World War II

Germany had the largest submarine fleet during World War II. Due to the Treaty of Versailles limiting the surface navy, the rebuilding of the German surface forces had only begun in earnest a year before the outbreak of World War II. Having no hope of defeating the vastly superior Royal Navy decisively in a surface battle, the German High Command immediately stopped all construction on capital surface ships save the nearly completed Bismarck class battleships and two cruisers and switched the resources to submarines, which could be built more quickly. Though it took most of 1940 to expand the production facilities and get the mass production started, more than a thousand submarines were built by the end of the war.

During World War II, U-boat warfare was the major component of the Battle of the Atlantic, which lasted the duration of the war. Germany had the largest submarine fleet in World War II, due to the Treaty of Versailles which limited the surface navy of Germany to six battleships (of less than 10,000 tonnes each), six cruisers and 12 destroyers. Winston Churchill, the United Kingdom's Prime Minister for most of the war, was quoted as saying, "The only thing that really frightened me during the war was the U-Boat peril."

In the early stages of the war the U-boats were extremely effective in destroying allied shipping, ranging from the Atlantic coast of the United States and Canada to the Gulf of Mexico, and from the Arctic to the West and Southern African coasts and even as far east as Penang. Because speed and range were severely limited underwater while running on battery power, U-boats were required to spend most of their time surfaced running on diesel engines, diving only when attacked or for rare daytime torpedo strikes. Contary to popular notion, the most common u-boat attack (especially in the early years of the war) was conducted on the surface and at night.

The Imperial Japanese Navy's I-400 class submarine, the largest submarine type of WWII

During World War II, the Kriegsmarine produced many different types of U-boats as technology evolved. Most notable are type VII as the "workhorse" of the fleet which was by far the most-produced type; type IX boats were larger and specifically designed for long-range patrols. With the type XXI "Elektroboot," German sub designers finally realized that the U-boat depended on its submerged ability both for survival and lethality. The XXI featured a revolutionary streamlined hull design and propulsion system which allowed it to cruise submerged for long periods and reach unprecedented submerged speeds. It also possessed a "Schnorchel" (snorkel) which allowed the boat to run its diesels while submerged at periscope depth, reducing the possibility of detection.

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