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. By the end of the war it was assumed that a 300 lb (136
kg) depth charge had to be less than 14 feet (4 meters) from a
submarine to buckle its hull, 30 feet (9 meters) to disable it, and 60
feet (20 meters) to do significant damage.
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 of Type 88 explosive (ammonium perchlorate and ferrosilicate). 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) of Type 97 explosive (70% TNA/30% HNDA) 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 Type 97 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 two types of depth charges. The Mark 6 depth charge had a 300 lb (136 kg) charge of TNT and could be set to explode at 30 to 300 feet (9 to 90 meters). The sinking rate was 8 feet per second (2.4 m/s). The U.S. Navy conducted tests with the Mark 6 and suggested it would inflict fatal damage within 30 feet (10 meters), serious damage within 60 feet (20 meters), and significant damage within 90 feet (30 m). The Mark 7 depth charge was similar to the Mark 6 but had a 600 lb (270kg) charge. Later models of both types 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 12 feet per second (3/7 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 Mark 6 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.
U.S. tactics at the beginning of the war were to drop a pattern of
seven depth charges over the likely location of a submarine. An
American destroyer of this period typically carried enough depth
charges for four such patterns. This proved inadequate, and some Bensons
were modified to carry enough depth charges for seven or more patterns.
Postwar analysis showed that a depth charge attack
in the Battle of the Atlantic had about a 6% chance of sinking the
target. The figure was likely higher against Japanese submarines in the
Pacific, since Japanese submarines could neither dive as deeply or as
quickly as the German U-boats. It
was likely lower against U.S. submarines, given the depth limitations
of early Japanese depth charges and the tendency of Japanese
antisubmarine forces to fail to press home their attacks. Less than 22
U.S. submarines were lost to depth charge attack in the Pacific, while
the Japanese lost as many as 85 submarines to depth charge attack.
Depth Charge
Throwers. 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.
The U.S. Mark 6 depth charge projector was
typical. It was essentially a fixed black powder mortar whose range was
adjusted by using different black powder charges. These gave ranges of
50, 75, and 120 yards (45, 70, 110 meters) with a Mark 6 depth charge
or 60, 90, or 150 yards (55, 80, or 140 m) with a Mark 9 depth charge.
References
Campbell
(1985)
Friedmann
(1995)
Keegan
(1988)
Lacroix
and Wells
(1997)
Ross (1988)
The Pacific War Online Encyclopedia © 2006, 2008-2009 by Kent G. Budge. Index
