A munition used by surface warships as a means of destroying submarines, typically an explosive-packed container dropped over the stern of a warship, armed by a fuse primed to detonate when it senses the water-pressure at a predetermined depth. From the middle of World War 2 the depth charge was supplanted by weapons which threw the explosive munition ahead of the anti-submarine warship. From the late 1950s, nuclear depth charges with explosive radii greater than one kilometre have been in service with nuclear-equipped navies. These require a device such as a missile or pilotless aircraft to take them a safe distance away from the launching warship.
The depth charge is the oldest anti-submarine weapon. It is usually a cylindrically-shaped object packed with explosives and a fuse set to go off at a pre-determined depth. Depth charges could be deployed by both ships and aircraft. The first effective depth charge, the “Type D,” developed in 1916, was a 300-pound (140 kg) barrel-like casing containing a high explosive, usually TNT. A “pistol” actuated by water pressure at a pre-selected depth detonated the charge.
The depth charge was such a successful device that it attracted the attention of the United States, who requested full working drawings of the devices in March 1917.
In 1943, Torpex, an explosive 50% more powerful than TNT, was introduced along with a more streamlined depth charge casing that sank faster. Although the explosions of the standard 600-pound 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 five meters. 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 charge detonations over a period of many hours; U-427 survived 678 depth charge blasts aimed at her in April, 1945, though many (if not all!) of these may have actually detonated nowhere near 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.
Later, special depth-charge projectors called “Y-guns” were developed, which used an explosive propellant charge to hurl charges about 150 feet (50 meters) to the sides of the attacker. Some Royal Navy trawlers used for anti-submarine work during 1917-1918 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. The first depth charge “projectors” were called Y-guns; Y-guns allowed for a wider “spread” of depth charges to be deployed by ships. The K-guns were often used together with stern racks to create patterns of six to ten charges. To remedy the failure of this weapon, the Royal Navy's 450 lb Mark VII depth charge was modified for aerial use by the addition of a streamlined nose fairing and stabilising fins on the tail. Later depth charges would be developed specifically for aerial use. Depth charges are especially useful for ‘flushing the prey' in the event a diesel submarine is lying on the bottom or otherwise hiding with all machinery shut down.
Effectiveness
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 upon location of the sub during the day and at night, then quickly attacking once it had been located, as the sub would normally crash-dive to escape attack.
As the Battle of the Atlantic wore on, British and Commonwealth forces in particular proved 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.
In the Pacific, Japanese depth charge attacks initially proved fairly unsuccessful against U.S. and Russian subs. Unless caught in shallow water, a U.S. submarine commander could normally dive to a deeper depth in order to escape destruction. At the press conference, May revealed that American submarines had a high survivability rate because Japanese depth charges were fused to explode at too shallow a depth. Soon, Japanese forces were resetting their depth charges to explode at a more effective average depth of 250 feet.
The shortcoming of the depth charge as deployed by surface ships was not the weapon itself, but how it was delivered. However, to drop its depth charges it had to pass over the contact to drop them over the stern.
Later Developments
For the reasons expressed above, the depth charge was generally replaced as an anti-submarine weapon. These weapons throw a pattern of warheads ahead of the attacking vessel to bracket a submerged contact. 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, United States and United Kingdom developed anti-submarine weapons using nuclear warheads and these are sometimes referred to as Nuclear Depth Bombs (NDB).
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