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Japanese submarines pioneered many innovations, being some of the largest and longest range vessels of their type and were armed with the Type 95 torpedo. However, they ended up having 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 three successful torpedo strikes on the US fleet carriers USS Saratoga and USS Wasp, the latter of which was abandoned and scuttled as a result of the attack.[34]

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 advantage 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. Japan's naval command was caught off guard; Japan had neither the anti-submarine technology or doctrine, nor the production capability to withstand a tonnage war of attrition, nor did she 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,[c] 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, while the Navy developed and introduced the Kyushu Q1W anti-submarine bomber into service in 1945.

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.[35] 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.[36][37]

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 better forward-throwing weapons, such as Mousetrap and Squid, 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, the innovations of the late war U-boats were quickly adopted by the major navies. Both the United Kingdom and The United States studied the German Type XXI and used the information to modify WW2 fleet boats, the US with the GUPPY program and the UK with the Overseas Patrol Submarines Project.[38] The Soviets launched new submarines patterned on Type XXIs, the Whiskey and Zulu classes. Britain also tested hydrogen peroxide fuels in Meteorite, Excalibur, and Explorer, with less success.

To deal with these more capable submarines new ASW weapons were essential. This new generation of diesel electric submarine, like the Type XXI before it, had no deck gun and a streamlined hull tower for greater underwater speed, as well as more storage battery capacity than a comparable WW2 submarine; in addition, they recharged their batteries using a snorkel and could complete a patrol without surfacing.[39] This led to the introduction of longer-ranged forward-throwing weapons, such as Weapon Alpha, Limbo, RBU-6000, and of improved homing torpedoes. Nuclear submarines, even faster still, and without the need to snorkel to recharge batteries, posed an even greater threat; in particular, shipborne helicopters (recalling the blimps of World War I)[18] have emerged as essential anti-submarine platforms. A number of torpedo carrying missiles such as ASROC and Ikara 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 II) 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 US in the GIUK gap and other strategically important places.

Airborne ASW forces developed better bombs and periscope depth – the Royal Navy, mostly operating from Malta, lost 41 submarines to the opposing German and Italian forces, including HMS Upholder and HMS Perseus.

Japanese submarines pioneered many innovations, being some of the largest and longest range vessels of their type and were armed with the Type 95 torpedo. However, they ended up having 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 three successful torpedo strikes on the US fleet carriers USS Saratoga and USS Wasp, the latter of which was abandoned and scuttled as a result of the attack.[34]

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 advantage the Allies had was the breaking

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 advantage 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. Japan's naval command was caught off guard; Japan had neither the anti-submarine technology or doctrine, nor the production capability to withstand a tonnage war of attrition, nor did she 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,[c] 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, while the Navy developed and introduced the Kyushu Q1W anti-submarine bomber into service in 1945.

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.[35] 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.[36][37]

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 better forward-throwing weapons, such as Mousetrap and Squid, 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.

In the immediate postwar period, the innovations of the late war U-boats were quickly adopted by the major navies. Both the United Kingdom and The United States studied the German Type XXI and used the information to modify WW2 fleet boats, the US with the GUPPY program and the UK with the Overseas Patrol Submarines Project.[38] The Soviets launched new submarines patterned on Type XXIs, the Whiskey and Zulu classes. Britain also tested hydrogen peroxide fuels in Meteorite, Excalibur, and Explorer, with less success.

To deal with these more capable submarines new ASW weapons were essential. This new generation of diesel electric submarine, like the Type XXI before it, had no deck gun and a streamlined hull tower for greater underwater speed, as well as more storage battery capacity than a comparable WW2 submarine; in add

To deal with these more capable submarines new ASW weapons were essential. This new generation of diesel electric submarine, like the Type XXI before it, had no deck gun and a streamlined hull tower for greater underwater speed, as well as more storage battery capacity than a comparable WW2 submarine; in addition, they recharged their batteries using a snorkel and could complete a patrol without surfacing.[39] This led to the introduction of longer-ranged forward-throwing weapons, such as Weapon Alpha, Limbo, RBU-6000, and of improved homing torpedoes. Nuclear submarines, even faster still, and without the need to snorkel to recharge batteries, posed an even greater threat; in particular, shipborne helicopters (recalling the blimps of World War I)[18] have emerged as essential anti-submarine platforms. A number of torpedo carrying missiles such as ASROC and Ikara 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 II) 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 US 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 HMS Venturer and U-864.[citation needed]

A significant detection aid that has continued in service is the Magnetic Anomaly Detector (MAD), a passive device. First used during the Second World War, 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.

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.

Anti-submarine warfare technologies

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

Sensors