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

- RAPSON

HARFIELD'S GEAR.

OLLIS'S GEAR.

TRAMWAY

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 battle-ships, which are full at the stern. It has the advantage of 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 versa.

SCREW GEAR

FIG. 83.

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 acting square to the tiller. This acts at a

of forces, a resultant force

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.

CHAIN

SPROCKET WHEEL

FIG. 84.-Rapson's slide steering gear.

h

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

FIG. 85.

P/cos

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

STERN POST

which engages with an eccentric pinion. This pinion is made to revolve as required. Assuming a constant moment acting on

this pinion, the force on the teeth is greatest at large angles, because the leverage from the turning centre is then smallest.

This large force acts on the cross-head, and at the large angles acts at a larger leverage than at small angles. Thus,

at the extreme angles we have a greater moment on the rudder obtained in this double way, a larger force on the teeth, and a

larger leverage. Thus the compensation desired is obtained. With this gear a hydraulic brake is fitted, which can be made to hold the rudder in any desired position. The safety of the gear depends absolutely on the teeth of the rack and pinion in contact, and on this account these are made of forged steel of ample strength. The gear is reversible, like the Rapson's slide, unless a worm and worm-wheel is introduced, which has been done in some ships.

3. Ollis's Steering Gear (Figs. 83 and 87).-This is a gear designed at the Admiralty by Mr. F. B. Ollis.1 It answers the same purpose as Harfield's gear, with simpler construction. The pinion and rack are both circular, but the connecting rods to the rudder cross-head are brought close together at the forward end. Assuming a constant moment on the pinion, it is seen that as the angle increases the connecting rods come closely together, and so the force along these rods increases, giving a larger moment at large angles than at small. This is the compensation desired. The gear is reversible, as in the previous cases, and it is necessary to provide a friction brake, to hold the gear in case it becomes disabled, and when changing from hand to steam, or vice versa.

4. Screw Steering Gear (Figs. 83 and 88).-This gear is quite different to the previous gears, being non-reversible, and being the reverse of compensating. It has, however, been very largely adopted. It consists of a right- and left-handed screw, which, on rotating, causes two nuts to slide along in opposite directions; these nuts are prevented from rotating by being made to slide along parallel bars. Connecting rods are led from these sliding nuts to the rudder cross-head.

The gear takes up very little room. Another great advantage in this gear is that it cannot reverse, and the rudder in any position is locked. It is because of this that it is being now fitted to all the new ships of large size in the Royal Navy. It is, however, the reverse of compensating, and a rudder not balanced would be difficult to move by the hand wheels at large angles when a high speed is reached. All the large ships at present building are, however, being fitted with rudders more or less balanced, and in these ships the screw steering gear should prove a satisfactory fitting. A friction brake has usually been fitted on the rudder cross-head, to hold the rudder in case anything has to be done to the gear.

1 Now Chief Constructor at Hong Kong.

Steam Steering Gear.-Whatever steering gear is adopted at the stern, the motive power for working it under ordinary circumstances is a steam steering engine. Formerly the engine was placed aft, just forward of the steering gear, but this is objectionable for the following reasons:—

(i.) The steam-engine heats the compartment, making special ventilation necessary.

(ii.) The steam and exhaust pipes led from the boilers heat the compartments at the after end of the ship, through which they pass.

(iii.) Attention of the engine-room staff is required at the engine.

In recent ships the steering engines, of which there are two, are placed on the after bulkhead of the engine-room, and shafts are led along the shaft passages to the steering gear. This entails a good deal of weight, but on the whole is more satisfactory than the former system.

The steam steering engines are of ample power, and in a large ship the time specified to take the rudder from hard-over to hardover is only 30 seconds when the vessel is going full speed. A worm and worm-wheel is fitted between the shaft and the steering engine, so that shocks from the rudder are kept from the engine.

The steam engine is operated by means of steering wheels placed as follows in a large ship:-forward bridge, forward conning tower, after conning tower, lower deck forward in the ammunition lobby, platform aft in the steering compartment. The engine is worked from these positions by means of controlling shafting. This shafting is led down inside the armoured tubes. from the conning towers to below the protective deck. Care is taken to avoid any rigid connection of the shafting to the protective deck or any portion of the vessel exposed to damage in action. Some ships have been fitted with telemotor controlling gear instead of the shafting. In this gear the motion is conveyed to the steering engine from the steering wheel by means of small pipes. A small piston is worked at the wheel; this causes motion of water throughout the system, and exactly similar motion at the steering engine.

Steering by Manual Power.-Most vessels have one position below, aft, for steering by manual power, in case the steam gear is not available. The hand wheels are four in number in the largest vessels, three and two being fitted in smaller ships. In