Convoy tactics, high frequency direction finding ( "Huff-Duff"), radar, sonar (ASDIC in Britain), depth charges, ASW spigot mortars ("hedgehog"), the cracking of the German Enigma code, the introduction of the Leigh Light, the range of escort aircraft (especially with the use of escort carriers), and the full entry of the U.S. into the war with its enormous shipbuilding capacity, all turned the tide against the Unterseeboot. In the end, the U-boat fleet suffered extremely heavy casualties, losing 743 U-boats and about 28,000 submariners (75% casualty rate). The U-boat offensive was a severe nuisance but never something that on it's own would be a cause of defeat