All About Engines

Engines for Ships 281 room cannot be placed in the rapidly narrowing por- tion of the ship near the stern, and a blunt stern reduces the speed. The transmission of power through a long shaft in this way causes twisting. Suppose the circle in Fig. 157 represents a solid shaft and the two diameters the amount of twist which occurs when power is being transmitted. It will be clear that the strain in the material increases with the distance from the centre, and that if the material on the outside is bearing as much / \ stress as it can do with safety, / the material near the centre is I bearing very little stress?. Con- L—*C***JJ sequently by removing the \ 7 centre and making the shaft x. S in the form of a tube a little ---------- larger than the solid shaft, the F1*‘ 157<~Dia^ram t0 illustrate stress m the material is more even and weight is saved. A marine engine shaft therefore, may be 12 to 24 inches or more in diameter with a 6-inch or 10-inch or larger hole through its entire length. It is made in sections, bolted together by means of flanges. The high-pressure cylinder of a marine engine is invariably fitted with a piston valve, because with this form there is the smallest friction and a most perfect balance under high pressure of steam. The intermediate and low-pressure cylinders have long double-ported D slide valves. In order to secure light- ness the pistons are of cast or pressed steel, conical in