The control of the submarine in the vertical plane may be accomplished by the manipulation of the rudders, either automatically, by means of some such arrangement as the hydrostatic valve or pendulum, or by hand, and she can be kept on an even keel and prevented from rising to the surface or sinking to the bottom, when running beneath the waves, by the pumping of water from a reservoir situated aft to one situated forward, or vice versâ, by the admission of water into trimming tanks, by shifting weights, &c. These operations can be carried out either automatically or by hand-operated mechanism.
It will be readily understood that while it is a comparatively simple matter to force a vessel beneath the surface to a depth previously determined, it is not so easy to ensure its keeping at this depth during the whole time it is submerged and maintaining throughout the run a perfectly even keel.
One of the greatest difficulties the inventor of submarine boats has to overcome is their lack of longitudinal stability. Submerged vessels are of two classes, those which are equal in weight to the water they displace, and those which are lighter. Both classes are subject to various disturbances which tend to upset their longitudinal stability and send them up to the surface and down towards the bottom. In Chapter XV. mention is made of the difficulties experienced by those who had to navigate the Nordenfelt.
The principal causes of disturbance have been summed up by Captain Hovgaard in a paper entitled, “The Motion of Submarine Boats in the Vertical Plane,” read before the Institution of Naval Architects at the Annual Meeting in 1901.
1. Faulty use of horizontal rudder. 2. Admission of water through leakages. 3. Expulsion of foul air and products of combustion. 3a. Firing of torpedoes and projectiles. 4. Movements of crew. 5. Existence of free surfaces of liquid. 6. Movements of loose weights, such as fuel. 7. Variations of buoyancy caused by varying density of sea water. 8. Grounding and collision. 9. Variations in speed.
Some of the most important of these disturbances may be briefly discussed.
1. Most modern submarines are provided with more than one pair of horizontal rudders, but if all the rudders should refuse to act and the boat is running down an inclined plane, the only thing to be done is to pump the water out of the tanks and thus bring the boat to the surface.
2. By the careful construction of the hull, and by strict control of all sea-valves, the admission of water may be prevented. If the boat is stove in and water enters in any quantity, she will inevitably sink. As an escape some inventors have provided their submarines with detachable boats.
3. Usually the length of the run under water will not be so great that the foul air will need to be got rid of. If necessary it can be expelled by drawing on the store of compressed air, and as the substances withdrawn will always be small, no change in longitudinal balance need be feared if precautions are taken. As all modern submarines are driven by electricity beneath the surface, the expulsion of products of combustion need not be considered.
3a. In the earliest submarine boats the torpedoes consisted of charges of explosive in cases, which were attached to the outside of the vessel to be attacked, or were towed against her sides.