The above [sketch] represents the engine built by a New York firm for such little craft. This is the smallest size made for the market. It has a steam-cylinder 3 inches in diameter and a stroke of piston of 5 inches, driving a screw 26 inches in diameter and of 3 feet pitch. The maximum power of the engine is four or five times the nominal power. The boiler is of the form shown in the illustrations of semi-portable engines, and has a heating-surface, in this case, of 75 square feet. The boat itself is like that seen on page 386, and is 25 feet long, of 5 feet 8 inches beam, and draws 21∕4 feet of water. These little machines weigh about 150 pounds per nominal horse-power, and the boilers about 300.
Some of these little vessels have attained wonderful speed. A British steam-yacht, the Miranda, 451∕2 feet in length, 53∕4 feet wide, and drawing 21∕2 feet of water, with a total weight of 33∕4 tons, has steamed nearly 181∕2 miles an hour for short runs. The boat was driven by an engine of 6 inches diameter of cylinder and 8 inches stroke of piston, making 600 revolutions per minute, driving a two-bladed screw 21∕2 feet in diameter and of 31∕3 feet pitch. Its machinery had a total weight of two tons. Another English yacht, the Firefly, is said to have made 18.94 miles an hour. A little French yacht, the Hirondelle, has attained a speed of 16 knots, equal to about 181∕2 miles, an hour. This was, however, a much larger vessel than the preceding. One of the most remarkable of these little steamers is a torpedo-boat built for the United States navy. This vessel is 60 feet long, 6 feet wide, and 5 feet deep; its screw is 38 inches in diameter and of 5 feet pitch, two-bladed, and is driven, by a very light engine and boiler, 400 revolutions per minute, the boat attaining a speed of 19 to 20 miles an hour. Another little vessel, the Vision, made nearly as great speed, developing 20 horse-power with engine and boiler weighing but about 400 pounds.
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 51∕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 101∕2 feet pitch, 135 revolutions a minute, giving the yacht a speed of 131∕2 knots an hour.
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.
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.
