This chapter will deal with the auxiliaries to be found in large vessels designed for peaceful or warlike uses. Many devices are common to ships of both classes, and some are confined to one type only, though the "steel wall" certainly has the advantage with regard to multiplicity.
All marine engines should be fitted with some apparatus which enables the engineer to reverse them from full speed ahead to full speed astern in a few seconds. The effort required to perform the operation of shifting over the valves is such as to necessitate the help of steam. Therefore you will find a special device in the engine-room which, when the engineer moves a small lever either way from the normal position, lets steam into a cylinder and moves rods reversing the main engine. By a link action (which could not be explained without a special diagram) the valves of the auxiliary are closed automatically as soon as the task has been performed; so that there is no constant pressure on the one or the other side of its piston. To prevent the reversal being too sudden, the auxiliary's piston-rod is prolonged, and fitted to a second piston working in a second cylinder full of glycerine or oil. This piston is pierced with a small hole, through which the incompressible liquid passes as the piston moves. Since its passage is gradual, the engines are reversed deliberately enough to protect their valves from any severe strains. These reversing engines can, if the steam serving them fails, be worked by hand.
When a ship is passing through a strong sea and pitches as she crosses the waves, the screw is from time to time lifted clear of the water, and the engines which a moment before had been doing their utmost, suddenly find their load taken off them. The result is "racing" of the machinery, which makes itself very unpleasantly felt from one end of the ship to the other. Then the screw, revolving at a speed much above the normal, suddenly plunges into the water again, and encounters great resistance to its revolution.
A series of changes from full to no "load," as engineers term it, must be harmful to any engines, even though the evil effects are not shown at once. Great strains are set up which shake bolts loose, or may crack the heavy standards in which the cranks and shaft work, and even seriously tax the shaft itself and the screw. On land every stationary engine set to do tasks in which the load varies--which practically means all stationary engines--are fitted with a governor, to cut off the steam directly a certain rate of revolution is exceeded. These engines are the more easily governed because they carry heavy fly-wheels, which pick up or lose their velocity gradually. A marine engine, on the other hand, has only the screw to steady it, and this is extremely light in proportion to the power which drives it; in fact, has scarcely any controlling influence at all as soon as it leaves the water.