U-boat (from the German unterseeboot), basic German diesel-electric submarine that revolutionized submarine warfare during World War I (1914-1918) and World War II (1939-1945). U-boats gained notoriety during World War I through Germany’s policy of unrestricted submarine warfare, which declared that any and all ships straying into disputed waters, including commercial shipping boats and civilian passenger lines, were to be regarded as military targets. As the U-boat improved, it quickly became a symbol of the military gains in undersea technology that occurred in the two decades between the end of World War I and the outbreak of World War II.
German engineers produced several types of U-boats between 1914 and 1945. The Type VII-C, constructed in large numbers from 1940 through 1945, is characteristic of the U-boats deployed by Germany. It was made of steel and displaced 871 metric tons of water, about a tenth of the 8300 metric tons displaced by the modern USS Seawolf nuclear attack submarine made in the United States. The Type VII-C could hold only 14 torpedoes in its bow and stern compartments, and like all U-boats, it depended on its rechargeable batteries to operate while submerged. When a U-boat surfaced, it used its air-breathing diesel engine to propel itself and to recharge its batteries. The range of U-boats was extended during World War II through the addition of a snorkel device that allowed a U-boat to run its diesel engines while submerged at periscope depth. By doing so, a U-boat could recharge its batteries with a better chance of remaining undetected.
Experts consider American and Japanese submarines of the same era to have performed in a similar manner, but the U-boat attained a superior reputation, due in part to the aggressiveness of many of Germany's submarine captains. One of them was Captain Gunther Prien, who slipped inside a Royal Navy anchorage north of Scotland in 1939 and sank the British battleship Royal Oak.
The hull of the Type VII-C was 67.4 m (221 ft) long with a beam of 6.2m (19 ft), divided into six compartments of equal length for its crew of 44 men. (Other types of U-boats had crews of as few as 22 or as many as 63.) The bow compartment contained four torpedo tubes, storage space for torpedoes, and berthing space for 25 sailors who shared collapsible bunks or hammocks. The rest of the submarine was divided into living and working spaces, with the control room located toward the middle of the ship, under the primitive sail structure (also called the conning tower). Shortages of space, fresh water, and refrigeration required crews to tolerate poor food, infrequent bathing, and cramped living conditions.
The principal weapon of the U-boats was the torpedo. During World War II German submariners used torpedoes powered by compressed air, and later used electric models that left no telltale wake of bubbles. The Type VII-C also had an 88-mm (3-in) naval deck gun for surface combat. This was used frequently to finish off torpedoed ships or to attack smaller vessels. Most U-boats also had antiaircraft guns mounted behind the conning tower. Early in World War II, U-boats individually patrolled regional areas looking for targets, but Germany eventually embraced the 'wolf pack' strategy of sending groups of submarines to confront enemy convoys. German U-boats ranged from the Arctic Ocean to the South Atlantic Ocean and the Mediterranean Sea throughout World War II, aided significantly by the construction of submarine bases on the French Atlantic coast after 1940. From early 1942 to mid-1943, U-boats attacked American ships along the U.S. eastern seaboard and destroyed cargo bound for Europe. These attacks threatened to strangle Britain's economy, which was then dependent on goods from North America. Despite its successes, the U-boat service had to fight for resources and support among competing German military services, particularly the German Army (Wehrmacht) and Air Force (Luftwaffe).
Germany began World War II with fewer than 100 U-boats, but it produced several thousand between 1939 and 1945. Military historians credit several factors for the defeat of the U-boat campaign in World War II, including secret Allied interception of German naval codes, which allowed antisubmarine warfare units to swarm to known U-boat locations; and the development of effective sonar and radio direction-finding gear to pinpoint their locations. About 40,000 members of the U-boat service fought for Nazi Germany during six years of war. Only 12,000 men survived, and 5000 of them were taken prisoner by U.S. and British forces. Germany lost 784 U-boats to Allied attacks, but the Allies lost more than 2800 merchant ships to the submarines. At the end of World War II, both the U.S. and Soviet navies obtained several advanced U-boats, and postwar diesel-electric submarines on both sides incorporated many of the features of the defeated German Navy's submarines.