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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.)
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.
I. 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
-ff-— f^r \ x~ 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.
Tlieory of Bapson'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
p of forces, a resultant force acting square to the tiller. This acts at a
leverage of —, so that the moment at the angle 9 is Kz, as compared
cos e cos J9
with the moment PA at the middle line. At 35°, —=z = 1*5, so that at the
'cos 2e extreme angle the moment is 50 per cent, greater than at 0° for the same
— Sprocket Chain— -sprocket Whkl
Kio. 84.—Rapson's slide steering gear.
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.
2. Harfield's Steering 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 FlQ gobtained as shown in Figs. 83 and 86. The forward cross-head is fitted with a curved rack,
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.
Fig. 88.—Screw steering gear.
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