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.
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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 WWI 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.
During WW2 more effective systems, and the first Anti-submarine mortars, were developed. These all had the common characteristic of throwing the charge(s) ahead of the attacking vessel, while it was still in sonar contact. The first was the famous "Hedgehog", which consisted of 24 small mortar rounds, each one 7 inches (180 mm) in diameter and weighing 65 pounds with a 35-pound warhead. Each projectile had a range of about 250 yards (230 m) and was fired in a circular pattern in front of a ship. While the warhead on a Hedgehog was much smaller than that of a depth charge it scored three times as many kills then its predecessors. This was due to the use of a contact fuse on the projectile, which would only detonate on impact with a target. Since the projectile would only explode on a hit the long periods of sonar "blackout" from the blast and turbulence of a conventional depthcharge explosion were eliminated. The Hedgehog was followed by the Squid three-barreled depth charge mortar, the two weapons remaining in service alongside each other in the later stages of WW2. Squid fired 390-lb depth charges to a range of 250 metres.
The homing torpedo has largely replaced the anti-submarine mortar in naval combat, although several examples still exist. The British Limbo system, with three gyro-stabilized barrels, fires 350-pound projectiles to a range of 1,000 yards (910 m). It remained in service with many British and Commonwealth navies until the 1980s. The Bofors anti-submarine mortar, which uses a rocket motor in place of an explosive charge, is still in use with the Swedish Navy. Due to the poor sonar conditions of the Baltic Sea the mortar still retains a place next to torpedoes. It has two or four barrels and fires a 550-pound projectile up to 3,800 yards (3,500 m). The former Soviet Navy (and by extension, the Russian Navy) is the largest user of anti-submarine mortars. Keeping with the Soviet idea that weapons should be simple and cheap, several versions of rocket-propelled anti-submarine mortars were developed. There were also conducted trials of destroying oncoming torpedoes with anti-submarine mortars. The most common is the RBU-6000, which fires twelve 160-pound projectiles in a horseshoe pattern up to 6,500 yards (5,900 m) away. There was also a more extreme version, the nuclear SUW-N-1, though this is more technically an anti-submarine rocket. It had anti-surface and land-attack uses as well.