is in order to diminish the resistance of these brackets, it having been proved by experiment that this form offers the least resistance when fully submerged. (It will be remembered that a torpedo has a blunt nose and a fine run.)

CHAPTER VIII.

STEERING GEARS.

THE steering gear, fitted at the stern adjacent to the rudder, is a most important fitting of a war-ship, and, except in the smallest classes, is always arranged under water and under protection. The stern has to be specially shaped to do this, and Figs. 71 to 76 show how this is done in different classes of ships. Under ordinary circumstances the steering gear is actuated by the steam steering engine, but hand wheels are provided as an auxiliary.

The twisting moment on the rudder-head (within the ordinary limits, viz. up to about 35°), which has to be overcome by the steering gear, depends on

(i.) The area of the rudder;

(ii.) The square of the speed of the water meeting the rudder; (iii.) The angle made by the rudder with the middle line, and (iv.) The distance of the centre of pressure from the axis. This last is of great importance, and has led to the introduction of balanced rudders, as Figs. 71 to 74, because in these rudders the centre of pressure is close to the axis. This makes the twisting moment small, even at high speeds.

The larger the angle (within the ordinary limits) the greater is the moment to be dealt with. A gear which, with a constant motive force, will operate on a rudder to overcome a large twisting moment at large angles, and a small moment at small angles, will be the most desirable. This is necessarily accompanied by a slower motion at large angles than at small angles. Such a steering gear is termed compensating. We shall consider three such gears, which have been largely fitted in vessels of the Royal Navy.

1. Rapson's Slide Steering Gear. This gear is shown in outline in Fig. 83, and in some detail in Fig. 84. It is a very heavy gear, and takes up a lot of room. It has been principally

used in battle-ships. A cross-head on the rudder-head is connected by parallel rods to a second cross-head, on to which a long tiller is

attached. The forward end of this tiller is of parallel section, and passes through

a block, which can swivel inside another block. This second block is made to travel across the ship on a thwartship path, by means of a sprocket chain. This chain passes to the sides of the ship, and down to the centre line, where it passes under a sprocket wheel, which fits into the chain. This sprocket wheel is made to revolve either by the hand or steam gear, and so the rudder moves as required. This gear takes up a lot of room, because of the travel of the tiller from side to side, and for this reason its use has been confined to It has the advantage of

battle-ships, which are full at the stern. compensation, but has the disadvantage of being reversible. If the chain or sprocket wheel broke, the tiller would swing from side to side with the movement of the rudder. On this account a friction brake is fitted, to hold the tiller if necessary. This brake is tightened up to hold the tiller when the gear has to be changed from hand to steam, or vice versâ.

Theory of Rapson's slide steering gear.-In Fig. 85 a tiller is shown, passing through a ball, which is made to move along the thwartship path. This is similar to the state of things that obtains in the actual gear. The constraint of the slide brings into action a side force on the ball, as shown, Q. The pull of the chain P, combined with the force Q, gives, by the parallelogram

with the moment Ph at the middle line. At 35°,

1 COS 20

=

1.5, so that at the

extreme angle the moment is 50 per cent. greater than at 0° for the same

pull on the chain. This increase in moment is accompanied by a slower

motion; thus, if a certain number of revolutions moves the tiller through 10° at the start, the same number will move it through about 7° at the end of the travel. The detail of the slide shown in Fig. 84 shows how the side thrust is provided for.

Steering

2. Harfield's Gear. This patent gear has been fitted in a number of ships, both battle-ships and cruisers. It has the advantage of compensation, which is obtained as shown in Figs. 83

and 86. The forward cross-head is fitted with a curved rack,