STEERING ENGINES
The uninitiated may think that the man on the bridge, revolving a spoked-wheel with apparently small exertion, is directly moving the rudder to port or to starboard as he wishes. But the helm of a large vessel, travelling at high speed, could not be so easily deflected were not some giant at work down below in obedience to the easy motions of the wheel.
Sometimes in a special little cabin on deck, but more often in the engine-room, where it can be tended by the staff, there is the steering engine, usually worked by steam-power. Two little cylinders turn a worm-screw which revolves a worm-wheel and a train of cogs, the last of which moves to right or left a quadrant attached to the chains or cables which work the rudder. All that the steersman has to do with his wheel is to put the engine in forward, backward, or middle gear. The steam being admitted to the cylinders quickly moves the helm to the position required.
A particularly ingenious steam gear is that made by Messrs. Harfield and Company, of London. Its chief feature is the arrangement whereby the power to move the rudder into any position remains constant. If you have ever steered a boat, you will remember that, when a sudden curve must be made, you have to put far more strength into the tiller than would suffice for a slight change of direction. Now, if a steam-engine and gear were so built as to give sufficient pressure on the helm in all positions, it would, if powerful enough to put the ship hard-a-port, evidently be overpowered for the gentler movements, and would waste steam. The Harfield gear has the last of the cog-train—the one which engages with the rack operating the tiller—mounted eccentrically. The rack itself is not part of a circle, but almost flat centrally, and sharply bent at the ends. In short, the curve is such that the rack teeth engage with the eccentric cog at all points of the latter's revolution.
When the helm is normal the longest radius of the eccentric is turned towards the rack. In this position it exerts least power; but least power is then needed. As the helm goes over, the radius of the cogs gradually decreases, and its leverage proportionately increases. So that the engine is taxed uniformly all the time.
Some war vessels, including the ill-fated Russian cruiser Variag, have been fitted with electric steering gear, operated by a motor in which the direction of the current can be varied at the will of the helmsman.
All power gears are so arranged that, in case of a breakdown of the power, a hand-wheel can be quickly brought into play.