<Li> Type XXIII: smaller version of the XXI used for coastal operations </Li> <Li> Midget submarines, including Biber, Hai, Molch, and Seehund </Li> <Li> Uncompleted U-boat projects </Li> <P> Advances in convoy tactics, high - frequency direction finding (referred to as "Huff - Duff"), radar, active sonar (called ASDIC in Britain), depth charges, ASW spigot mortars (also known as "hedgehog"), the intermittent cracking of the German Naval Enigma code, the introduction of the Leigh light, the range of escort aircraft (especially with the use of escort carriers), the use of mystery ships, and the full entry of the U.S. into the war with its enormous shipbuilding capacity, all turned the tide against the U-boats . In the end, the U-boat fleet suffered extremely heavy casualties, losing 793 U-boats and about 28,000 submariners (a 75% casualty rate, the highest of all German forces during the war). </P>

How many sailors on a german u boat