Anti-submarine warfare

• 1. Introduction

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  Anti-submarine warfare began with men leaning over the sides of ships with hammers, trying to knock off the periscopes of German U-boats. That was the state of the art in 1914. By the Second World War, Allied forces were breaking encrypted naval codes, deploying aircraft equipped with radar that could detect a surfaced submarine at night, and using listening devices so sensitive they could track submarines across entire ocean basins. The story of how that gap closed reveals as much about the ingenuity of engineers and codebreakers as it does about the desperation of naval commanders who watched their ships disappear beneath the waves.

• 2. Hammers and Guncotton

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  Robert Whitehead, a British engineer, invented the first practical self-propelled torpedo in 1866. Within decades, the weapon had spread to every major navy, and the submarine capable of delivering it had followed close behind. The Nordenfelt I, the first submarine fitted with a torpedo, was built in 1884-1885. By the time the Russo-Japanese War broke out in 1904, all the large navies except Germany's had acquired submarines.

  Still, those early boats were fragile and limited enough that naval planners classified them as experimental rather than operational. That view changed decisively in the First World War. German U-boats ranged as far as the North Atlantic, striking targets far beyond the protected coastal waters earlier submarines had been confined to.

  The early British response was improvised to the point of absurdity. Explosive grapnel sweeps were tried; the Royal Navy's torpedo establishment HMS Vernon found they sank perhaps four or five U-boats across the entire war. A floating cable strung with 70-lb charges, fired electrically, inspired Admiral Edward Evans to remark that any U-boat sunk by it deserved its fate. Sailors threw hand-grenades called guncotton bombs, each weighing 18.5 lb. A device called the Lance Bomb mounted a 35-40 lb cone of steel on a five-foot shaft, designed to be flung at a submarine.

  The idea that actually mattered came from a 1913 Royal Navy Torpedo School report proposing a countermining device it called a "dropping mine." At the request of Admiral John Jellicoe, a standard Mark II mine was fitted with a hydrostatic pistol developed by Thomas Firth and Sons of Sheffield in 1914, preset to fire at 45 feet, and launched from a stern platform. That device weighed 1,150 lb and was effective at 100 feet. It was also, if the ship dropped it too slowly, a potential hazard to the crew releasing it. From that awkward ancestor came the depth charge.

• 3. The Convoy and the Code Room

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  During June 1915, the Royal Navy began operational trials of the Type D depth charge, a 300-lb charge of TNT with a hydrostatic pistol that could be set to fire at 40 or 80 feet. The first recorded sinking of a submarine by depth charge was U-68, destroyed by a Q-ship off County Kerry, Ireland, on the 22nd of March 1916. By July 1917, depth charge settings had been extended to a range of 50-200 feet, a design that would persist largely unchanged through the end of the Second World War.

  In July 1915, the British Admiralty set up the Board of Invention and Research to evaluate public suggestions as well as pursue its own lines of inquiry. Some 14,000 proposals were received on the question of combating submarines. In December 1916, the Royal Navy created its own Anti-Submarine Division, the body from which the term "Asdic" eventually derived. After 1917, that division took over most ASW research.

  But the weapon that mattered most in the First World War was not a device that destroyed submarines. It was a tactic that made them irrelevant. The introduction of escorted convoys reduced shipping losses in the German war zone around the British Isles from 25 percent to less than 1 percent. The historian Paul E. Fontenoy wrote that the convoy system defeated the German submarine campaign outright. A second, quieter factor reinforced that success: Room 40 of the British Admiralty was intercepting and breaking German submarine radio signals. Intelligence and logistics, not weaponry, decided the First World War beneath the waves.

  By the war's end, 211 of the 360 U-boats had been destroyed. Mines accounted for 58 of those losses, depth charges for 30, and submarine torpedoes for 20. The most lethal single tool was also one of the simplest.

• 4. Sonar and the Interwar Lull

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  Between the wars, the most consequential development was active sonar, which the British designated ASDIC. The technology integrated into a complete weapons system allowed a ship to broadcast a sound pulse and listen for the echo that bounced back from a submarine's hull. The introduction of electronics for amplifying and displaying those signals was central to making the system practical. A device called the range recorder stored a memory of the target's position over time, which was critical because submarine propellers were loud in water in ways that weren't apparent from the surface.

  The bathythermograph, invented in 1937, became a standard fixture on ASW ships within a few years of its introduction. It measured temperature gradients at different depths, information that turned out to matter enormously for sound propagation. Water of different temperatures bends and scatters sound in ways that could hide a submarine from the very sonar designed to find it. Understanding the ocean itself was part of learning to hunt beneath it.

  By 1928, a small escort ship had been designed and plans drawn up to arm trawlers and produce ASDIC sets in large numbers. Both the Royal Navy and the U.S. Navy fitted their destroyers with active sonar during this period. The improvements were real, but they concealed a significant gap: ASDIC was only effective against submarines running submerged. Against a U-boat running on the surface at night, the technology offered nothing.

• 5. The Battle of the Atlantic

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  When the Second World War opened, most navies had developed little beyond the approach of locating a submarine by sonar and dropping depth charges on it. Almost immediately, that approach revealed its limitations. U-boats routinely operated on the surface at night, precisely because sonar couldn't touch them there. The survival of Britain depended on imports of food, oil, and other war materials, and German submarines were attacking the ships that carried them.

  Radar changed the arithmetic. Allied airborne radar technology consistently outpaced German countermeasures. The first generation of airborne radar used a 1.7-meter wavelength. By the second half of 1942, U-boats had deployed a detector called Metox to warn them of approaching radar-equipped aircraft. The Allies then switched to cavity magnetron-based radar operating at 10 centimeters, known as ASV III, which Metox could not detect. The Germans eventually fielded a detector called Naxos that could pick up 10-centimeter radar, but it had a very short range and gave a U-boat only limited time to dive. Between 1943 and 1945, radar-equipped aircraft accounted for the bulk of Allied kills against U-boats.

  The Leigh Light, an airborne searchlight used in conjunction with radar, allowed aircraft to illuminate and attack surfaced submarines at night. The Hedgehog and Squid were forward-throwing weapons that let an escort vessel maintain sonar contact with a submarine while attacking it, solving a critical problem: traditional depth charges required a ship to pass directly over a submarine, during which it lost contact entirely.

  On the intelligence side, codebreakers at Bletchley Park were reading German Naval Enigma communications. When the Germans added a fourth rotor to their Enigma machines in 1943, convoy losses rose significantly until the new configuration was solved. The British, concerned the Germans would guess Enigma had been broken, planted a false story about a special infrared camera being used to locate U-boats. German engineers responded by developing a submarine paint intended to replicate the optical properties of seawater.

  Commander F. J. "Johnnie" Walker of the Royal Navy developed a creeping attack technique in which one destroyer would track a U-boat acoustically while a second vessel attacked it, preventing the submarine from using a course change to break the attacker's sonar contact. The only recorded instance of one submerged submarine sinking another occurred in 1945, when HMS Venturer torpedoed U-864 off the coast of Norway after its captain tracked the target on hydrophones for several hours and manually calculated a three-dimensional firing solution before launching four torpedoes.

• 6. The Pacific and the Torpedo Scandal

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  Japanese submarines were among the most capable vessels of their type in the world. They were large, long-ranged, and armed with the Type 95 torpedo. Yet they had limited impact on the war's outcome. Japanese naval doctrine followed the Mahanian tradition, directing submarines against warships rather than merchant shipping. Warships were fast, maneuverable, and defended; merchant ships were not. The campaign against Allied supply lines that might have strangled the Pacific war effort was never seriously pursued.

  The American submarine campaign against Japan moved in the opposite direction, but it started badly. US torpedoes routinely failed to detonate on impact, ran too deep, or went off course. Japanese commanders, who faced an apparently weak submarine threat early in the war, became complacent and did not develop serious anti-submarine capability. US Vice Admiral Charles A. Lockwood pushed the ordnance department to acknowledge and fix the torpedo problems. When they initially ignored his complaints, he ran his own tests to prove the weapons' unreliability. He also replaced cautious submarine commanders with more aggressive ones.

  By the latter half of 1943, the results were dramatic. US submarines were sinking Japanese ships at a sharply higher rate, accounting for nearly half of the Japanese merchant fleet. Japan lacked the anti-submarine doctrine, the production capacity, and the organizational framework to respond effectively. Its destroyer escorts, better suited to convoy protection than fleet action, arrived too late and were too few.

  A separate and costly failure came from an unexpected direction. A June 1943 press conference held by US Congressman Andrew J. May revealed that American submarines could dive below 150 feet. Japanese depth charges had been set too shallow to reach them. After May's disclosure, enemy depth charges were reset to explode as deep as 250 feet. Vice Admiral Lockwood later estimated that the revelation cost the navy as many as ten submarines and 800 crewmen.

• 7. Nuclear Submarines and Cold War Detection

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  After the war, both the United States and Britain studied German Type XXI submarines, which had introduced streamlined hulls, high underwater speed, and snorkel-based battery recharging that allowed a patrol to be completed without surfacing. The US modified its fleet through the GUPPY program; Britain pursued the Overseas Patrol Submarines Project. The Soviets built new submarines patterned directly on the Type XXI, producing the Whiskey and Zulu classes.

  Nuclear submarines raised the threat further. They were faster than diesel-electric boats, did not need to snorkel, and could stay submerged indefinitely. Because they were noisy, passive sonar became the primary detection method. The SOSUS network, a system of bottom-mounted hydrophones, was deployed by the United States in the GIUK gap and other strategic passages. From a land-based processing facility, it could monitor submarines crossing those chokepoints without ever exposing itself.

  Helicopters emerged as essential ASW platforms during the 1960s because they could operate from almost any warship and carry sonar, sonobuoys, and torpedoes. Unlike a ship, a helicopter could fly a search pattern offset from the vessel it supported and relay sonar data to the ship's combat information center. Torpedo-carrying missiles such as ASROC and Ikara extended the range at which a surface ship could deliver a weapon to a sonar contact.

  The Magnetic Anomaly Detector, first used in the Second World War, continued in service into the modern era. MAD uses the Earth's magnetosphere as a baseline, detecting the anomaly produced by the large metallic mass of a submarine. On fixed-wing aircraft it is mounted in a tail boom; on helicopters it hangs from a deployable tow line, kept well away from the aircraft's engines and avionics to reduce interference. A 2024 study found that climate change may reduce the detectability of submarines in certain ocean locations, introducing a new and unresolved variable into ASW planning.