Anti-submarine warfare
If you were searching for A/S, you might have meant aksjeselskap, a Norwegian stock company form.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.
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
|
Officers on the bridge of a destroyer on convoy escort duties keep a sharp look out for enemy submarines during the Battle of the Atlantic, October 1941 |
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.
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A depth charge being loaded onto a depth charge thrower on board the corvette HMS Dianthus, 14 August 1942 |
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.
|
Hedgehog, a 24 barrelled anti-submarine mortar, mounted on the forecastle of the destroyer HMS Westcott |
* 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.
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A Leigh Light used for spotting U-boats on the surface at night fitted to a Liberator aircraft of Royal Air Force Coastal Command, 26 February 1944 |
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A Vought SB2U scout bomber from USS Ranger (CV-4) flies anti-submarine patrol over the Convoy WS12, en route to Capetown, 27 November 1941 |
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
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An MH-60R conducts an airborne low frequency sonar (ALFS) operation during testing and evaluation. |
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.
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*
Modern Naval tactics*
Naval tactics in the Age of Steam*
Anti-submarine weapon
