Depth charge
A depth charge is an anti-submarine warfare
Anti-submarine warfare
Anti-submarine warfare is a branch of naval warfare that uses surface warships, aircraft, or other submarines to find, track and deter, damage or destroy enemy submarines....

 (ASW) weapon intended to destroy or cripple a target submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 by the shock of exploding near it. Most use explosives and a fuze
Fuze Beverage, commercially referred to as just Fuze , is a manufacturer of teas and non-carbonated fruit drinks enriched with vitamins. Currently the brand consists of five vitamin-infused lines: Slenderize, Refresh, Tea, Defensify, and Vitalize...

 set to go off at a preselected depth in the ocean. Depth charges can be dropped by either surface ships, patrol aircraft, or from helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

s. The depth charge has now largely been replaced by anti-submarine homing torpedo
Anti-submarine weapon
An anti-submarine weapon is any one of a range of devices that are intended to act against a submarine, and its crew, to destroy the vessel or to destroy or reduce its capability as a weapon of war...


A depth charge fitted with a nuclear warhead is known as a nuclear depth bomb
Nuclear Depth Bomb
A Nuclear Depth Bomb is the nuclear equivalent of the conventional depth charge and can be used in Anti-Submarine Warfare for attacking submerged submarines...

. These were designed to be dropped from a patrol plane or deployed by Anti-submarine missile
Anti-submarine missile
An anti-submarine missile is a standoff weapon including a rocket designed to rapidly deliver an explosive warhead or homing torpedo from the launch platform to the vicinity of a submarine.-History:...

 from a surface ship, or another submarine, located a safe distance away. All nuclear anti-submarine weapons were withdrawn from service by the United States, the United Kingdom, France, Russia, and China in or around 1990. They were replaced by conventional weapons that boasted ever-increasing accuracy and range as ASW technology improved.


The idea of firing charges at set depths was not new. The first attempt was by aircraft bombs attached to lanyards which would trigger their charges; a similar idea was a 16 lb (7.3 kg) 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 20 ft (6.1 m).

The best concept arose in a 1913 RN Torpedo School report, describing a device intended for countermining
Demining or mine clearance is the process of removing either land mines, or naval mines, from an area, while minesweeping describes the act of detecting of mines. There are two distinct types of mine detection and removal: military and humanitarian.Minesweepers use many tools in order to accomplish...

, 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 and Sons of Sheffield) preset for 45 ft (13.7 m) firing, to be launched from a stern platform. Weighing 1150 lb (521.6 kg), and effective at 100 ft (30.5 m), the "cruiser mine" was a potential hazard to the dropping ship, but was also on the right track. The design work was carried out by Herbert Taylor at the RN Torpedo and Mine School, HMS Vernon
HMS Vernon (shore establishment)
HMS Vernon was a shore establishment or 'stone frigate' of the Royal Navy. Vernon was established on 26 April 1876 as the Royal Navy's Torpedo Branch and operated until 1 April 1996, when the various elements comprising the establishment were split up and moved to different commands.-Foundation...

. The first effective depth charge, the Type D, became available in January 1916. These were barrel-like casings containing a high explosive, usually TNT (amatol
Amatol is a highly explosive material made from a mixture of TNT and ammonium nitrate. Its name originates from the words ammonium and toluene...

, when TNT became scarce). There were initially two sizes--Type D, with a 300 lb (136.1 kg) charge for fast ships, and Type D* with a 120 lb (54.4 kg) charge for ships too slow to clear the danger area of the more powerful charge.

A hydrostatic pistol actuated by water pressure at a pre-selected depth detonated the charge. Initial depth settings were 40 or. Because production could not keep up with demand, anti-submarine vessels initially carried only two depth charges, to be released from a chute at the stern of the ship. The first success was the sinking of U-68 off Kerry
County Kerry
Kerry means the "people of Ciar" which was the name of the pre-Gaelic tribe who lived in part of the present county. The legendary founder of the tribe was Ciar, son of Fergus mac Róich. In Old Irish "Ciar" meant black or dark brown, and the word continues in use in modern Irish as an adjective...

, Ireland, on 22 March 1916 by the Q-ship
Q-ships, also known as Q-boats, Decoy Vessels, Special Service Ships, or Mystery Ships, were heavily armed merchant ships with concealed weaponry, designed to lure submarines into making surface attacks. This gave Q-ships the chance to open fire and sink them...

HMS Farnborough
HMS Farnborough, also known as , was a Q-ship of the British Royal Navy that saw service in the First World War. Farnborough was a heavily armed merchant ship with concealed weaponry that was designed to lure submarines into making surface attacks. Farnborough sank two submarines in her service in...

 Germany became aware of the depth charge following unsuccessful attacks on U-67 on 15 April 1916 and U-69 on 20 April 1916. UC-19
SM UC-19
SM UC-19 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy during World War I. The U-boat was ordered on 26 August 1915 and was launched on 15 March 1916...

 and UB-29
SM UB-29
SM UB-29 was a German Type UB II submarine or U-boat in the German Imperial Navy during World War I. The U-boat was ordered on 30 April 1915 and launched on 10 February 1916...

 were the only other submarines sunk by depth charge during 1916.

Numbers of depth charges carried per ship increased to 4 in June 1917, to 6 in August, and 30-50 by 1918. The weight of charges and racks caused instability problems unless guns and torpedo tubes were landed to compensate. Improved pistols allowed greater depth settings in 50-foot (15-meter) increments, from 50 to 200 ft (15.2 to 61 m). Even slower ships could safely use the Type D at below 100 ft (30.5 m) and at 10 kn (19.6 km/h; 12.2 mph) or more, so the relatively ineffective Type D* was withdrawn. Monthly use of depth charges increased from 100 to 300 per month during 1917 to an average of 1745 per month during the last 6 months of World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. The Type D could be detonated as deep as 300 ft (91.4 m) by that date. By war's end, 74,441 depth charges had been issued by RN, and 16,451 fired, scoring 38 kills in all, and aiding in 140 more.

The depth charge was such a successful device it attracted the attention of the United States, who requested full working drawings of the devices in March 1917. Having received them, Commander Fullinwider of the U.S. Bureau of Naval Ordnance and U.S. Navy engineer Minkler made some modifications and then patented it in the U.S. It has been argued this was done to avoid paying the original inventor.

The Royal Navy Type D depth charge was designated the Mark VII by 1939. Initial sinking speed was 7 ft/s (2.1 m/s) with a terminal velocity of 9.9 ft/s (3 m/s) at a depth of 250 ft (76.2 m) if rolled off the stern, or upon water contact from a depth charge thrower. Cast iron weights of 150 lb (68 kg) were attached to the Mark VII at the end of 1940 to increase sinking velocity to 16.8 ft/s (5.1 m/s). New hydrostatic pistols increased the maximum detonation depth to 900 ft (274.3 m). The Mark VII's 290 lb (131.5 kg) Amatol charge was estimated capable of splitting a 7/8 in submarine pressure hull at a distance of 20 ft (6.1 m), and forcing the submarine to surface at twice that. Change of explosive to Torpex
Torpex is a secondary explosive 50% more powerful than TNT by mass. Torpex is composed of 42% RDX, 40% TNT and 18% powdered aluminium. It was used in the Second World War from late 1942. The name is short for Torpedo Explosive', having been originally developed for use in torpedoes...

 (or Minol) at the end of 1942 was estimated to increase those distances to 26 and 52 ft (7.9 and 15.8 m).

The British Mark X depth charge weighed 3000 pounds (1400 kg) and was launched from 21-inch (53 cm) torpedo tube
Torpedo tube
A torpedo tube is a device for launching torpedoes. There are two main types of torpedo tube: underwater tubes fitted to submarines and some surface ships, and deck-mounted units installed aboard surface vessels...

s of older destroyers to achieve a sinking velocity of 21 ft/s (6.4 m/s). The launching ship needed to clear the area at 11 knots to avoid damage, and the charge was seldom used. Only 32 were actually fired, and it was known to be troublesome.

The teardrop-shaped United States Mark 9 depth charge entered service in the spring of 1943. The charge was 200 lb (90.7 kg) of Torpex with a sinking speed of 14.4 ft/s (4.4 m/s) and depth settings up to 600 ft (182.9 m). Later versions increased depth to 1000 ft (304.8 m) and sinking speed to 22.7 ft/s (6.9 m/s) with increased weight and improved streamlining.
Although the explosions of the standard United States' 600 lb (272.2 kg) Mark 4 or Mark 7 depth charge used in World War II were nerve-wracking to the target, an undamaged U-boat’s pressure hull would not rupture unless the charge detonated closer than about 15 ft (4.6 m). Placing the weapon within this range was entirely a matter of chance and quite unlikely as the target maneuvered evasively during the attack. Most U-boats sunk by depth charges were destroyed by damage accumulated from a long barrage rather than by a single carefully aimed attack. Many survived hundreds of depth charges over a period of many hours; U-427
Unterseeboot 427
German submarine U-427 was a Type VIIC U boat of the German Kriegsmarine during World War II.Built by Danziger Werft, Danzig, the U-boat was laid down on 27 July 1942, and launched on 6 February 1943, with a crew of 53 under their Austrian commander Oberleutnant Graf Carl-Gabriel von Gudenus. It...

 survived 678 depth charges in April 1945, though many may have detonated a considerable distance from the target.

Delivery mechanisms

The first delivery mechanism was to simply roll the "ashcans" off racks at the stern of the attacking vessel. Originally depth charges were simply placed at the top of a ramp and allowed to let roll. Improved racks, which could hold several depth charges and release them remotely with a trigger, were developed towards the end of the First World War
Armistice Day
Armistice Day is on 11 November and commemorates the armistice signed between the Allies of World War I and Germany at Compiègne, France, for the cessation of hostilities on the Western Front of World War I, which took effect at eleven o'clock in the morning—the "eleventh hour of the eleventh day...

. These racks remained in use throughout World War II, because they were simple and easy to reload.

Some Royal Navy trawler
Naval trawler
A naval trawler is a vessel built along the lines of a fishing trawler but fitted out for naval purposes. Naval trawlers were widely used during the First and Second world wars. Fishing trawlers were particularly suited for many naval requirements because they were robust boats designed to work...

s used for anti-submarine work during 1917–1918 had a thrower on the forecastle
Forecastle refers to the upper deck of a sailing ship forward of the foremast, or the forward part of a ship with the sailors' living quarters...

 for a single depth charge, but there do not seem to be any records of it being used in action. Specialized depth charge projectors were developed to generate a wider dispersal pattern when used in conjunction with rack-deployed charges. The first of these was developed from a British Army
British Army
The British Army is the land warfare branch of Her Majesty's Armed Forces in the United Kingdom. It came into being with the unification of the Kingdom of England and Scotland into the Kingdom of Great Britain in 1707. The new British Army incorporated Regiments that had already existed in England...

 trench mortar, 1277 were issued, 174 installed in auxiliaries during 1917 and 1918. The bombs they launched were too light to be truly effective; only one U-boat is known to have been sunk by them.

Thornycroft was a United Kingdom-based vehicle manufacturer which built coaches, buses, and trucks from 1896 until 1977.-History:Thornycroft started out with steam vans and lorries. John Isaac Thornycroft, the naval engineer, built his first steam lorry in 1896...

 created an improved version able to throw a charge 40 yd (36.6 m). The first was fitted in July 1917 and became operational in August. In all, 351 TBDs and 100 other craft were equipped. Projectors called Y-guns (in reference to their basic shape), developed by the U.S. Navy's Bureau of Ordnance
Bureau of Ordnance
The Bureau of Ordnance was the U.S. Navy's organization responsible for the procurement, storage, and deployment of all naval ordnance, between the years 1862 and 1959.-History:...

 from the Thornycroft thrower, became available in 1918. Mounted on the centerline of the ship with the arms of the "Y" pointing outboard, two depth charges were cradled on shuttles inserted into each arm. An explosive propellant charge was detonated in the vertical column of the Y-gun to propel a depth charge about 45 yd (41.1 m) over each side of the ship. The main disadvantage of the Y-gun is needed to be mounted on the centerline of a ship's deck, which could otherwise be occupied by superstructure, masts, or guns. The first were built by New London Ship Engine Company beginning 24 November 1917.

The K-gun, standardardized in 1942, replaced the Y-gun as the primary depth charge projector. K-guns could be mounted on the periphery of a ship's deck, thus freeing up valuable centerline space. The K-guns were often used together with stern racks to create patterns of six to ten charges. In all cases, the attacking ship needed to be moving above a certain speed or it would be damaged by the force of its own weapons.

Depth charges can also be dropped from an attacking aircraft against submarines. At the start of World War II, Britain's aerial anti-submarine weapon was the 100 lb (45.4 kg) anti-submarine bomb. This weapon was too light and ultimately, a failure. Indeed, 5 September 1939, a Royal Air Force
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

 Avro Anson
Avro Anson
The Avro Anson is a British twin-engine, multi-role aircraft that served with the Royal Air Force, Fleet Air Arm and numerous other air forces prior to, during, and after the Second World War. Named for British Admiral George Anson, it was originally designed for maritime reconnaissance, but was...

 of No. 233 Squadron
No. 233 Squadron RAF
No. 233 Squadron RAF was a Royal Air Force squadron that operated from 1918–1919, 1937–1945, 1952 - 1957 and 1960–1964. The squadron was formed from several Royal Naval Air Service flights and took part in the tail end of World War I before being disbanded. The squadron was reformed with the...

 was destroyed when its own A/S bomb skipped off the surface of the water and detonated under the aircraft. To remedy the failure of this weapon, the Royal Navy's 450 lb (204.1 kg) Mark VII depth charge was modified for aerial use by the addition of a streamlined nose fairing and stabilising fins on the tail.

The first to use depth charges on airplanes in actual combat were the Finns
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...

, though. Experiencing the same problems as RAF with insufficient charges on anti-submarine bombs, Captain Birger Ek of Finnish Air Force
Finnish Air Force
The Finnish Air Force is one of the branches of the Finnish Defence Forces. Its peacetime tasks are airspace surveillance, identification flights, and production of readiness formations for wartime conditions...

 squadron LeLv 6 contacted one of his Navy friends and suggested testing aerial use of standard Finnish Navy depth charges. The tests proved successful, and the Tupolev SB
Tupolev SB
The Tupolev ANT-40, also known by its service name Tupolev SB , and development co-name TsAGI-40, was a high speed twin-engined three-seat monoplane bomber, first flown in 1934....

 bombers of LeLv 6 were modified in early 1942 to carry depth charges. The success of the anti-submarine missions reached also RAF Coastal Command
RAF Coastal Command
RAF Coastal Command was a formation within the Royal Air Force . Founded in 1936, it was the RAF's premier maritime arm, after the Royal Navy's secondment of the Fleet Air Arm in 1937. Naval aviation was neglected in the inter-war period, 1919–1939, and as a consequence the service did not receive...

, which promptly began modifying depth charges for aerial use.

Later depth charges would be developed specifically for aerial use. Such weapons still have utility today and are in limited use, particularly for shallow-water situations where a homing torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...

 may not be suitable. Depth charges are especially useful for "flushing the prey" in the event of a diesel submarine lying on the bottom or otherwise hiding, with all machinery shut down. Homing torpedoes can be used for the same purpose, but the cost is prohibitive and aircraft and shipboard inventories limited. An example of such a weapon is the BAE Systems
BAE Systems
BAE Systems plc is a British multinational defence, security and aerospace company headquartered in London, United Kingdom, that has global interests, particularly in North America through its subsidiary BAE Systems Inc. BAE is among the world's largest military contractors; in 2009 it was the...

 Mark 11, deployed by the British Fleet Air Arm
Fleet Air Arm
The Fleet Air Arm is the branch of the British Royal Navy responsible for the operation of naval aircraft. The Fleet Air Arm currently operates the AgustaWestland Merlin, Westland Sea King and Westland Lynx helicopters...



The effective use of depth charges required the combined resources and skills of many individuals during an attack. Sonar, helm, depth charge crews and the movement of other ships had to be carefully coordinated. Aircraft depth charge tactics depended on the aircraft using its speed to rapidly appear from over the horizon and surprising the sub on the surface (where it spent most of its time) during the day or night (using radar to detect the target and a Leigh Light
Leigh light
The Leigh Light was a British World War II era anti-submarine device used in the Second Battle of the Atlantic.It was a powerful carbon arc searchlight of 24 inches diameter fitted to a number of the British Royal Air Force's Coastal Command patrol bombers to help them spot surfaced...

 to illuminate just prior to the attack), then quickly attacking once it had been located, as the sub would normally crash dive
Crash dive
A crash dive is a maneuver performed by a submarine to submerge as quickly as possible to avoid attack. Crash diving from the surface to avoid attack has been largely rendered obsolete with the advent of nuclear-powered submarines as they operate constantly submerged and are unlikely to be found on...

 to escape attack.

As the Battle of the Atlantic wore on, British and Commonwealth
Commonwealth of Nations
The Commonwealth of Nations, normally referred to as the Commonwealth and formerly known as the British Commonwealth, is an intergovernmental organisation of fifty-four independent member states...

 forces became particularly adept at depth charge tactics, and formed some of the first destroyer hunter-killer groups to actively seek out and destroy German U-boats.

The shortcoming of the depth charge as deployed by surface ships was not the weapon itself, but how it was delivered. An attacking vessel would usually detect a submerged contact using its sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

 (or in British parlance, ASDIC). However, to drop its depth charges it had to pass over the contact to drop them over the stern. As such, sonar contact would be lost immediately prior to attack, thus rendering the hunter blind at the crucial moment. A skillful submarine commander therefore had an opportunity to take successful evasive action. This situation would be remedied by the adoption of the ahead-throwing weapon, hedgehog
Hedgehog (weapon)
The Hedgehog was an anti-submarine weapon developed by the Royal Navy during World War II, that was deployed on convoy escort warships such as destroyers to supplement the depth charge. The weapon worked by firing a number of small spigot mortar bombs from spiked fittings...

, which allowed contacts to be engaged at a "stand-off" distance while still in sonar contact.

Pacific theater

In the Pacific, Japanese depth charge attacks initially proved fairly unsuccessful against U.S. and British submarines. Unless caught in shallow water, a U.S. submarine commander could normally dive to a deeper depth in order to escape destruction.

The deficiencies of Japanese depth-charge tactics were revealed in a press conference held by U.S. Congressman
United States House of Representatives
The United States House of Representatives is one of the two Houses of the United States Congress, the bicameral legislature which also includes the Senate.The composition and powers of the House are established in Article One of the Constitution...

 Andrew J. May
Andrew J. May
Andrew Jackson May was a Kentucky attorney and influential New Deal-era politician, best known for his chairmanship of the House Military Affairs Committee during World War II, and his subsequent conviction for bribery...

, a member of the House Military Affairs Committee
United States House Committee on Armed Services
thumb|United States House Committee on Armed Services emblemThe U.S. House Committee on Armed Services, commonly known as the House Armed Services Committee, is a standing committee of the United States House of Representatives...

 who had visited the Pacific theater and received many intelligence and operational briefings. Incredibly, May mentioned the highly sensitive fact that American submarines had a high survivability rate because Japanese depth charges were fuze
Fuze Beverage, commercially referred to as just Fuze , is a manufacturer of teas and non-carbonated fruit drinks enriched with vitamins. Currently the brand consists of five vitamin-infused lines: Slenderize, Refresh, Tea, Defensify, and Vitalize...

d to explode at too shallow a depth.

Various press associations sent this leaked news story over their wires, compounding the danger, and many newspapers (including one in Honolulu, Hawaii
Honolulu, Hawaii
Honolulu is the capital and the most populous city of the U.S. state of Hawaii. Honolulu is the southernmost major U.S. city. Although the name "Honolulu" refers to the urban area on the southeastern shore of the island of Oahu, the city and county government are consolidated as the City and...

) published it. Soon, Japanese forces were resetting their depth charges to explode at a more effective average depth of 75 m (250 feet), to the detriment of American submariners. Vice Admiral Charles A. Lockwood
Charles A. Lockwood
Charles Andrews Lockwood was an admiral of the United States Navy. He is known in submarine history as the legendary commander of Submarine Force Pacific Fleet during World War II...

, commander of the U.S. submarine fleet in the Pacific, later estimated that May's revelation cost the United States Navy as many as ten submarines and 800 seamen killed in action
Killed in action
Killed in action is a casualty classification generally used by militaries to describe the deaths of their own forces at the hands of hostile forces. The United States Department of Defense, for example, says that those declared KIA need not have fired their weapons but have been killed due to...


Later developments

For the reasons expressed above, the depth charge was generally replaced as an anti-submarine weapon. Initially, this was by ahead-throwing weapons such as the British-developed Hedgehog
Hedgehog (weapon)
The Hedgehog was an anti-submarine weapon developed by the Royal Navy during World War II, that was deployed on convoy escort warships such as destroyers to supplement the depth charge. The weapon worked by firing a number of small spigot mortar bombs from spiked fittings...

 and later Squid
Squid (weapon)
Squid was a British World War II ship-mounted anti-submarine weapon. It consisted of a three-barrelled mortar which launched depth charges. It replaced the Hedgehog system, and was in turn replaced by the Limbo system....

. These weapons threw a pattern of warheads ahead of the attacking vessel to bracket a submerged contact. Hedgehog was contact fuze
Fuze Beverage, commercially referred to as just Fuze , is a manufacturer of teas and non-carbonated fruit drinks enriched with vitamins. Currently the brand consists of five vitamin-infused lines: Slenderize, Refresh, Tea, Defensify, and Vitalize...

d, while Squid fired a pattern of three large (200 kg) depth-charges with clockwork detonators. Later developments included the Mark 24 "Fido" acoustic homing torpedo (and later such weapons) or the SUBROC, which was armed with a nuclear depth charge. The USSR
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 developed anti-submarine weapons using nuclear warheads
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

 and these are sometimes referred to as Nuclear Depth Bomb
Nuclear Depth Bomb
A Nuclear Depth Bomb is the nuclear equivalent of the conventional depth charge and can be used in Anti-Submarine Warfare for attacking submerged submarines...

s (NDB).

Underwater explosions

The high explosive in a depth charge undergoes a rapid chemical reaction at an approximate rate of 8,000 meters per second (25,000 ft/s). The gaseous products of that reaction momentarily occupy the volume previously occupied by the solid explosive, but at very high pressure. This pressure is the source of the damage and is proportional to the explosive density and the square of the detonation velocity. A depth charge gas bubble expands to reach the pressure of the surrounding water.

This gas expansion propagates a shock wave. The density difference of the expanding gas bubble from the surrounding water causes the bubble to rise toward the surface. Unless the explosion is shallow enough to vent the gas bubble to the atmosphere during its initial expansion, the momentum of water moving away from the gas bubble will create a gaseous void of lower pressure than the surrounding water. Surrounding water pressure then collapses the gas bubble with inward momentum causing excess pressure within the gas bubble. Re-expansion of the gas bubble then propagates another potentially damaging shock wave. Cyclical expansion and contraction continues until the gas bubble vents to the atmosphere.

Consequently, explosions where the depth charge is detonated at a shallow depth and the gas bubble vents into the atmosphere very soon after the detonation are quite ineffective, even though they are more dramatic and therefore preferred in movies. A sign of an effective detonation depth is that the surface just slightly rises and only after a while vents into a water burst.

Very large depth charges, including nuclear weapons, may be detonated at sufficient depth to create multiple damaging shock waves. However, very large depth charges can produce damage at distance, if reflected shock waves from the ocean floor and/or ocean surface converge to amplify radial shock waves. Submarines or surface ships may be damaged if operating in convergence zones of their own depth-charge detonations.

The damage that an underwater explosion inflicts on a submarine comes from a primary and a secondary shock wave. The primary shock wave is the initial shock wave from the depth charge, and will cause damage to personnel and equipment inside the submarine if detonated close enough. The secondary shock wave is a result from the cyclical expansion and contraction of the gas bubble and will bend the submarine back and forth and cause catastrophic hull breach, in a way that can be best described as bending a plastic ruler back and forth until it snaps. Up to sixteen cycles of the secondary shock wave have been recorded in tests. The effect of the secondary shock wave can be reinforced if another depth charge detonates on the other side of the hull in a close proximity in time of the first detonation, which is why depth charges normally are launched in pairs with different pre-set detonation depths.

The killing radius of a depth charge depends on the payload of the depth charge and the size and strength of the submarine hull. A depth charge of approximately 100 kg of TNT (400 MJ) would normally have a killing radius (hull breach) of only 3–4 m (9.8–13.1 ) against a conventional 1000 long tons (1,016.1 t) submarine, while the disablement radius (where the submarine is not sunk but put out of commission) would be approximately 8 metre. A higher payload only increases the radius by a few meters because the effect of an underwater explosion decreases with the distance cubed. The killing range would be greater against a larger submarine and shorter against a smaller submarine. It is doubtful if the hull of a midget submarine
Midget submarine
A midget submarine is any submarine under 150 tons, typically operated by a crew of one or two but sometimes up to 6 or 8, with little or no on-board living accommodation...

 with a titanium hull could be sunk by a depth charge by anything less than a direct hit, even though it could be decommissioned with less.

See also

  • Bouncing bomb
    Bouncing bomb
    A bouncing bomb is a bomb designed specifically to bounce to a target across water in a calculated manner, in order to avoid obstacles such as torpedo nets, and to allow both the bomb's speed on arrival at the target and the timing of its detonation to be pre-determined...

    , the specialized air-delivered bomb used for the RAF's Operation Chastise
    Operation Chastise
    Operation Chastise was an attack on German dams carried out on 16–17 May 1943 by Royal Air Force No. 617 Squadron, subsequently known as the "Dambusters", using a specially developed "bouncing bomb" invented and developed by Barnes Wallis...

  • Naval mine
    Naval mine
    A naval mine is a self-contained explosive device placed in water to destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel...

External links

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