Four-inch gun, Hedgehog, and depth-charge patterns

The four-inch gun punished submarines caught on the surface, while depth charges attacked boats after an escort passed overhead. Early depth-charge production was so limited that ships carried only a handful, and later increases imposed topweight and stability costs on small escorts. K-guns and stern racks eventually spread six-to-ten-charge patterns across a submarine's possible maneuvering space. Hedgehog changed the geometry again by letting an ASDIC-equipped corvette fire ahead while maintaining contact, reducing the need for several escorts to prosecute one target. That mattered at convoy scale because more ships could remain with the merchants or engage other members of a wolf pack.