Anti - submarine mortar Anti - submarine mortars are artillery pieces deployed on ships for the purpose of sinking submarine s 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 . Beginnings . 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 charge s , 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 . Early anti - submarine mortars . After WWI depth charge throwers were developed , which could hurl depth charges some 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 in diameter and weighing 65 pounds with a 35 - pound warhead . Each projectile had a range of about 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 . Modern uses . 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 . It remained in service with many British and Commonwealth navies until the 1980s . The Bofors anti - submarine rocket launcher was used until 1980 with the Swedish Navy . It had two or four barrels and fireds a 550 - pound projectile up to . Due to the poor sonar conditions of the Baltic Sea mortars or rocket projectors still retain a place next to torpedoes . 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 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 .