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'Anti-submarine warfare' (ASW or in older forms 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 superior technology, experience and luck. Sophisticated sonar equipment for first detecting, and then 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.

Contents

Development of anti-submarine warfare

World War I

World War II

Battle of the Atlantic

Pacific Theatre

HMS Venturer and the development of Attack Submarines

Post-war

Modern anti-submarine warfare

Technologies used

See also

References

Development of anti-submarine warfare

World War I

During the First World War submarines were a new menace. 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. The hydrophone, an underwater microphone, was used to listen for submarines; the German U-boat, ''UC-3'', was sunk with the aid of hydrophone on April 23 1916, in company with the first depth charges. By early 1917 the Royal Navy had also developed indicator loops 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'. Seaplanes and airships were also used to patrol for submarines, with ''Fregatten-Leutnant'' Zelezny scoring the first submarine kill by aircraft on 15 September 1916. While dipping hydrophones appeared before war's end; the trials were abandoned.^[1]
However, the most effective anti-submarine measure was the introduction of convoys.

World War II

Battle of the Atlantic

:Main articles: Battle of the Atlantic (1939-1945)

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 that every major navy had a large, modern submarine fleet.
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. But sonar proved much less effective than expected, and was no use at all against submarines operating on the surface at night. The Royal Navy had continued to develop indicator loops between the wars but this was a passive form of harbour defence 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.
Allied anti-submarine tactics developed to defend convoys, aggressively hunt down U-boats 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 that faster ships were less exposed.

★ Larger convoys, which allowed more escorts to be allocated to each convoy.

★ Huge construction programmes to mass-produce the small warships needed for convoy defence, such as corvettes, frigates, destroyer escorts, and escort carriers.

★ Support groups of escort ships that could be sent to reinforce the defence 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.
Aircraft and Radar

★ Air raids on the German U-boat pens at Brest and La Rochelle. (This was singularly ineffectual.)

★ 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) to pinpoint the location of an enemy submarine from its radio transmissions.

★ The introduction of seaborne radar.

★ Airborne radar.

★ The Leigh light airborne searchlight, in conjunction with airborne radar to surprise and attack enemy submarines on the surface at night.
Weaponry

★ Torpedo countermeasures such as Foxer acoustic decoy.

★ The development of forward-throwing anti-submarine weapons such as Hedgehog and the Squid.

★ The FIDO (Mk 24 'mine') homing torpedo.
Intelligence

★ One of the best-keep Allied secrets was the breaking of enemy codes including the German Enigma code (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: however, whenever codes changed, convoy losses rose significantly. By the end of the war, the Allies were regularly breaking and reading German naval codes.
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, Consolidated B-24 Liberator, Short Sunderland and Vickers Wellington. U-boats were not defenceless, 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.
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 and criticism was aimed at the British. The Americans favoured 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 this tactic did little to reduce or contain U-boat numbers. The British view was influenced by the fact that they had had to fight the battle of the Atlantic alone for much of the war, with very limited resources. There were no spare escorts for extensive hunts, and it was only important to neutralise 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.
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 defence 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 Frederic John Walker were able to develop integrated tactics which made the deployment of hunter-killer groups a practical proposition.

Pacific Theatre

:Main articles: Imperial_Japanese_Navy_submarines#World_War_II

:Main articles: Pacific_War#The_submarine_war_in_the_Pacific

Japanese submarines pioneered many innovations, being some of the largest and longest range vessels of their type. They were armed with the deadly Type 93 torpedo, nicknamed the "Long Lance" which were by far the best torpedoes on any side during the war. However, they ending up played 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[1]. 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 mass production 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.
In 1942 and early 1943, US submarines played 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 ordinance 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 commanders that were younger and more aggressive. 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 significantly upgraded their anti-submarine technology, nor did they have the production capability or planning to withstand a tonnage war of attrition (unlike the Allies in the Atlantic).
Japanese antisubmarine forces consisted mainly of their destroyers, with sonar and depth charges. However, Japanese destroyer design emphasized surface nightfighting and torpedo delivery over anti-submarine duties. By the time they finally developed a destroyer escort which was more economical and better suited to convoy protection, it was too late to save their shipping lanes. 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 emphasised fleet action, not convoy protection, so the best ships and crews went elsewhere.^[2] 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.^[3][4]
Much later in the war, active and passive sonobuoys were developed for aircraft use.

HMS Venturer and the development of 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 U864 in the North Sea, just off Keil Harbour.
The U864 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. The U864 was separated from its escort, and, recognising that it was being trailed by an enemy submarine, submerged and began to zig-zag. This was a course of action which would normally render it invulnerable, but Jimmy Launders, the captain of HMS Venturer (P68), submerged as well and tracked it using hydrophone signals.
For several hours the cat-and-mouse hunt progressed, until Lander decided to perform the complex calculations necessary to obtain a firing solution in three dimensions. These were done manually, predicting the likely manoeveuers of the target, and a spread of four torpedoes at 17 second intervals and varying depths was fired. The U864 dived into the path of one of these, and was blown in half.
This 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 torpedos are guided, but in all other respects the essentials of submerged anti-submarine warfare have remained the same.

Post-war

Since the introduction of submarines capable of carrying ballistic missiles, great efforts have been made to counter the threat they pose. In particular, the helicopter has emerged as a prime anti-submarine platform.
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. A system like this was deployed on the GIUK gap.
Seaborne forces developed better bombs and depth charges and a range of towed sonar devices to overcome the problem of ship-mounting that required ships to pass directly over the attacked submarine. Helicopters can fly courses offset from the ships and transmit sonar information to their combat information centre. 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 HMS Venturer (P68) and U864 (see above).
A significant detection aid that has continued in service is the Magnetic Anomaly Detector (MAD), a passive device. First used in World War II, the 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 that exploited the submarine's need to perform radar sweeps and to 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 that can penetrate the ocean's surface to reach submarines wherever they might be.

Modern anti-submarine warfare

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.
Today many nations cultivate offshore seabeds of listening devices capable of tracking submarines within the coverage area of the devices. It is known to be possible to detect man-made marine noises as far as right across the southern Indian Ocean from South Africa to New Zealand.

Technologies used

There are a large number of technologies used in modern anti-submarine warfare:

★ Acoustics particularly in active and passive sonar, sonobuoys and fixed hydrophones and in the concealment of vessels. Active Sonar used in such operations is often of "mid-frequency", approximately 3.5 kHz.

★ Pyrotechnics in the use of markers, flares and explosive devices

★ Searchlights

★ Radar

★ Electronic countermeasures and Acoustic Countermeasures such as noisemakers

★ Passive 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
=Notes=
1. Price, Alfred, ''Aircraft'' versus ''the Submarine'' (William Kimber, 1973)
2. Parillo, ''Japanese Merchant Marine in World War II''. U.S. Naval Institute Press, 1993
3. Blair, Clay, Silent Victory (Vol.1), The Naval Institute Press, 2001
4. Lanning, Michael Lee (Lt. Col.), Senseless Secrets: The Failures of U.S. Military Intelligence from George Washington to the Present, Carol Publishing Group, 1995

See also

★ Modern Naval tactics

★ Naval tactics in the Age of Steam

★ Anti-submarine weapon

References

★ Price, Alfred. ''Aircraft'' versus ''the Submarine''. London: William Kimber, 1973.

★ Parillo, Mark. ''Japanese Merchant Marine in World War II''. Annapolis: U.S. Naval Institute Press, 1993.

★ Blair, Clay, ''Silent Victory ''. Philadelphia: Lippincott, 1975.

★ Lanning, Michael Lee (Lt. Col.), ''Senseless Secrets: The Failures of U.S. Military Intelligence from George Washington to the Present'', Carol Publishing Group, 1995