MARINE ENGINES. 389
feet of water. There are two engines, having steam-cylin-
ders 14 inches in diameter and of the same length of stroke,
direct-acting, condensing, and driving a screw, of 7 feet
diameter and of 10 feet pitch, 135 revolutions a minute,
giving the yacht a speed of 13 knots an hour.
In larger vessels, as in yachts, in nearly all cases, the ordinary screw-engine is direct-acting. Two engines are placed side by side, with cranks on the shaft at an an- gle of 90° with each other. In merchant-steamers the steam-cylinders are usually vertical and directly over the crank-pins, to which the cross-heads are coupled. The con- denser is placed behind the engine-frame, or, where a jet- condenser is used, the frame itself is sometimes made hol- low, and serves as a condenser. The air-pump is worked by a beam connected by links with the cross-head. The gen- eral arrangement is like that shown in Figs. 137 and 138. For naval purposes such a form is objectionable, since its height is so great that it would be exposed to injury by shot. In naval engineering the cylinder is placed horizon- tally, as in Fig. 136, which is a sectional view, representing an horizontal, direct-acting naval screw-engine, with jet- condenser and double-acting air and circulating pumps. A is the steam-cylinder, B the piston, which is connected to the crank-pin by the piston-rod, D, and connecting-rod, E.
FIG. 186. Horizontal, Direct-acting Naval Screw-Engine.
