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In 1912, E-boats, the first U.S. diesel-engine submarines, were launched. They were 135 ft (41 m) long, had a crew of 23, and were the first to cross the Atlantic. Development continued, and in World War I submarines were for the first time used extensively by both sides. The Germans used 200-ton submarines (U-boats), and later they employed 2,100-ton craft armed with as many as 19 torpedoes. To halt the heavy destruction of shipping by these U-boats the Allied powers resorted to depth charges, Q-ships (armed vessels disguised as merchantmen), and escorted convoys. With the crucial additions of sonar (which uses high-frequency sound waves to find submarines through echo tracking) and radar-equipped air escorts (often carried on small aircraft carriers) these defenses were also used in World War II.
A typical U.S. Navy submarine in World War II was a 300-ft (91-m) craft of 1,450 tons displacement and had a crew of 55. It ran on diesel engines (while surfaced) at a speed of up to 17 knots and on electric motors (while submerged) at a speed of up to 8 knots. The ship was armed with one 3-in. (7.6-cm) dual-purpose gun, several light automatic weapons, and 10 21-in. (53-cm) torpedo tubes. A periscope is an integral part of every submarine. It extends up through the water and by a mirror arrangement provides the observer below with a view of the surface of the sea. Similar in appearance but totally different in purpose is the snorkel apparatus first employed by the Germans and now in general use. It admits air but not water and, by supplying a flow of fresh air and an outlet for foul air, makes it possible for a submarine to remain submerged for as much as nine tenths of a voyage.
In World War II the Allies and neutrals lost some 4,770 ships to submarines, mostly German U-boats; Axis submarines were a significant strategic threat until late in the war. U.S. submarines sank over 550 Japanese ships. Submarines were also used to insert commandos in enemy territory and for rescue operations. The Germans and Japanese exchanged military plans, equipment, and precious metals by submarine as well.
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