Naval
Historical Center #NH 99897
The depth charge was the basic antisubmarine weapon of the Second World War. It was a large (200 lb/90 kg or more) explosive charge with a hydrostatic fuse that could be set to the estimated depth of the submarine. It was dropped off the stern of an antisubmarine vessel over the estimated location of the submarine and would sink to its fused depth and explode. At very close range, the resulting shock wave was capable of buckling a submarine pressure hull, resulting in catastrophic failure and the destruction of the submarine. Such close detonations proved difficult to achieve, and the submarine usually had to be battered to death with dozens of depth charges.
Japanese depth charges were rather light and were usually set too shallow in the early days of the war, allowing many American submarines to escape. The Japanese were apparently unaware that the more modern American subs could dive to below 300 feet. Japanese escort commanders also had a tendency to assume a kill at the first sign of floating oil or debris. However, one of the stupidest men to ever darken the doors of the United States Congress revealed in a press conference that U.S. submarines were capable of diving deeper than the Japanese thought, and Japanese kills of U.S. submarines promptly increased. The Congressman in question was politically powerful and was able to avoid any immediate consequences, but was convicted on an unrelated charge of accepting bribes after the war ended.
All Japanese depth charges had dimensions of 30.5" by 17.7" (77.5cm by 45cm). The Type 95 was standard at the time war broke out, with a 220 lb (100 kg) charge. Its fuse had a water inlet that detonated the charge when a certain amount of water had entered. There were just two depth settings, 40 feet (30m) and 200 feet (60m), the latter well above the dive depth of American subs. Later the charge was increased to 324 lb (147kg) and a 300 foot (90m) setting was introduced. Slow ships could drop the charge with a parachute to retard its sinking until the ship was clear, but this reduced the depth setting to a maximum of 40 feet (30m). The Type 2, introduced later in the war, was similar to British depth charges, with 230 lbs (105kg) of explosive and depth settings down to 480 feet (145m). Later versions of the Type 2 had up to 357 lbs (162kg) of explosives. The Japanese experimented with a depth charge having 220 lbs (100 kg) of explosives and a magnetic influence fuse, but this weapon did not reached production before the surrender.
The Americans began the war with the Mark 7 depth charge, which had
a 600 lb (270kg) charge, sinking rate of 9 feet per second (2.7 m/s)
and depth settings from 30 to 300 feet (9 to 90 meters). Later models
could be set to 600 feet (183m) to reach the deep-diving German U-boats, and had additional
ballast to increase the sinking rate to 13 feet per second (4 m/s). The
Mark 8 entered service in early 1943 and had a magnetic influence fuse
that proved as unreliable as that of American torpedoes. It had a backup
hydrostatic fuse that could be set as deep as 500 feet (152 meters).
The Mark 9 had a teardrop shape and fins to make it spin and thus
stabilize its underwater trajectory, but this came at the cost of
reducing the charge to 200 lbs (90 kg) of Torpex, in order to allow the
weapon to fit in existing tracks and throwers. Sinking rate was 14.5
feet per second (4.4 m/s). Some of the later versions could be set as
deep as 1000 feet (300m). The Americans also experimented with an
acoustic fuse, which proved much more effective than the magnetic
influence fuse, but this was not ready before the surrender.
A weakness of early depth charge attacks was that sonars of the period were unable to
track a submarine directly under the escort vessel. This gave the
submarine a chance to evade at the last moment. The Allies developed
cooperative tactics in which a second escort guided the attacking
escort over the target, and also developed depth charge throwers that
could throw a depth charge some distance to the side of the attacking
ship. The Japanese also installed a limited number of depth charge
throwers on their more modern destroyers.
References
The Pacific War Online Encyclopedia (c) 2006, 2008 by Kent G. Budge. Index
