All About Engines

2^2 All About Engines form, as shown in Fig. 156, and the cylinder covers are designed to match as nearly as possible the upper and lower surfaces of the pistons in order to reduce the amount of “ clearance.” Since the vessel is required to move both ahead and astern the engines must be fitted with reversing motion, and Stephenson’s gear is generally employed. Further, as the engine tends to race when, in rough weather, the screw rises out of the water, there must be a governor, and this is usually fixed on the crank shaft.1 In the older engines the condenser was attached to the A standards on one side while the air pump was driven from a lever pinned to the low-pressure cross- head as in Fig. 156. But the modern plan is to have the condensing plant entirely separate. Nothing, in fact, is so startling as the amount of auxiliary machinery on a steamship, and the consideration of air pumps and condensers, circulating pumps, evapo- rators, ash extractors, etc., connected with the main engines, together with the steering engines, winches, and what not, deserve a book all to themselves. For the present let us concern ourselves with the main engines, tracing the steam from the boiler to the condenser and beyond. Steam is, of course, raised more quickly if water tube boilers are installed, but these are to be found only in the highest class 1 A centrifugal governor is not really sensitive enough to deal with the large alterations of speed which occur when a rough sea is running. The best arrange- ment is one in which the admission of steam is controlled by the pressure of water in the neighbourhood of the propeller. When the stern sinks the pressure in- creases, and this, acting on a piston, causes the throttle valve to open. Similarly, when the stern rises the reduction of pressure closes the valve.