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Anti-submarine warfare
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Anti-submarine warfare (ASW, or in older form A/S) is a branch of naval warfare that uses surface warships, aircraft, or other submarines to find, track and then 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.
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Encyclopedia
Anti-submarine warfare (ASW, or in older form A/S) is a branch of naval warfare that uses surface warships, aircraft, or other submarines to find, track and then 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 of 1776 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 propelled torpedo was invented in 1863 and launched from surface craft. The first submarine with a torpedo was the Nordenfeld II built in 1886, 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. Ships were built with an armour band as protection against torpedoes.
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 guns 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 battleships, as defense against torpedoes. 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 U15 was sunk in August 1914.
In July 1915 the British set up the civilian "Board of Invention and Research" to evaluate suggestions from the public as well as carrying out their own investigations. Some 14000 suggestions were received about combating submarines and wireless. 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 US 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 US 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 hydrophone, an underwater microphone, was used to listen for submarines; the German U-boat, UC-3, was sunk with the aid of hydrophones on April 23 1916, in company with the first depth charges. By early 1917 the Royal Navy had also developed 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 mines 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'. While dipping hydrophones appeared before war's end, the trials were abandoned.
Seaplanes and airships were also used to patrol for submarines. A number of successful attacks were made
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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 merchantman, 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:
- Mines 58
- Depth charges 30
- Submarine torpedoes 20
- Gunfire 20
- Ramming 19
- Unknown 19
- Accidents 7
- Sweeps 3
- Other (including bombs) 2
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, despite the fact every major navy had a large, modern submarine fleet, because all had fallen in the grip of a faulty Mahanian 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 charges 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 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 around the world. Sonar was far more effective and loop technology died straight after the war.
Allied anti-submarine tactics developed to defend convoys (the Royal Navy's preferred method), aggressively hunt down U-boats (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:
Vessels
- Allocating ships to convoys according to speed, so faster ships were less exposed.
- Larger convoys, which allowed escorts to better defend each convoy (the same number of total escorts for the same number of ships could actually cover smaller sectors in large convoys while it was mathematically determined by operations research that the enemy would not be able to inflict significantly more losses with the increase of targets provided the number of attacking U-Boats was unchanged).
- Huge construction programmes to mass-produce the small warships needed for convoy defense, such as corvettes, frigates, and destroyer escorts. These were more economical than using destroyers, which were better suited to fleet duties.
- Ships that could carry aircraft, such as the CAM ships, the merchant aircraft carrier, and eventually the purpose-built escort carriers.
- Support groups of escort ships that could be sent to reinforce the defense of convoys under attack. Free from the obligation to remain with the convoys, support groups could continue hunting a submerged submarine until its batteries and air supplies were exhausted and it was forced to surface.
- Hunter-killer groups, whose job was to actively seek out enemy submarines, as opposed to waiting for the convoy to come under attack. Later hunter-killer groups were centered around escort carriers.
- Huge construction programmes to mass-produce the transports and replace their losses. Once shipbuilding had ramped up to full efficiency, transports could be built faster than U-boats could sink them, playing a crucial role in the Allies winning the "Tonnage war".
Aircraft
- Air raids on the German U-boat pens at Brest and La Rochelle. (This was entirely ineffective.)
- Long-range aircraft patrols to close the Mid-Atlantic gap.
- Escort carriers to provide the convoy with air cover, as well as close the mid-Atlantic gap.
- High frequency direction finding (HF/DF), including shipborne sets, to pinpoint the location of an enemy submarine from its radio transmissions.
- The introduction of seaborne radar which could enable the detection of surfaced U-boats.
- Airborne radar.
- The Leigh light airborne searchlight, in conjunction with airborne radar to surprise and attack enemy submarines on the surface at night.
- Magnetic anomaly detection
- Diesel exhaust sniffers
- Sonobuoys
Weaponry
- Torpedo countermeasures such as the Foxer acoustic decoy.
- The development of forward-throwing anti-submarine weapons such as Hedgehog and the Squid.
- The FIDO (Mk 24 'mine') homing torpedo.
- Pattern running torpedoes
Intelligence
- One of the best kept Allied secrets was the breaking of enemy codes including some of the German Naval Enigma codes (information gathered this way was dubbed Ultra) at Bletchley Park in England. This enabled the tracking of U-boat packs to allow convoy re-routings; whenever the Germans changed their codes (and when they added a fourth rotor to the Enigma machines in 1943), convoy losses rose significantly. By the end of the war, the Allies were regularly breaking and reading German naval codes.
- To prevent the Germans from guessing that Enigma had been cracked, the British planted a false story about a special infrared camera being used to locate U-boats. The British were subsequently delighted to learn that the Germans responded by developing a special paint for submarines that exactly duplicated the optical properties of seawater.
Tactics
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. 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. 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.
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 ships and merchant aircraft carriers. 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 sinking in the latter part of the war. The British developments of ASDIC, Centimetric Radar and the Leigh Light also reached the point of being able to support U-Boat hunting towards the end of the war, while at the beginning technology 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.
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. 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. 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 Mahanian 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 escorts, 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 gradients (thermoclines). 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, COMSUBPAC, later estimated May's revelation cost the navy as many as ten submarines and 800 crewmen.
Much later in the war, active and passive sonobuoys were developed for aircraft use, together with MAD devices. Toward the end of the war, the Allies developed much better ATWs, such as 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, faced with the prospect of large numbers of Soviet submarines as capable as the Type XVIIs and XXIs, or better, new ASW weapons were essential. This led to the introduction of longer-ranged ATWs, such as Ikara, Weapon Alpha and ASROC. Nuclear submarines, even faster still, posed an even greater threat; in particular, shipborne helicopters (recalling the blimps of World War One) 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 missiles, great efforts have been made to counter the threat they pose; here, maritime patrol aircraft (as in World War Two) 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 bombs and depth charges, 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 centres. 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 submarines 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:
Sensors
- Acoustics particularly in active and passive sonar, sonobuoys and fixed hydrophones and in the reduction of radiated noise.
- Pyrotechnics in the use of markers, flares and explosive devices
- Searchlights
- Radar
- Low frequency spread-spectrum electromagnetic surface wave devices
- Active spread-spectrum magnetic techniques
- Hydrodynamic pressure wave detection
- Blue-green laser airborne and satellite LIDAR
- Neutrino (and anti-neutrino) detection of radioactive materials
- Electronic countermeasures and Acoustic Countermeasures such as noisemakers
- Passive acoustic countermeasures such as concealment and design of sound-absorbing materials to coat reflecting underwater surfaces
- Magnetic anomaly detection
- Active and (more commonly) passive infra-red detection
In modern times infra-red (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 periscopes 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 sonars is actually detrimental to sonar performance in that such sonars are reverberation limited.
Weapons
- Mines,
- Torpedoes, acoustic, wire-guided, and wake homing.
- Pulsed blue-green lasers
- Subatomic particle beam weapons
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 provide both a sensor and weapons platform as do some helicopters, with sonobuoys and/or dipping sonars as well as torpedoes. 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.
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. However, with the ending of the Cold War, some of the SOSUS arrays have been turned over to civilian use and are now used for marine research.
Attack Submarines
During World War II submarines were typically hunted on the surface and only engaged underwater if contact was maintained when the submarine dived. There was no expectation of submarines tracking other submarines underwater and engaging in 'torpedo dogfights'. This type of anti-submarine action became a possibility after the duel between HMS Venturer and U-864 in the North Sea, just off Kiel Harbour.
U-864 was carrying secret German weapons technology intended to assist Japan against US bombing raids. An ULTRA intercept alerted the Royal Navy, who dispatched a submarine to intercept the cargo. She was separated from her escort and, recognizing she was being trailed by an enemy submarine, submerged and began to zig-zag. This was a course of action which would normally render her invulnerable, but Jimmy Launders, the captain of Venturer submerged as well and tracked on hydrophones.
For several hours the cat-and-mouse hunt progressed, until Launders decided to perform the complex calculations necessary to obtain a firing solution in three dimensions. These were done manually, predicting the likely maneuvers of the target, and a spread of four torpedoes at 17 second intervals and varying depths was fired. U-864 dived into the path of one of these, and was blown in half.
This possibly was the first attempt by one submarine to sink another while both were submerged, and the only successful one ever recorded up to the present day. It was a hugely influential action in the history of anti-submarine warfare, and modern attack submarine's tactics of attempting to track ballistic missile submarines from their bases underwater are directly derived from it. Modern computers provide the calculations which were originally done manually, and modern torpedoes are guided, but in all other respects the essentials of submerged anti-submarine warfare have remained the same.
See also
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