On the Attack:
Depth Charges & Hedgehogs
Depth charges and hedgehogs were two of the most important weapons the Allies used to attack U-boats hiding underwater. Depth charges were powerful explosive devices that were dropped into the water after being preset to detonate at a specified depth. They didn't have to hit a sub to cause damage — by striking close enough, the blasts could shatter equipment inside a U-boat and even damage its hull. Hedgehogs detonated only upon direct contact with the targeted sub, or with the seafloor in the case of a miss. In 1943, the U.S. Navy started packing depth charges and hedgehogs with a new explosive called Torpex, which was 50% stronger than the previously used Trinitrotoluene (TNT).
The Mark 9 and Mark 4
Mark 9 depth charges and Mark 4 hedgehogs were used in the attack against the U-505. Mark 9 depth charges had a pressure-activated fuse that detonated 200 pounds of Torpex at a predetermined depth anywhere from 30-600 feet below the surface. Mark 4 hedgehogs were filled with 35 pounds of Torpex. They were launched in a salvo, or cluster, 24 at a time, from spiked fittings that gave the launcher a hedgehog-like appearance.