Anti-submarine warfare
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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.
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[edit] Development of anti-submarine warfare
[edit] 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]
[edit] World War II
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:
- Allocating ships to convoys according to speed, so that faster ships were less exposed.
- Huge construction programmes to mass-produce the small warships needed for convoy defence, such as corvettes, frigates, destroyer escorts, and escort carriers.
- The development of forward-throwing anti-submarine weapons such as Hedgehog and the Squid.
- High frequency direction finding (HF/DF) to pinpoint the location of an enemy submarine from its radio transmissions.
- The introduction of seaborne radar.
- Air raids on the German U-boat pens at Brest and La Rochelle. (This was singularly ineffectual.)
- Long-range aircraft patrols to find U-boats and either sink them or force them to submerge and lose contact with the convoy.
- Airborne radar.
- Torpedo countermeasures such as Foxer acoustic decoy.
- The Leigh light airborne searchlight, in conjunction with airborne radar to surprise and attack enemy submarines on the surface at night.
- Larger convoys, which allowed more escorts to be allocated to each convoy.
- The formation of 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.
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.
The provision of air cover was essential. At first, the British developed temporary solutions such as merchant aircraft carriers and CAM ships. These were superseded by mass-produced, relatively cheap escort carriers built by the United States and operated by the US Navy and Royal Navy.
At this point 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, this tactic did little to reduce or contain U-boat numbers. There were rarely sufficient escorts for extensive hunts, and it was certain U-boats would be found in the vicinity of convoys. In the event, the tactics were complementary, suppressing and destroying U-boats.
An important Allied advantage was provided by the breaking of German naval 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: however, whenever codes changed, convoy losses rose significantly.
German sub sinkings were significantly increased after Allied forces began combining hunter-killer tactics with newly-developed anti-submarine weapons such as Hedgehog and FIDO (Mk 24 'mine') homing torpedo.
In the Pacific, Japanese antisubmarine forces often found themselves overmatched when trying to defend both naval ships and merchant shipping from attacks by U.S. and Allied submarines. The primary weapon of attack continued to be the depth charge, dropped from aircraft or deployed by surface ships. Japanese sub detection gear was not as advanced as that of some other nations. Their primary Japanese anti-submarine weapon for most of WWII was the depth charge, and 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]
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.
Much later in the war, active and passive sonobuoys were developed for aircraft use.
[edit] 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.
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.
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.
[edit] 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 snorts 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.
[edit] Technologies used
There are a large number of technologies used in modern anti-submarine warfare:
- Acoustics particularly in active and passive sonar, sonobuoys and 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
- Passive concealment and countermeasures such as chaff and design of sound-absorbing materials to coat reflecting underwater surfaces
- Magnetic anomaly detection
- Active and (more commonly) passive infra-red detection
[edit] Notes
- ^ Price, Alfred, Aircraft versus the Submarine (William Kimber, 1973)
- ^ Parillo, Japanese Merchant Marine in World War II. U.S. Naval Institute Press, 1993
- ^ Blair, Clay, Silent Victory (Vol.1), The Naval Institute Press, 2001
- ^ Lanning, Michael Lee (Lt. Col.), Senseless Secrets: The Failures of U.S. Military Intelligence from George Washington to the Present, Carol Publishing Group, 1995
[edit] See also
[edit] 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
