TORPEDO BOAT, SUBMARINE. Modern sub marine torpedo boats are of two kinds, the sub merged and the submergible. The submerged, when in light cruising condition, moves with only a small percentage of the hull above water; the submergible cruises on the surface much like an ordinary torpedo boat, which it resembles exter nally. The difference in principle between the two types is slight, but in construction details it is very marked. Submerged boats are usually nearly cylindrical with pointed ends, the general shape being much like that of a Whitehead tor pedo. Submergence is effected by admitting water to the ballast tanks or by means of in clined rudders or both. Submergible boats have two Mills, one inside the other. The outer hull resembles closely that of the ordinary torpedo boat, but has as few projections as possible rising from the general outline, in order to present a smooth surface when submerged. Inside this there is a second hull of nearly circular cross section and as large as the shape of the outer boat permits. To effect submergence water is first admitted to the space between the hulls, and this brings the boat to the 'awash' condition. Further submergence is effected by permitting the ballast tanks to fill. The advantages of each type are apparent. The submerged boat is ready for nearly instant service. but cannot cruise with comfort to the crew. The submergible boat af fords fair speed and comfort when cruising, but considerable time is required to effect its sub mergence. The section (Fig. 4) of the Holland submarine boat shows the general features of a submerged boat; similar boats have also been built for the British Navy. France, which is building more submarines than any other nation, is the only one using the submergible boats, and they form but a small part of those she has completed or under construction. For the number of boats completed and under construc tion in the various navies of the world in 1903, see NAVIES.
The difficulties in the way of submarine navi gation are: (a ) difficulty of securing safety; (b) of obtaining a fair speed; (c) of steering; (d) of securing habitability; (e) of insuring sta bility; to which is to be added, for submarine torpedo boats, the difficulty of directing and discharging the torpedo. In the present state
of development submarine boats can be made fairly safe. The problem of reaching a speed in excess of 10 knots when wholly submerged is a difficult one. Storage batteries are too heavy at least all types now known) for the purpose of furnishing sufficient power; steam is inadmissible for obvious reasons; and the only practicable ma chine seems to be an explosive motor developing a high pressure in the engine. So far no efficient. means of keeping the exhaust gases from escap ing into the boat has yet been devised; and even if such means were found it is possible that the escaping bubbles and the smell would enable the course of the boat to be ascertained and followed. As to steering and directing the course, there remain several imperfectly solved problems. It is impossible when under water to see more than a few feet ahead with the boat at rest, and the difficulty increases with the speed. Even when near the surface of the water the hull of a large ship is undiscernible beyond 100 feet. A sensi tive compass with adequate directive force is possible only if the hull is made of copper, bronze, aluminum, or some other non-magnetizable metal. The most promising device is the periscope, which (like a camera obscura) consists of a tube with a prism or inclined mirror at the upper end with a reflecting table below. The upper part of the periscope is designed to project several feet above water when the boat is entirely sub merged. The unsteadiness of the boat, however, interferes seriously with the effectiveness of this instrument; moreover, the projecting tube may betray the boat's position. In place of a compass a gyroscope has been experimented with, but the results were not satisfactory. All boats are fitted with small conning towers projecting a short distance above the hull and having glass covered peep holes of considerable size. This conning tower can be used when cruising at the surface and its top may be opened if the water is very smooth. In most boats the opening in the top of the conning tower forms the principal way of ingress and egress.