Another hollow shaft is slipped over the first shaft and one end is connected to the engine and to the other and outside end of the shaft another propeller is keyed on. This shaft is driven in the opposite direction to the solid shaft inside of it by gears that is, a set of cog-wheels.
The Propellers.—The propellers have four blades; and the reason two propellers are used is to counteract the force of each other. That is to say, if only one propeller is used, the force of the blades striking the water tends to tilt the craft to one side, and hence by using two propellers this effect is very largely offset.
The Steering and Diving Rudders.—The vertical or steering rudder and the diving or horizontal rudders are hinged to a frame around the propellers as shown in [Fig. 42].
As it takes a lot of power to move these rudders against the varying water currents, an air engine must be used to work them instead of levers that are connected directly with the gyro, pendulum, or water pressure controls.
How a Torpedo Is Shot at a Ship.—And now comes the most exciting moment of all, and that is when a torpedo is shot from a submarine at an enemy ship.
The latest type of submarine usually has eight torpedo tubes, four of them fore, and four aft. These tubes are shaped as shown in [Fig. 43], and are built right into the hull of the craft.
Now, when we say that a torpedo is shot from a submarine we do not mean that it is fired from it, for it does not explode until it hits the ship it is aimed at, but we simply mean that it is forced out of, or driven from, the tube of the boat.
FIG. 42. HOW A REAL TORPEDO IS MADE.
The force that drives the torpedo from the tube is our old friend compressed air, and this is taken from the torpedo air tanks of the submarine; they are placed just above the torpedo tube and are connected with the rear part of it.