The depth charge is the original dedicated Anti-Submarine Warfare (ASW) weapon. It was an extraordinarily crude weapon - a can filled with explosives and a fuze that detonated at a preset depth based on hydrostatic pressure. This was adequate in WWI as submarines did not operate at great depths. Developed by the Royal Navy in 1916, the quantities used per kill in WWI gave no appreciation for what would be required in WWII.
The USN began to develop a depth charge of their own in early 1917, which was too weak to be successful. After the U.S. entry into WWI they adopted the Royal Navy depth charge fitted with their own hydrostatic fuze. The final U.S. WWI depth charge could detonate at up to 300ft depth and carried 300lbs of explosives. There was little development between the wars except for a 600lb variant.
At the start of WWII, depth charges were essentially the same weapon as from the end of WWI. Development concentrated on increasing the depth at which a submarine might be successfully attacked and improvements to the sinking speed of the depth charges.
The MK9 became the main improvement over the MK6. This was teardrop-shaped, capable of detonating at 1000ft and entered service in 1943. The early MK9s still did not yet sink sufficiently fast enough. Lead ballast and fins were added to speed sinking. These modifications were at the sacrifice of explosive power and it now carried only 200 lbs of TNT.
Depth charges were detonated by a spring-loaded firing pin released by a water pressure driven bellows system. The mechanism could be set to various depths based on the attacking vessel's estimate of the depth of the submarine. A late war variant included a magnetic detonator which automatically exploded the depth charge when it reached the proximity of a submarine.
Mark 6: An older depth charge that was cylindrical in shape, about 28 inches long and 18 inches in diameter. They contained 300 pounds of TNT. These were primarily used in racks but could also be fired from K-guns. A heavier 600 lb. version was also utilized from the depth charge racks to stern and the rails could be adjusted to accommodate either the 300 or 600 lb. variants.
MK6 (Early War)
Mark 9 and 14: These were the later type of depth charges and had a teardrop shape, with a weighted nose to increase their sinking rate and improve underwater trajectory. They contained about 200 pounds of TNT or of HBX and have the same overall dimensions as the 300 pound cylindrical charges. These were primarily used in K-Guns but could be used in racks with minor modifications to the tracks.
MK9 (Late War)
Patterns: Destroyer escorts and other vessels likely to engage submarines carried depth charges. Generally, several charges are released in rapid succession to form a pattern in depth, width and length, and thus increase the probability of destroying the submarine. The pattern is obtained by dropping some charges from release gear on the stern and firing others abeam from projectors, with appropriate depth settings made on the charges before launching.