Yachts of high speed require such weight and bulk of engine that but little space is left for cabins, and they are usually exceedingly uncomfortable vessels. In the Miranda the weight of machinery is more than one-half the total weight of the whole. An illustration of the more comfortable and more generally liked pleasure-yacht is the Day Dream. The length is 105 feet, and the boat draws 5-1/2 feet of water. There are two engines, having steam-cylinders 14 inches in diameter and of the same length of stroke, direct-acting, condensing, and driving a screw, of 7 feet diameter and of 10-1/2 feet pitch, 135 revolutions a minute, giving the yacht a speed of 13-1/2 knots an hour. [Illustration: FIG. 136.--Horizontal, Direct-acting Naval Screw-Engine.] 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 angle 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 condenser is placed behind the engine-frame, or, where a jet-condenser is used, the frame itself is sometimes made hollow, and serves as a condenser. The air-pump is worked by a beam connected by links with the cross-head. The general 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 horizontally, 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_. _F_ is the cross-head guide. The eccentrics, _G_, operate the valve, which is of the "three-ported variety," by a Stephenson link. Reversing is effected by the hand-wheel, _C_, which, by means of a gear, _m_, and a rack, _k_, elevates and depresses the link, and thus reverses the valve. [Illustration: FIG. 137.--Compound Marine Engine. Side Elevation.] The trunk-engine, in which the connecting-rod is attached directly to the piston and vibrates within a trunk or cylinder secured to the piston, moving with it, and extending outside the cylinder, like an immense hollow piston-rod, is frequently used in the British navy. It has rarely been adopted in the United States. [Illustration: FIG. 138.--Compound Marine Engine. Front Elevation and Section.]