Mark IX depth charges on the starboard stern of the USS Cassin Young DD 793
Photo. https://en.wikipedia.org/wiki/Depth_charge#/media/File:USS_Cassin_Young_depth_charges.jpg
A depth charge is an anti-submarine warfare weapon. It is intended to destroy a submarine by being dropped into the water nearby and detonating, subjecting the target to a powerful and destructive hydraulic shock. Most depth charges use high explosive charges and a fuze set to detonate the charge, typically at a specific depth. Depth charges can be dropped by ships, patrol aircraft, and helicopters.
A 1913 Royal Navy Torpedo School report described 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 and Sons of Sheffield) preset for 45 ft (14 m) firing, to be launched from a stern platform. Weighing 1,150 lb (520 kg), and effective at 100 ft (30 m), the "cruiser mine" was a potential hazard to the dropping ship. The design work was carried out by Herbert Taylor at the RN Torpedo and Mine School, HMS Vernom.
The first effective depth charge, the Type D, became available in January 1916. It was a barrel-like casing containing a high explosive (usually TNT, but amatol was also used when TNT became scarce). There were initially two sizes—Type D, with a 300 lb (140 kg) charge for fast ships, and Type D* with a 120 lb (54 kg) charge for ships too slow to leave the danger area before the more powerful charge detonated.
A hydrostatic pistol actuated by water pressure at a pre-selected depth detonated the charge. Initial depth settings were 40 or 80 ft (12 or 24 m). 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, Ireland, on 22 March 1916, by the Q-ship Farnborough. Germany became aware of the depth charge following unsuccessful attacks on U-67 on 15 April 1916, and U-69 on 20 April 1916. The only other submarines sunk by depth charge during 1916 were UC-19 and UB-29.
Numbers of depth charges carried per ship increased to four in June 1917, to six in August, and 30-50 by 1918. The weight of charges and racks caused ship instability unless heavy guns and torpedo tubes were removed to compensate. Improved pistols allowed greater depth settings in 50-foot (15-meter) increments, from 50 to 200 ft (15 to 61 m). Even slower ships could safely use the Type D at below 100 ft (30 m) and at 10 kn (19 km/h; 12 mph) or more, so the relatively ineffective Type D* was withdrawn.
The Royal Navy Type D depth charge was designated the "Mark VII" in 1939. Initial sinking speed was 7 ft/s (2.1 m/s) with a terminal velocity of 9.9 ft/s (3.0 m/s) at a depth of 250 ft (76 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).
The British Mark X depth charge weighed 3,000 pounds (1,400 kg) and was launched from 21-inch (53 cm) torpedo tubes 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 they were known to be troublesome.
Although the explosions of the standard United States 600 lb (270 kg) Mark 4 and Mark 7 depth charge used in World War II were nerve-wracking to the target, an U-boat’s undamaged 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 charge. Many survived hundreds of depth charges over a period of many hours; U-427 survived 678 depth charges fired against it in April 1945.
Delivery mechanisms The first delivery mechanism was to simply roll the "ashcans" off racks at the stern of the moving attacking vessel. Originally depth charges were simply placed at the top of a ramp and allowed to 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. These racks remained in use throughout World War II, because they were simple and easy to reload.
Some Royal Navy trawlers used for anti-submarine work during 1917 and 1918 had a thrower on the forecastle for a single depth charge, but there do not seem to be any records of it being used in action. Specialized depth charge throwers 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 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 created an improved version able to throw a charge 40 yd (37 m). The first was fitted in July 1917 and became operational in August. In all, 351 torpedo boat destroyers 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 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 m) over each side of the ship. The main disadvantage of the Ygun was that it had 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 and Engine Company beginning on 24 November 1917.
The first to deploy depth charges from airplanes in actual combat were the Finns. Experiencing the same problems as the RAF with insufficient charges on anti-submarine bombs, Captain Birger Ek of Finnish Air Force squadron LeLv 6 contacted one of his navy friends and suggested testing the aerial use of standard Finnish Navy depth charges. The tests proved successful, and the Tupolev SB bombers of LeLv 6 were modified in early 1942 to carry depth charges. The success of the anti-submarine missions reached RAF Coastal Command, which promptly began modifying depth charges for aerial use.
Depth charges could 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 kg) antisubmarine bomb. This weapon was too light and ultimately, a failure. Indeed, on 5 September 1939, a Royal Air Force Avro Anson of No. 233 Squadron was destroyed when its own antisubmarine bomb skipped off the surface of the water and detonated under the aircraft.
Above, Brazilian crewmembers from Submarine Chaser Guajara launch depth charges during a drill in south Atlantic. Photo. https://en.wikipedia.org/wiki/Battle_of_the_Atlantic
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 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 Mark 11 Depth Charge, deployed by the British Fleet Air Arm.
Effectiveness As the Battle of the Atlantic wore on, British and Commonwealth 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 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 submarine 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 to illuminate just prior to the attack), then quickly attacking once it had been located, as the submarine would normally crash dive to escape attack.
Surface ships usually used ASDIC (sonar) to detect submerged submarines. However, to deliver its depth charges a ship had to pass over the contact to drop them over the stern; sonar contact would be lost just before attack, rendering the hunter blind at the crucial moment. This gave a skilful submarine commander an opportunity to take evasive action. In 1942 the forward-throwing "hedgehog" mortar, which fired a spread salvo of bombs with contact fuzes at a "stand-off" distance while still in sonar contact, was introduced and proved to be effective.
By https://en.wikipedia.org/wiki/Depth_charge
IWM caption : A depth charge explodes after it had been dropped from HMS CEYLON. The ship had just made a call at Colombo the capital of Ceylon.
Photo. https://upload.wikimedia.org/wikipedia/commons/7/7b/HMS_Ceylon_depth_charge.jpg