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 "Type D" could be detonated as deep as 300 feet (100 meters).
The first delivery mechanism was to simply roll the "barrels" off racks at the stern of the attacking vessel. Later, special depth-charge projectors or "K-guns" were developed, which used an explosive propellant charge to hurl charges about 150 feet (50 meters) to the sides of the attacker. K-guns were often used together with stern racks to create patterns of six to ten charges. The attacking ship needed to be moving above a certain speed or it would be damaged by its own weapons.
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. 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 carefully-aimed attack. Many survived hundreds of depth charge detonations over a period of many hours; 678 depth charges were dropped onto U-427 in April, 1945. The U-boat survived.