Surface ships usually used ASDIC (sonar) to detect submerged submarines. However, to deliver its depth charges a ship had to pass over the contact to drop them over the stern; sonar contact would be lost just before attack, rendering the hunter blind at the crucial moment. This gave a skilful submarine commander an opportunity to take evasive action. In 1942 the forward-throwing "hedgehog" mortar, which fired a spread salvo of bombs with contact fuzes at a "stand-off" distance while still in sonar contact, was introduced, and proved to be effective.
Surface ships usually used .ASDIC (sonar) to detect submerged submarines However, to deliver its depth charges a ship had to pass over the contact to drop them over the stern; sonar contact would be lost just before attack, rendering the hunter blind at the crucial moment. This gave a skilful submarine commander an opportunity to take evasive action. In 1942 the forward-throwing "hedgehog" mortar, which fired a spread salvo of bombs with contact fuzes at a "stand-off" distance while still in sonar contact, was introduced, and proved to be effective.