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Anti-submarine warfare (ASW) 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.
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 that used guns and good luck. They mainly relied on the fact that a submarine of the day was often on the surface for a range of reasons, such as charging the batteries or crossing long distances at a higher speed. The first depth charges for attacking submarines at depth were used. The first approach to protect warships with chainlink nets strung from the sides of battleships would protect against torpedoes fired from submarines. Nets were also deployed across the mouth of a harbour or naval base to stop submarines entering or to stop torpedoes that were 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. The first sonars were deployed in 1916. 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 a 'guard loops'.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 course of 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 escort construction programmes to mass-produce the small warships needed for convoy defence, such as corvettes, frigates, destroyers, and escort carriers.
- The development of new ahead-throwing anti-submarine weapons such as the 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 bases at Brest and La Rochelle.
- Long-range aircraft patrols to find German U-boats and either sink them or force them to submerge and lose contact with the convoy.
- Airborne radar.
- Torpedoes active countermeasures such as Foxer acoustic decoy.
- The Leigh light airborne searchlight which was used 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.
In the air many different aircraft from lighter-than-air airships to four-engined seaplanes and land-planes were used. Some of the more successful anti-submarine aircraft were the Lockheed Ventura, PBY Catalina, Consolidated B-24 Liberator, Short Sunderland and Vickers Wellington.
The provision of seaborne 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 by the 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 that this tactic did little to reduce or contain U-boat numbers. In the event, the tactics were complementary, suppressing and destroying U-boats.
The critical 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.
Much later, in the war, active and passive sonobuoys were developed for aircraft use.
Post-war
Since the introduction of submarines capable of carrying ballistic missiles, great efforts have been made to counter the threat they pose.In some areas of the ocean, where land forms natural barriers, long strings of sonobuoys 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 also transport homing torpedoes to positions many miles away from the detecting ships.
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 used in the 1960s was the so-called MAD detector. This used the earth's magnetosphere as a standard and detected anomalies caused by large steel vessels, such as submarines. These devices are obvious as long tail extensions from the aircraft, housing the device as far from aircraft influences as possible.
More reliance was being placed on electronic warfare detection devices that used the submarine's need to do radar sweeps and to transmit responses to radio signals from home base. As frequency surveillance and direction finding became more sophisticated these devices enjoyed some successes, but submariners learned techniques of not relying on such transmitters. Home bases then used 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 to rise 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.
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 35,000hz. Such sonar has been argued to be a cause of whale beaching.
- 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
See also
"Anti-submarine warfare"