Modern submarines are of four types, coast defense, cruising, fleet and mine laying. While generally similar, they necessarily differ in details according to the work they are called upon to perform. Those used for coast defense require only a small cruising radius, since they can readily return to their base for fresh supplies of fuel, food and ammunition. For a similar reason, auarters for officers and crew need be only of the simplest necessarily be larger than the coastal type, and even berore the war boats of 800 to 1,200 tons had been built. Dur ing the war, both the United States and Great Britain built several of 1,500 to 2,000 tons, and similar large boats were owned by Germany. On the average, the submarine cruiser is 250 to 300 feet long, with a beam of 18 to 25 feet or more. They are swifter than the coastal type, and can attain a speed of 20 knots on the surface, and 14 to 15 knots sub merged. More recent vessels have a cruising radius of 10,000 miles. Their armament has steadily increased, and some of the larger boats are equipped with two 5-inch or even 6-inch guns. In the earlier years of the war the German boats were fitted with one or two 3-inch guns, to which very soon was added an kind. Compared with the cruising type they are small, varying from 250 to 600 tons, being 150 to 200 feet long with a beam of 15 to 20 feet. They can attain a speed of as much as 16 knots on the surface, but when submerged this is re duced to 8-11 knots. They have a cruis ing radius of 1,000 to 2,000 miles, and are usually armed with one 3-inch gun, and sometimes with an anti-aircraft gun in addition. They carry also from four to eight torpedo tubes.
The Cruiser was the type which showed probably the great est development during the war. These formed the most active branch of the German navy, and the enormous de struction of Allied and neutral shipping was carried out by them. Because they are away from their base for several days, or even weeks, they must contain sleeping and living quarters for officers and crew, with space for the prepara tion and cooking of food. In addition, they have to carry a comparatively large number of torpedoes and other needed supplies. It follows that they Trust anti-aircraft gun. These were, of course, in addition to the torpedo tubes, which numbered as many as eight. They car ried a crew of from 40 to 50 men.
The Fleet Submarine, as its name implies, is intended to accompany the fleet in action. Its radius of action need not necessarily be high, as it can be accompanied by a mother ship carry ing supplies of fuel, extra ammunition, food for the crew and other necessities. This type can therefore be smaller than the cruiser, but it must be speedy and also possess seaworthiness. The sub marine mine-layer was a type evolved by Germany during the war, and proved of great value to her in laying mines off the coast of Great Britain. They car
ried 16 mines in inclined tubes, which were open at the lower end. The tubes were loaded from the deck, the mines being liberated by a special device under water at the open end.
All types of submarines are now driven by Diesel or gasoline engines when trav eling on the surface, but rely on electric motors when submerged. The current for the motors is supplied by storage batteries. An arrangement is made sim ilar to that on the ordinary automobile (see "Electric Battery"), by which the storage batteries can be recharged from a generator driven by the Diesel or gaso line engine when the boat is traveling on the surface. The newer types of cruis ing submarines can travel upward of 100 miles on a single charge of the bat teries. It will be seen, therefore, that even they can remain beneath the surface for comparatively short periods of time, even if other considerations permitted them to keep submerged for any greater period. One of the most difficult prob lems which confronted early inventors was that of keeping the boat on an even keel. This is now accomplished by means of ballast tanks and horizontal runners, or by means of the gyroscope. But the matter still remains one which requires constant watchfulness. It will be clearly seen that under the conditions existing in a submarine, where the vessel is so to speak in a state of delicate balance, the moving of equipment or of torpedoes, and especially the discharge of torpedoes, presents problems of peculiar difficulty. As indicated above, the problems have to a large extent been solved, but the sta bility of the submarine still leaves room for improvement. Submerging is ef fected by admitting water into tanks specially designed for the purpose. Con versely, the boat is brought to the sur face by expelling the water from these tanks either by means of compressed air or with the aid of pumps. The speed with which a boat can submerge or rise has increased considerably in recent years, but even now it varies somewhat with conditions. On the average, from one and one-half to three minutes are re quired for a boat to emerge sufficiently far from the water to use her guns. The periscope, by means of which the ob serving officer can watch what is taking place on the surface of the water while the boat remains almost completely sub merged, is an essential part of submarine equipment, and the principle of reflecting mirrors, on which it depends, is familiar to all. Submarines are costly to build; ton for ton they cost more than twice as much as surface battleships or cruisers. The pre-war cost was estimated at, ap proximately, $1,000 per ton, but it is cer tain that the cost of those built under war-time conditions was much higher than this.