Air-dropped depth charges. British air dropped depth charge on a Sunderland.
The attempt to locate and track submarines in order to neutralize them before they are able to deploy weapons, discharge passengers or payload, or conduct surveillance. This endeavor is currently fulfilled by surface vessels, aircraft, satellites, submerged electronic barriers, naval intelligence, and other submarines.
The efficacy of a vessel capable of operating beneath the surface of the ocean had been recognized for centuries, and a submersible did prove it was capable of destroying a surface ship when the Confederate vessel H. L. Hunley sank the Union warship Housatonic with a spar torpedo on 17 February 1864. It was not until the late nineteenth century, however, that truly practical submarines first emerged, notably the types designed by John Holland. When the self-propelled torpedo was united with the submarine, first accomplished in an external tube aboard the Swedish-designed Nordstrom 1 in 1882, the major offensive capability of the submarine was established for the next seven decades. Torpedoes improved in stability, performance, and explosive power, and soon submarine designs included internal torpedo tubes.
Although the devastating potential of submarines against shipping was noted by some naval officers in policy positions, the early submarines were fragile and thus their presence did not set off a general alarm. The first submarines, being slow, of short range, mechanically unreliable, and difficult to maneuver, did not pose a great threat to much faster and more heavily armed surface ships. In addition, a few British naval theorists believed that submarines would not be deployed in a war because of prohibitions against using weapons of stealth that went against common agreements of how wars were conducted.
The relatively cheap cost of submarines and their improved performance in shallow coastal waters, such as the English Channel and the Mediterranean, made submarines an attractive option for countries that wished to challenge large navies such as Great Britain’s but lacked resources to support an equivalent navy financially or logistically. In the early twentieth century the submarine became a common naval weapon, and measures to counter these vessels were sought.
The transmission of sound through water was well known by the time submarines first appeared, and it was not long before various underwater listening devices, hydrophones, were employed to pick up the sounds of submarines beneath the surface. Hydrophones constituted a passive system that did not provide information on bearing or range, however. They were also subject to interference from the sounds emanating from the host ship itself.
The first exercises officially sanctioned to demonstrate ASW techniques were conducted by the British in 1904; the chief strategy was to attempt to come alongside a submarine and attach an explosive. Nets were also used to snare submarines, but these proved impractical. An aircraft was employed to spot a submarine in 1912, a harbinger of a very successful ASW technique.
At the onset of World War I, all the leading naval powers deployed submarines. On 11 August 1914, the British light cruiser Birmingham sliced in half the German submarine U-15, the first submarine casualty of the war. On 5 September, the U-21 became the first submarine to destroy an enemy vessel and survive when it torpedoed and sank the British cruiser Pathfinder. But it was the obsolete paraffin-powered U-9 that forced Britain to consider its ASW tactics carefully. On 22 September 1914, this U-boat torpedoed and sank the three old British cruisers Aboukir, Hogue, and Cressy in quick succession, with heavy loss of life.
ASW during World War I foreshadowed the kinds of techniques and weapons deployed in the next world war, but ASW remained generally ineffective compared to the destruction caused by submarines. One of the first ASW measures was to have ships steam in a zigzag pattern at top cruising speeds, making it difficult for submarines to predict a ship’s course and hence calculate where to fire a torpedo. Disruptive camouflage schemes also helped disguise the direction, speed, and type of vessel as seen from the low vantage of a submarine periscope or conning tower. Antitorpedo nets were deployed, but these proved less than satisfactory. Q-ships, armed vessels outfitted as merchantmen in order to lure unsuspecting submarines within gunfire range, were utilized with some success, although submarines soon resorted to simply destroying any merchant ship so as not to risk falling victim to this ruse. Ramming surfaced submarines was also a standard ASW technique of the period and accounted for 19 U-boats sunk.
Submarines proved to be good hunters themselves, with British submarines destroying 18 German U-boats. Depth charges were first deployed in January 1916, but it was not until 1917 that a satisfactory charge was perfected. These devices were rolled off the sterns of surface ships, but projectors mounted on a ship’s deck soon appeared to launch them in precise patterns from the ship. Destroyers, originally designed to counter torpedo boats, now became associated with ASW since they were fast, highly maneuverable, and could unload depth charges without much disruption of their deck plans or stability.
The most effective ASW weapon of World War I, however, was the submerged mine, usually deployed in high-density barriers. The Allies sowed them at varying depths and in great numbers, most notably in the North Sea barrage. This was designed to make a submarine’s transit from Germany to the Atlantic extremely risky. Choke points in the English Channel were also employed. Mines accounted for the sinking of at least 75 U-boats.
Despite these techniques, German U-boats continued to sink Allied ships at alarming rates, and only the belated introduction of the convoy system enabled Britain to survive. It proved the best defense against submarines and became the most reliable ASW measure of the war.
Once shipping was satisfactorily protected through convoys, Allied ships and planes were free to pursue U-boats with improved ASW weapons and techniques, including more active use of hydrophones. Torpedoes remained highly effective, but submarines ironically destroyed most unarmed merchant shipping with their heavy-caliber deck guns while surfaced; once submerged, submarines were less effective because of their slower speed and poor endurance. Hence they only resorted to underwater approaches when there was a threat from armed surface vessels. If a submarine could be located and forced under, shipping could more readily be protected.
The war ended before early ASDIC (sonar) sets could be deployed, but these instruments demonstrated great potential for recognizing submarines beneath the water as well as for providing range and bearing.
The success of sonar led in the interwar period to complacency about the need for further ASW research, since echo-ranging appeared to compromise a submarine’s ability to remain undetected. The fact that most submarine attacks had actually occurred on the surface, where sonar was irrelevant, was not taken into consideration. Submarines of World War II were faster, able to sustain greater depths, and had longer range and more powerful weapons than submarines of the previous world war.
With the beginning of World War II in September 1939, German submarines were once again deployed around Great Britain. They were not charged with destroying shipping, but rather with attacking naval vessels. Over the course of the next year, however, the rules of engagement were expanded and U-boats began concerted efforts against shipping. The Allies instituted convoy tactics at the onset of hostilities, but the fall of Norway, the Low Countries, and France in 1940 gave German U-boats better access to the Atlantic convoy lanes, something conspicuously missing during the last war. This greatly expanded the area where submarine attacks could be expected.
Increasing numbers of Allied escort vessels, long-range aircraft, and small carriers to accompany convoys infringed upon submarines’ ability to attack shipping. Depth charges and launchers became more reliable and powerful. Sonar also greatly improved, and was ultimately able to determine a submarine’s approximate bearing and depth. Such information could be exploited by the newly invented ahead-throwing weapons, the hedgehog and squid. Airborne ordnance, including sonobuoys, homing torpedoes, and devices such as the magnetic anomaly detector (MAD), were becoming commonplace.
Radar, both on surface ships and in aircraft, proved the most important ASW device of the war; radio direction finding (HF/DF) was also extremely important against German submarines. Decoded intelligence also played a major role in the defeat of the U-boats. Whole convoys could be routed around known German wolf packs, while hunter-killer groups composed of escort carriers, destroyers, and destroyer escorts could intercept and sink U-boats. Germany developed countermeasures, but the sheer scale of Allied saturation techniques, combined with the extraordinary production of shipping, made it very difficult for diesel submarines to prosecute the war.
In the Pacific, American submarines waged an even more destructive war against Japanese shipping. Japanese ASW was markedly inferior to that of the Allies, partly as a result of deeply ingrained doctrine that submarines were fleet vessels and deployed only against other naval ships, not merchant ships. Consequently, the Japanese navy had few ASW measures to protect shipping in place before their 7 December 1941 attack on Pearl Harbor.
Following the war, true submersibles appeared, thanks to inventions such as the snorkel. These allowed submarines to operate at high speeds beneath the ocean surface for extended periods of time and be equipped with long-range weapons. This completely redefined ASW. It changed from defending and preventing submarine attacks on shipping or reducing the effectiveness of a submarine’s weapons once fired to active early detection of submarine movements at sea. The primary objective of a modern submarine equipped with nuclear or air independent propulsion (AIP) and carrying nuclear ballistic missiles is to avoid contact so that it can remain part of a strategic force.
Antishipping patrols are no longer the primary task of submarines and, although that capability remains part of their role, their overriding offensive objective is to deliver nuclear and nonnuclear warheads to targets at great distances. Therefore, current ASW is chiefly charged with close monitoring of submarine movements rather than the protection of resources at sea. Radar would be ineffective against submarines that remained underwater for much of their patrols; MAD had short range, and sonar found it more difficult to obtain targets and bearing on increasingly quiet and fast submarines. Surface ships would have difficulty prosecuting contacts made at any distance at the necessary speed; therefore, aircraft—both fixed wing and helicopters—and missiles would have to be deployed.
This German technology was incorporated into both the Russian and western navies, with the newly created NATO preparing for a large Soviet submarine fleet that might attack western sea lanes en masse. Early on it was recognized that it would be easier to track submarines close to their bases than at sea, so barriers close to bases that could provide data on submarine movements or potentially lethal obstructions became an integral part of postwar ASW.
A guided missile was first launched from a submarine in 1947, but submarines had to surface in order to fire these weapons. The first nuclear-powered submarine, the U.S. Navy’s Nautilus, was commissioned in 1954, and was quickly followed by similar USSR submarines. Nuclear power enabled submarines to cruise for weeks at a time without surfacing—a major tactical advantage.
The first successful launch of a ballistic missile from a submerged submarine occurred in July 1960, thereby radically revising ASW requirements in less than 15 years from the end of World War II. The chief objective of ASW remained the early and accurate detection of submarines, although the timing had now changed. Given the vast destructive power of a single submarine, it became paramount for ASW to prevent a submarine from achieving weapon launch. The primary western ASW strategy remained containment, achieved through barriers such as SOSUS, a network of hydrophones placed at key areas along the ocean floor; long-range air patrols capable of deploying sonobuoys and improved MAD devices; Atlantic and Pacific distant early warning (DEW) lines that stationed radar-equipped ships at key points far at sea; specialized hunter-killer submarines lurking near bases in order to detect and tail submarines as they began a patrol (as revealed through intelligence information); and prosecution upon detection, which required fast, long-range weapons capable of destroying a submarine before it would be able to respond.
Rocket-launched homing torpedoes fired from surface ships (ASROC) or submarines (SUBROC) were developed and deployed. The number of vessels and aircraft necessary for continual monitoring of the Soviet submarine fleet was achieved through multiple nation participation, upgrading equipment of existing ships, and large expenditures on production of new aircraft, helicopters, and submarines. Very long-range, low-frequency sonars, capable of fitting into or being towed by submarines and surface ships, were also developed that increased their ability to detect enemy submarines at great distances.
The Cold War was the high point of large-scale ASW tactical and weaponry development. The end of the war has reduced the likelihood of a massive underwater attack for the moment, but ASW remains one of the most important tasks of modern navies, since a single submarine—perhaps at the behest of a rogue state—can have immense destructive power, capable of threatening any spot on the globe.
An explosive device designed to sink a submarine by detonating in its vicinity. The weapon may be triggered by water pressure, the target’s magnetic or acoustic signature, or a variety of timers. From 1916 until 1943 the depth charge was the principal antisubmarine weapon of all navies, and it still remains in the antisubmarine arsenal, although it has largely been superseded by more sophisticated weapons with greater range.
Before World War I the Royal Navy experimented with a variety of explosive antisubmarine devices, especially explosive sweeps towed behind destroyers. In 1915 development began of depth charges triggered by hydrostatic pistols. The Type D Mark III, with a 300-pound charge, entered service in 1916. The U.S. Navy developed similar charges in 1917. Antisubmarine vessels discharged depth charges from roll-off racks and mortars that projected them some 40 yards to the side. Although submarine detectors were primitive, depth charges were remarkably effective during World War I, sinking 30 German U-boats between 1916 and 1918.
Depth charges developed little between the world wars. In 1939 most navies used similar weapons with about a 300-pound charge, a sink-rate of 6–10 feet per second, and a hydrostatic trigger set for depths between 25 and 300 feet. Such charges had a lethal radius of 20 feet and, exploding within 40 feet, could force a submarine to surface.
Development during World War II concentrated on the areas of more lethal charges, faster sink-rates, and greater depths. More powerful explosives, such as Minol and Torpex, replaced guncotton, while charges also increased in size. In 1942 the Royal Navy introduced the massive Mark X weapon with a 2,000-pound charge, so large it was fired from torpedo tubes. Sink-rates rose to 22–50 feet per second, either by adding weights to conventional charges or by streamlining the cases. Modifications also allowed depth charges to explode deeper, first doubling the maximum depth to 600 feet and then increasing it to as much as 1,500 feet for the British Mark X**.
Equally important were more effective dropping patterns. Modified casings resulted in more reliable and predictable underwater trajectories, newer projectors increased surface ranges to 150 yards, and mathematical analysis generated patterns with greater kill probabilities.
Air-dropped depth charges played an important role during World War II and subsequently. First designs were modifications of existing surface types, which limited their efficacy, because their weight reduced the number that could be carried; also they were subject to restrictions on dropping height and speed. The purpose-designed types that followed were lighter and less subject to dropping restrictions.
During 1944 newer weapons, such as Hedgehog, Squid, and homing torpedoes, surpassed depth charges in killing submarines. Nevertheless, the depth charge was still an important antisubmarine weapon until after the war’s end. It saw a new lease of life in the 1950s when it was fitted with a nuclear warhead, although this also required an antisubmarine rocket (ASROC) for its discharge. It remained popular with smaller navies.
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