Anti-submarine warfare (ASW, or in the older form A/S) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, or destroy enemy submarines. Such operations are typically carried out to protect friendly shipping and coastal facilities from submarine attacks and to overcome blockades.
Successful ASW operations typically involve a combination of sensor and weapon technologies, along with effective deployment strategies and sufficiently trained personnel. Typically, sophisticated sonar equipment is used for first detecting, then classifying, locating, and tracking a target submarine. Sensors are therefore a key element of ASW. Common weapons for attacking submarines include torpedoes and naval mines, which can both be launched from an array of air, surface, and underwater platforms. ASW capabilities are often considered of significant strategic importance, particularly following provocative instances of unrestricted submarine warfare and the introduction of submarine-launched ballistic missiles, which greatly increased the lethality of submarines.
At the beginning of the 20th century, ASW techniques and submarines themselves were primitive. During the First World War, submarines deployed by Imperial Germany proved themselves to be a capable threat to shipping, being able to strike targets even out in the North Atlantic Ocean. Accordingly, multiple nations embarked on research into devising more capable ASW methods, resulting in the introduction of practical depth charges and advances in sonar technology; the adoption of the convoy system also proved to be a decisive tactic.
After a lull in progress during the interwar period, the Second World War saw submarine warfare and ASW alike advance rapidly, particularly during the critical Battle of the Atlantic, during which Axis submarines sought to prevent Britain from effectively importing supplies. Techniques such as the Wolfpack achieved initial success, but became increasingly costly as more capable ASW aircraft were introduced. Technologies such as the Naxos radar detector gained only a temporary reprieve until detection apparatus advanced yet again. Intelligence efforts, such as Ultra, also played a major role in curtailing the submarine threat and guiding ASW efforts towards greater success.
During the post-war era, ASW continued to advance, as the arrival of nuclear submarines had rendered some traditional techniques less effective. The superpowers of the era constructed sizable submarine fleets, many of which were armed with nuclear weapons; in response to the heightened threat posed by such vessels, various nations chose to expand their ASW capabilities. Helicopters, capable of operating from almost any warship and equipped with ASW apparatus, became commonplace during the 1960s. Increasingly capable fixed-wing maritime patrol aircraft were also widely used, covering vast areas of ocean. The magnetic anomaly detector (MAD), diesel exhaust sniffers, sonobuoys and other electronic warfare technologies also became a staple of ASW efforts. Dedicated attack submarines, purpose-built to track down and destroy other submarines, became a key component as well. Torpedo carrying missiles, such as ASROC and Ikara, were another area of advancement.
History Origins The first attacks on a ship by an underwater vehicle are generally believed to have been during the American Revolutionary War, using what would now be called a naval mine but what was then referred to as a torpedo.Gilbert, Jason A., L/Cdr, USN. "Combined Mine Countermeasures Force", Naval War College paper (Newport, RI, 2001), p. 2. Even so, various attempts to produce submarines had been made prior to this.
In 1866, British engineer Robert Whitehead invented the first effective self-propelled torpedo, the eponymous Whitehead torpedo; French and German inventions followed soon thereafter. The first submarine with a torpedo was Nordenfelt I, built in 1884–1885, though it had been proposed earlier. By the outbreak of the Russo-Japanese War in 1904, all the large navies except the Germans had acquired submarines. Nevertheless, at that time, all powers still defined the submarine as an experimental vessel and did not put it into operational use.
There were no means to detect submerged U-boats, and attacks on them were limited at first to efforts to damage their periscopes with hammers. The Royal Navy torpedo establishment, HMS Vernon, studied explosive grapnel sweeps; these sank four or five U-boats in the First World War. A similar approach featured a string of charges on a floating cable, fired electrically; an unimpressed Admiral Edward Evans considered any U-boat sunk by it deserved to be.
Another primitive technique of attacking submarines was the dropping of hand-thrown guncotton bombs. The Lance Bomb was also developed; this featured a cone-shaped steel drum on a shaft, intended to be thrown at a submarine. Firing Lyddite shells, or using trench mortars, was tried. Use of nets to ensnare U-boats was also examined, as was a destroyer, , fitted with a spar torpedo. To attack at set depths, aircraft bombs were attached to lanyards which would trigger their charges; a similar idea was a guncotton charge in a lanyarded can; two of these lashed together became known as the Depth Charge Type A. Problems with the lanyards tangling and failing to function led to the development of a chemical pellet trigger as the Type B. These were effective at a distance of around .
Perhaps the best early concept arose in a 1913 Royal Navy Torpedo School report, describing a device intended for countermining, a "dropping mine". At Admiral John Jellicoe's request, the standard Mark II mine was fitted with a hydrostatic pistol (developed in 1914 by Thomas Firth & Sons of Sheffield) preset for firing, to be launched from a stern platform. Weighing , and effective at , the "cruiser mine" was also a potential hazard to the dropping ship.
First World War
During the First World War, submarines were a major threat. They operated in the Baltic, North Sea, Black Sea and Mediterranean as well as the North Atlantic. 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 using guns and good luck. They mainly relied on the fact a submarine of the day was often on the surface for a range of reasons, such as charging batteries or crossing long distances. The first approach to protect warships was chainlink nets strung from the sides of battleships, as defense against torpedoes. Nets were also deployed across the mouth of a harbour or naval base to stop submarines entering or to stop torpedoes of the Whitehead type fired against ships. British warships were fitted with a ram with which to sink submarines, and U-15 was thus sunk in August 1914.
During June 1915, the Royal Navy began operational trials of the Type D depth charge, with a charge of TNT (amatol, as TNT supplies became critical) and a hydrostatic pistol, firing at either , and believed to be effective at a distance of ; the Type D*, with a charge, was offered for smaller ships.
In July 1915, the British Admiralty set up the Board of Invention and Research (BIR) to evaluate suggestions from the public as well as carrying out their own investigations. Some 14,000 suggestions were received about combating submarines. In December 1916, the Royal Navy set up its own Anti-Submarine Division (ASD), from which came the term "Asdic", but relations with the BIR were poor. After 1917, most ASW work was carried out by the ASD. In the U.S., a Naval Consulting Board was set up in 1915 to evaluate ideas. After American entry into the war in 1917, they encouraged work on submarine detection. The U.S. National Research Council, a civilian organization, brought in British and French experts on underwater sound to a meeting with their American counterparts in June 1917. In October 1918, there was a meeting in Paris on "supersonics", a term used for echo-ranging, but the technique was still in research by the end of the war.
The first recorded sinking of a submarine by depth charge was U-68, sunk by Q-ship off County Kerry, Ireland on 22 March 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 as 'guard loops'. By July 1917, depth charges had developed to the extent that settings of between were possible. This design would remain mainly unchanged through the end of World War II. While dipping hydrophones appeared before the war's end, the trials were abandoned.
Seaplanes and airships were also used to patrol for submarines. A number of successful attacks were made, but the main value of air patrols was in driving the U-boat to submerge, rendering it virtually blind and immobile.
However, the most effective anti-submarine measure was the introduction of escorted convoys, which reduced the loss of ships entering the German war zone around the British Isles from 25% to less than 1%. The historian Paul E. Fontenoy states that "[t]he convoy system defeated the German submarine campaign."Paul E. Fontenoy, "Convoy System", The Encyclopedia of World War I: A Political, Social and Military History, Volume 1, Spencer C. Tucker, ed. (Santa Barbara: ABC-CLIO, 2005), 312–314. A major contributing factor was the interception of German submarine radio signals and breaking of their code by Room 40 of the British Admiralty.
To attack submerged boats, a number of anti-submarine weapons were derived, including the sweep with a contact-fused explosive. Bombs were dropped by aircraft and depth charge attacks were made by ships. Prior to the introduction of dedicated depth charge throwers, charges were manually rolled off the stern of a ship. The Q-ship, a warship disguised as a merchantman, was used to attack surfaced U-boats,Beyer, Kenneth M.: Q-Ships versus U-Boats. America's Secret Project. Naval Institute Press. Annapolis, Maryland, US. 1999. while the R1 was the first ASW submarine.
211 of the 360 U-boats were sunk during the war, using a variety of ASW methods: Mines 58 Depth charges 30 Submarine torpedoes 20 Gunfire 20 Ramming 19 Unknown 19 Accidents 10 Sweeps 33 Other (including bombs) 2
Inter-war period
This period saw the development of active sonar (ASDIC) and its integration into a complete weapons system by the British, as well as the introduction of radar. During the period, there was a great advance due to the introduction of electronics for amplifying, processing, and displaying signals. In particular, the "range recorder" was a major step that provided a memory of target position. Because the propellers of many submarines were extremely loud in the water (although they did not seem so from the surface), range recorders were able to gauge the distance from the U-boat by sound. This would allow mines or bombs around that area to be detonated. New materials for sound projectors were developed. Both the Royal Navy and the U.S. Navy fitted their destroyers with active sonars. In 1928, a small escort ship was designed and plans made to arm trawlers and to mass-produce ASDIC sets.