(4) Electricity.
On some ships we may find all four working side by side to drive the multifarious auxiliaries, since each has its peculiar advantages and disadvantages. At the same time, marine engineers prefer to reduce the number as far as possible, since each class of transmission needs specially trained mechanics, and introduces its special complications.
Let us take the four agents in order and briefly consider their value.
Steam is so largely used in all departments of engineering that its working is better understood by the bulk of average mechanics than hydraulic power, compressed air, or electricity. But for marine work it has very serious drawbacks, especially on a war vessel. Imagine a ship which contains a network of steam-pipes running from end to end, and from side to side. The pipes must, on account of the many obstacles they encounter, twist and turn about in a manner which might be avoided on land, where room is more available. Every bend means friction and loss of power. Again, the condensation of steam in long pipes is notorious. Even if they are well jacketed, a great deal of heat will radiate from the ducts into the below-deck atmosphere, which is generally too close and hot to be pleasant without any such further warming. So that, while power is lost, discomfort increases, with a decided lowering of human efficiency. We must not forget, either, the risk attending the presence of a steam-pipe. Were it broken, by accident or in a naval engagement, a great loss of life might result, or, at least, the abandonment of all neighbouring machinery.
For these reasons there is, therefore, a tendency to abolish the direct use of steam in the auxiliary machinery of a modern vessel.
High-pressure water is free from heating and danger troubles, and consequently is used for much heavy work, such as training guns, raising ashes and ammunition, and steering. One of its great advantages is its inelasticity, which prevents the overrunning of gear worked by it. Water, being incompressible, gives a "positive" drive; thus, if the pump delivers a pint at each stroke in the engine-room a pint must pass into the motor, assuming that all joints are tight, and the work due from the passage of one pint is done. Air and steam—and electricity too, if not very delicately controlled—are apt to work in fits and starts when operating against varying resistance, and "run away" from the engineer.
An objection to hydraulic power is, that all leakage from the system must be replaced by fresh water manufactured on board, which, as we have seen, is no easy task.
Compressed air, like steam, may cause explosions; but when it escapes in small quantities only it has a beneficial effect in cooling and freshening the air below decks. The exhaust from an air-driven motor is welcome for the same reason, that it aids ventilation. On a fighting ship it is of the utmost importance that the personnel should be in good physical condition; and when the battle-hatches have been battened down for an engagement any supply of fresh oxygen means an increased "staying power" for officers and crew. Poisoned air brings mental slackness, and weakening of resolve; so that if the motive power of heavy machinery can be made to do a second duty, so much the better for all concerned.
Compressed air also proves useful as a water-excluder. If a vessel contain, as it should, a number of water-tight compartments, any water rushing into one of these can be expelled by injecting air until the pressure inside is equal to that of the draught of water of the vessel outside.