Anti-submarine mortar

Anti-submarine mortars are artillery pieces deployed on ships for the purpose of sinking submarines by a direct hit with a small explosive charge. They are often larger versions of the mortar used by infantry and fire a projectile in relatively the same manner. They were created during World War II as a development of the depth charge and work on the same principle.

Anti-submarine warfare did not become an issue of great concern until World War I, when Germany used submarines in an attempt to strangle British shipping in the Atlantic Ocean and elsewhere. The earliest way to counter a submarine was in the form of depth charges, which were large canisters filled with explosives, rolled off the back of a ship and detonated by a hydrostatic fuse. Depth charges served well throughout World War I but were not without flaws. A ship had to pass directly over a submarine to score an effective hit, because of this depth charges were dropped in lines instead of more effective clusters and could only be carried in ships fast enough to avoid the concussion of the explosion. The depth charges were also not as effective as one might think at sinking a submarine - only a very close detonation would sink a submarine, and the problems of scoring a direct hit meant that a submarine was more often damaged then destroyed by depth charges.

After World War I depth charge throwers were developed, which could hurl depth charges some 100 feet (30 m) off the side of a ship. These were a significant improvement over the old method, permitting the use of large 'patterns' of up to 10 depth charges from the throwers and stern depth charge rails used together. However they still required a ship to pass very close to a submarine, which entailed loss of sonar (ASDIC) contact during the final stages of the approach. Submarines could and did use this dead interval to take evasive action.

It was the British who developed the first anti-submarine mortars. Several versions appeared in 1917, most notably the BL 7.5 inch Naval Howitzer; however the anti-submarine mortar did not become a truly successful weapon until the advent of the multi-barrelled Hedgehog 25 years later.

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