Anti - submarine warfare Anti - submarine warfare ( ASW , or in older form A / S ) is a branch of naval warfare that uses surface warship s , aircraft , or other submarine s to find , track and deter , damage or destroy enemy submarines . Like many forms of warfare , successful anti - submarine warfare depends on a mix of sensor and weapon technology , training , experience and luck . Sophisticated sonar equipment for first detecting , then classifying , locating and tracking the target submarine is a key element of ASW . To destroy submarines both the torpedo and mine are used , launched from air , surface and underwater platforms . Other means of destruction have been used in the past but are now obsolete . ASW also involves protecting friendly ships . History . The first attacks on a ship by an underwater vehicle are generally believed to have been during the American Revolutionary War , using what would now be called a naval mine but what then was called a torpedo , though various attempts to build submarines had been made before this . The first self - propelled torpedo was invented in 1863 and launched from surface craft . The first submarine with a torpedo was " Nordenfelt I " built in 1884 - 1885 , though it had been proposed earlier . In the Russo - Japanese War of 1904 - 5 , the submarine was a significant threat . By the start of the First World War nearly 300 submarines were in service . Some warships were fitted with an armoured belt as protection against torpedoes . There were , however , no means to detect submerged U - boats , and attacks on them were limited at first to efforts to damage their periscopes with hammers . The Royal Navy torpedo establishment , HMS " Vernon " , studied explosive grapnel sweeps ; these sank four or five U - boats in the First World War . A similar approach featured a string of charges on a floating cable , fired electrically ; an unimpressed Louis Mountbatten considered any U - boat sunk by it deserved to be . Also tried were dropping hand - thrown guncotton bombs . The Lance Bomb was developed , also ; this featured a cone - shaped steel drum on a shaft , intended to be thrown at a submarine . Firing Lyddite shells , or using trench mortar s , was tried . Use of nets to ensnare U - boats was also examined , as was a destroyer , HMS " Starfish " , fitted with a spar torpedo . To attack at set depths , aircraft bombs were attached to lanyards which would trigger their charges ; a similar idea was a guncotton charge in a lanyarded can ; two of these lashed together became known as the Depth Charge Type A . Problems with the lanyards tangling and failing to function led to the development of a chemical pellet trigger as the Type B . These were effective at a distance of around . The best concept arose in a 1913 RN Torpedo School report , describing a device intended for countermining , a " dropping mine " . At Admiral John Jellicoe ' s request , the standard Mark II mine was fitted with a hydrostatic pistol ( developed in 1914 by Thomas Firth & Sons of Sheffield ) preset for firing , to be launched from a stern platform . Weighing , and effective at , the " cruiser mine " was a potential hazard to the dropping ship , but was also on the right track . World War I . During the First World War , submarines were a major menace . They operated in the Baltic , North Sea , Black Sea and Mediterranean as well as the North Atlantic . Previously they had been limited to relatively calm and protected waters . The vessels used to combat them were a range of small , fast surface ships using gun s and good luck . They mainly relied on the fact a submarine of the day was often on the surface for a range of reasons , such as charging batteries or crossing long distances . The first approach to protect warships was chainlink nets strung from the sides of battleship s , as defense against torpedo es . Nets were also deployed across the mouth of a harbour or naval base to stop submarines entering or to stop torpedoes fired against ships . British warships were fitted with a ram with which to sink submarines , and " U - 15 " was sunk in August 1914 . RN in June 1915 began operational trials of the Type D depth charge , with a charge of TNT ( amatol , as TNT supplies became critical ) and a hydrostatic pistol , firing at either , and believed to be effective at a distance of ; the Type D * , with a charge , was offered for smaller ships . In July 1915 , the British Admiralty set up the Board of Invention and Research to evaluate suggestions from the public as well as carrying out their own investigations . Some 14 , 000 suggestions were received about combating submarines . In December 1916 , the RN set up its own Anti - Submarine Division ( from which came the term " Asdics " ) but relations with the BIR were poor . After 1917 most ASW work was carried out by ASD . In the U . S . , a Naval Consulting Board was set up in 1915 to evaluate ideas . After American entry into the war in 1917 , they encouraged work on submarine detection . The U . S . National Research Council , a civilian organization , brought in British and French experts on underwater sound to a meeting with their American counterparts in June 1917 . In October 1918 , there was a meeting in Paris on " supersonics " , a term used for echo - ranging , but the technique was still in research by the end of the war . The first recorded sinking of a submarine by depth charge was " U - 68 " , sunk by Q - ship HMS " Farnborough " off Kerry , Ireland 22 March 1916 . By early 1917 , the Royal Navy had also developed indicator loop s which consisted of long lengths of cables lain on the seabed to detect the magnetic field of submarines as they passed overhead . At this stage they were used in conjunction with controlled mine s which could be detonated from a shore station once a ' swing ' had been detected on the indicator loop galvanometer . Indicator loops used with controlled mining were known as ' guard loops ' . By July 1917 , depth charges had developed to the point settings of between were possible . This design would remain mainly unchanged through the end of World War II . While dipping hydrophones appeared before war ' s end , the trials were abandoned . Seaplane s and airship s were also used to patrol for submarines . A number of successful attacks were made but the main value of air patrols was in driving the U - boat to submerge , rendering it virtually blind and immobile . However , the most effective anti - submarine measure was the introduction of escorted convoys , which reduced the loss of ships entering the German ' s War Zone around the British Isles from 25 % to less than 1 % . To attack submerged boats a number of anti - submarine weapons were derived , including the sweep with a contact - fused explosive . Bombs were dropped by aircraft and depth charge attacks were made by ships . Initially these were simply dropped off the back of a ship but then depth charge throwers were introduced . The Q - ship , a warship disguised as a merchant ship , was used to attack surfaced U - boats while the " R1 " was the first ASW submarine . A major contribution was the interception of German submarine radio signals and breaking of their code by " Room 40 " of the Admiralty . 178 of the 360 U - boats were sunk during the war , from a variety of ASW methods : Inter - war period . This period saw the development of active sonar ( ASDIC ) and its integration into a complete weapons system by the British , as well as the introduction of radar . During the period there was a great advance due to the introduction of electronics for amplifying , processing and display of signals . In particular the " range recorder " was a major step that provided a memory of target position . New materials for sound projectors were developed . Both the Royal Navy and the US Navy fitted their destroyers with ASDIC . In 1928 a small escort ship was designed and plans made to arm trawlers and to mass - produce ASDIC sets . Depth sounders were developed that allowed measurement by moving ships and an appreciation obtained of the properties of the ocean affecting sound propagation . The bathythermograph was invented in 1937 , which was soon fitted to ASW ships . There were few major advances in weapons . However , the performance of torpedoes continued to improve . World War II . Battle of the Atlantic . During the Second World War , the submarine menace revived , threatening the survival of island nations like Britain and Japan which were particularly vulnerable because of their dependence on imports of food , oil , and other vital war materials . Despite this vulnerability , little had been done to prepare sufficient anti - submarine forces or develop suitable new weapons . Other navies were similarly unprepared , even though every major navy had a large , modern submarine fleet , because all had fallen in the grip of Mahan ian doctrine which held " guerre de course " could not win a war . At the beginning of the war , most navies had few ideas how to combat submarines beyond locating them with sonar and then dropping depth charge s on them . Sonar proved much less effective than expected , and was no use at all against submarines operating on the surface , as U - boats routinely did at night . The Royal Navy had continued to develop indicator loops between the wars but this was a passive form of harbour defense that depended on detecting the magnetic field of submarines by the use of long lengths of cable lain on the floor of the harbour . Indicator loop technology was quickly developed further and deployed by the US Navy in 1942 . By then there were dozens of loop stations around the world . Sonar was far more effective and loop technology died straight after the war . The use and improvement of radar technology was one of the most important proponents in the fight against submarines . Locating submarines was the first step in being able to defend against and destroy them . Throughout the war , Allied radar technology was much better than their German counterparts . German U - Boats struggled to have proper radar detection capabilities and keep up with the successive generations of Allied airborne radar . The first generation of Allied airborne radar used a 1 . 7 meter wavelength and had a limited range . By the second half of 1942 the " Metox " radar detector was used by U - boats to give some warning from airborne attack . In 1943 the Allies began to deploy aircraft equipped with new cavity magnetron - based 10 - centimeter wavelength radar ( ASV III ) , which was undetectable by " Metox " , in sufficient numbers to yield good results . Eventually the " Naxos " radar detector was fielded that could detect 10 - cm wavelength radar , but it had a very short range and only gave a U - Boat limited time to dive . From 1943 - 1945 radar equipped aircraft would account for the bulk of Allied kills against U - Boats . Allied anti - submarine tactics developed to defend convoys ( the Royal Navy ' s preferred method ) , aggressively hunt down U - boat s ( the U . S . Navy approach ) , and to divert vulnerable or valuable ships away from known U - boat concentrations . During the Second World War , the Allies developed a huge range of new technologies , weapons and tactics to counter the submarine danger . These included : Many different aircraft from airships to four - engined sea - and land - planes were used . Some of the more successful were the Lockheed Ventura , PBY ( Catalina or Canso , in British service ) , Consolidated B - 24 Liberator ( VLR Liberator , in British service ) , Short Sunderland , and Vickers Wellington . As more patrol planes became equipped with radar , U - Boats began to be surprised at night by aircraft attacks . U - Boats were not defenseless , since their deck guns were a very good anti - aircraft weapon . They claimed 212 Allied aircraft shot down for the loss of 168 U - boats to air attack . The German naval command struggled to find a solution to the aircraft attacks . ' U - Flak ' submarines , equipped with extra anti - aircraft weapons , were tried unsuccessfully . At one point in the war , there was even a ' shoot back order ' requiring U - boats to stay on the surface and fight back , in the absence of any other option . Some commanders started charging batteries during the day to gain more warning from air attack , and perhaps gain time to submerge . One solution was the snorkel , which allowed a U - boat to stay submerged and still charge its batteries . A snorkel made a U - boat more survivable and losses to aircraft went down . However the low snorkeling speeds of 5 to 6 knots greatly limited the mobility of the U - Boats . The provision of air cover was essential . The Germans at the time had been using their Focke - Wulf Fw 200 " Condor " long range aircraft to attack shipping and provide reconnaissance for U - boats , and most of their sorties occurred outside the reach of existing land - based aircraft that the Allies had ; this was dubbed the " Mid - Atlantic gap " . At first , the British developed temporary solutions such as CAM ship s and merchant aircraft carrier s . These were superseded by mass - produced , relatively cheap escort carriers built by the United States and operated by the US Navy and Royal Navy . There was also the introduction of long - ranged patrol aircraft . Many U - boats feared aircraft , as the mere presence would often force them to dive , disrupting their patrols and attack runs . There was a significant difference in the tactics of the two navies . The Americans favored aggressive hunter - killer tactics using escort carriers on search and destroy patrols , whereas the British preferred to use their escort carriers to defend the convoys directly . The American view was that defending convoys did little to reduce or contain U - boat numbers , while the British were constrained by having to fight the battle of the Atlantic alone for the early part of the war with very limited resources . There were no spare escorts for extensive hunts , and it was only important to neutralize the U - boats which were found in the vicinity of convoys . The survival of convoys was critical , and if a hunt missed its target a convoy of strategic importance could be lost . The British also reasoned that since submarines sought convoys , convoys would be a good place to find submarines . Once America joined the war , the different tactics were complementary , both suppressing the effectiveness of and destroying U - boats . The increase in Allied naval strength allowed both convoy defense and hunter - killer groups to be deployed , and this was reflected in the massive increase in U - boat kills in the latter part of the war . The British developments of centimetric radar and the Leigh Light , as well as increased numbers of escorts , reached the point of being able to support U - boat hunting towards the end of the war , while earlier on , the advantage was definitely on the side of the submarine . Commanders such as F . J . " Johnnie " Walker of the Royal Navy were able to develop integrated tactics which made the deployment of hunter - killer groups a practical proposition . Walker developed a " creeping attack " technique , where one destroyer would track the U - boat while another attacked . Often U - boats would turn and increase speed to spoil the depth charge attack , as the escort would lose sonar contact as it steamed over the submarine . With the new tactic , one escort vessel would attack while another would track the target . Any course or depth change could be relayed to the attacking destroyer . Once a U - boat was caught , it was very difficult to escape . Since Hunter - Killer groups were not limited to convoy escort , they could continue an attack until a U - Boat was destroyed or had to surface from damage or lack of air . The earliest recorded sinking of one submarine by another while both were submerged occurred in 1945 when HMS " Venturer " torpedoed " U - 864 " off the coast of Norway . The captain of " Venturer " tracked " U - 864 " on hydrophones for several hours and manually calculated a three - dimensional firing solution before launching four torpedoes . Mediterranean . Italian and German submarines operated in the Mediterranean on the Axis side while French and British submarines operated on the side of the Allies . The German Navy sent 62 U - Boats to the Mediterranean , all were lost in combat or scuttled . German subs first had to pass through the highly defended Straits of Gibraltar , where 9 were sunk , and a similar number damaged so severely they had to limp back to base . The Mediterranean is calmer than the Atlantic , which made escape for U - Boats more difficult and was ringed with Allied air bases . Similar ASW methods were used as in the Atlantic but an additional menace was the use by Italians of midget submarines . Pacific Theatre . Japanese submarines pioneered many innovations , being some of the largest and longest range vessels of their type and were armed with the Long Lance torpedo . However , they ended up playing little impact , especially in the latter half of the war . Instead of commerce raiding like their U - boat counterparts , they followed the Mahan ian doctrine , serving in offensive roles against warships , which were fast , maneuverable and well - defended compared to merchant ships . In the early part of the Pacific War , Japanese subs scored several tactical victories , including two successful torpedo strikes on the US fleet carriers " Saratoga " and " Wasp " , the latter of which was sunk abandoned and scuttled as a result of the attack . However , these are mostly considered incidental successes , due to limited resources in the US Navy at the time . Once the US was able to ramp up construction of destroyers and destroyer escort s , as well as bringing over highly effective anti - submarine techniques learned from the British from experiences in the Battle of the Atlantic , they would take a significant toll on Japanese submarines , which tended to be slower and could not dive as deep as their German counterparts . Japanese submarines , in particular , never menaced the Allied merchant convoys and strategic shipping lanes to any degree that German U - boats did . One major advantages the Allies had was the breaking of the Japanese " Purple " code by the US , so allowing friendly ships to be diverted from Japanese submarines and allowing Allied submarines to intercept Japanese forces . In 1942 and early 1943 , US submarines posed little threat to Japanese ships , whether warships or merchant ships . They were initially hampered by poor torpedoes , which often failed to detonate on impact , ran too deep , or even ran wild . As the US submarine menace was slight in the beginning , Japanese commanders became complacent and as a result did not invest heavily into ASW measures or upgrade their convoy protection to any degree to what the Allies in the Atlantic did . Often encouraged by the Japanese not placing a high priority on the Allied submarine threat , US skippers were relatively complacent and docile compared to their German counterparts , who understood the " life and death " urgency in the Atlantic . However , US Vice Admiral Charles A . Lockwood pressured the ordnance department to replace the faulty torpedoes ; famously when they initially ignored his complaints , he ran his own tests to prove the torpedoes ' unreliability . He also cleaned out the " deadwood " , replacing many cautious or unproductive submarine skippers with younger ( somewhat ) and more aggressive commanders . As a result , in the latter half of 1943 , US subs were suddenly sinking Japanese ships at a dramatically higher rate , scoring their share of key warship kills and accounting for almost half of the Japanese merchant fleet . Japanese naval command was caught off guard , as they had not the anti - submarine technology or doctrine , nor did the production capability to withstand a tonnage war of attrition , nor did they develop the organizations needed ( unlike the Allies in the Atlantic ) . Japanese antisubmarine forces consisted mainly of their destroyers , with sonar and depth charges . However , Japanese destroyer design , tactics , training , and doctrine emphasized surface nightfighting and torpedo delivery ( necessary for fleet operations ) over anti - submarine duties . By the time Japan finally developed a destroyer escort which was more economical and better suited to convoy protection , it was too late ; coupled to incompetent doctrine and organization , it could have had little effect in any case . Late in the war , the Japanese Army and Navy used Magnetic Anomaly Detector ( MAD ) gear in aircraft to locate shallow submerged submarines . The Japanese Army also developed two small aircraft carriers and Ka - 1 autogyro aircraft for use in an antisubmarine warfare role . The Japanese depth charge attacks by its surface forces initially proved fairly unsuccessful against U . S . fleet submarines . Unless caught in shallow water , a U . S . submarine commander could normally escape destruction , sometimes using temperature gradient s ( thermocline s ) . Additionally , IJN doctrine emphasized fleet action , not convoy protection , so the best ships and crews went elsewhere . Moreover , during the first part of the war , the Japanese tended to set their depth charges too shallow , unaware U . S . submarines could dive below 150 feet ( 45m ) . Unfortunately , this deficiency was revealed in a June 1943 press conference held by U . S . Congressman Andrew J . May , and soon enemy depth charges were set to explode as deep as 250 feet ( 76m ) . Vice Admiral Charles A . Lockwood , , later estimated May ' s revelation cost the navy as many as ten submarines and 800 crewmen . Much later in the war , active and passive sonobuoy s were developed for aircraft use , together with MAD devices . Toward the end of the war , the Allies developed much better ATWs , such as Squid , Limbo and Mousetrap , in the face of new , much better German submarines , such as the Type XVII and Type XXI . British and Dutch submarines also operated in the Pacific , mainly against coastal shipping . Post - war . In the immediate postwar period , the innovations of the late war U - boats were quickly adopted by the major navies . The United States studied the German Type XXI and used the information to modify WW2 fleet boats with the GUPPY program . The Soviets launched new submarines patterned on Type XXIs , the Whiskey and Zulu classes . Britain tested hydrogen peroxide fuels in " Meteroite " , " Excalibur " , and " Explorer " , with less success . To deal with these more capable submarines new ASW weapons were essential . This new generation of diesel electric submarine , like the Type XXI before it , had no deck gun and a streamlined hull tower for greater underwater speed , as well as more storage battery capacity than a comparable WW2 submarine ; in addition , they recharged their batteries using a snorkel and could complete a patrol without surfacing . This led to the introduction of longer - ranged ATWs , such as Ikara , Weapon Alpha , ASROC , and to improved homing torpedoes . Nuclear submarine s , even faster still , and without the need to snorkel to recharge batteries , posed an even greater threat ; in particular , shipborne helicopter s ( recalling the blimps of World War I ) have emerged as essential anti - submarine platforms . A number of torpedo carrying missiles were developed , combining ahead - throwing capability ( or longer - range delivery ) with torpedo homing . Since the introduction of submarines capable of carrying ballistic missile s , great efforts have been made to counter the threat they pose ; here , maritime patrol aircraft ( as in World War II ) and helicopters have had a large role . The use of nuclear propulsion and streamlined hulls has resulted in submarines with high speed capability and increased maneuverability , as well as low " indiscretion rates " when a submarine is exposed on the surface . This has required changes both to the sensors and weapons used for ASW . Because nuclear submarines were noisy , there was an emphasis on passive sonar detection . The torpedo became the main weapon ( though nuclear depth charges were developed ) . The mine continued to be an important ASW weapon . In some areas of the ocean , where land forms natural barriers , long strings of sonobuoys , deployed from surface ships or dropped from aircraft , can monitor maritime passages for extended periods . Bottom mounted hydrophones can also be used , with land based processing . A system like this SOSUS was deployed by the USA in the GIUK gap and other strategically important places . Airborne ASW forces developed better bomb s and depth charge s , while for ships and submarines a range of towed sonar devices were developed to overcome the problem of ship - mounting . Helicopters can fly courses offset from the ships and transmit sonar information to their combat information centre s . They can also drop sonobuoys and launch homing torpedoes to positions many miles away from the ships actually monitoring the enemy submarine . Submerged submarines are generally blind to the actions of a patrolling aircraft until it uses active sonar or fires a weapon , and the aircraft ' s speed allows it to maintain a fast search pattern around the suspected contact . Increasingly anti - submarine submarines , called attack submarine s or hunter - killers , became capable of destroying , particularly , ballistic missile submarines . Initially these were very quiet diesel - electric propelled vessels but they are more likely to be nuclear - powered these days . The development of these was strongly influenced by the duel between " Venturer " and " U - 864 " . A significant detection aid that has continued in service is the Magnetic Anomaly Detector ( MAD ) , a passive device . First used in World War II , MAD uses the Earth ' s magnetosphere as a standard , detecting anomalies caused by large metallic vessels , such as submarines . Modern MAD arrays are usually contained in a long tail boom ( fixed - wing aircraft ) or an aerodynamic housing carried on a deployable tow line ( helicopters ) . Keeping the sensor away from the plane ' s engines and avionics helps eliminate interference from the carrying platform . At one time , reliance was placed on electronic warfare detection devices exploiting the submarine ' s need to perform radar sweeps and transmit responses to radio messages from home port . As frequency surveillance and direction finding became more sophisticated , these devices enjoyed some success . However , submariners soon learned not to rely on such transmitters in dangerous waters . Home bases can then use extremely low frequency radio signals , able to penetrate the ocean ' s surface , to reach submarines wherever they might be . Modern warfare . The military submarine is still a threat , so ASW remains a key to obtaining sea control . Neutralizing the SSBN has been a key driver and this still remains . However , non - nuclear - powered submarines have become increasingly important . Though the diesel - electric submarine continues to dominate in numbers , several alternative technologies now exist to enhance the endurance of small submarines . Previously the emphasis had been largely on deep water operation but this has now switched to littoral operation where ASW is generally more difficult . Current technologies . There are a large number of technologies used in modern anti - submarine warfare : In modern times forward looking infrared ( FLIR ) detectors have been used to track the large plumes of heat that fast nuclear - powered submarines leave while rising to the surface . FLIR devices are also used to see periscope s or snorkels at night whenever a submariner might be incautious enough to probe the surface . The active sonar used in such operations is often of " mid - frequency " , approximately 3 . 5 kHz . Because of the quietening of submarines , resulting in shorter passive detection ranges , there has been interest in low frequency active for ocean surveillance . However , there have been protests about the use of medium and low frequency high - powered active sonar because of its effects on whales . Others argue the high power level of some LFA ( Low Frequency Active ) sonars is actually detrimental to sonar performance in that such sonars are reverberation limited . Platforms . Satellites have been used to image the sea surface using optical and radar techniques , and it is claimed these might be used for indirect detection of submarines , as could thermal imaging . Fixed - wing aircraft , such as the P - 3 Orion provide both a sensor and weapons platform as do some helicopters like the SH - 60 Seahawk , with sonobuoys and / or dipping sonars as well as aerial torpedo es . In other cases the helicopter has been used solely for sensing and rocket delivered torpedoes used as the weapon . Surface ships continue to be a main ASW platform because of their endurance , now having towed array sonars . Submarines are the main ASW platform because of their ability to change depth and their quietness , which aids detection . In the future unmanned vehicles may be used in the ASW role . In early 2010 DARPA began funding the ACTUV programme to develop a semi - autonomous ocean going unmanned naval vessel . Today some nations have seabed listening devices capable of tracking submarines . It is known to be possible to detect man - made marine noises across the southern Indian Ocean from South Africa to New Zealand . Some of the SOSUS arrays have been turned over to civilian use and are now used for marine research .