Although there were some kills from "dead on" hits from DC's, most of the time it was accumulated damage which forced a U-boat to the surface that resulted in the loss of the boat. For example, cracked battery cells released their contents into the bilge, which mixed with sea water and created chlorine gas, forcing an emergency surface. Later in the war, when the Allies could take the time to prosecute a contace to conclusion, U-boats were forced to surface due to low air or batteries. Aerial depth charges initially caused little damage, as their depth settings were too shallow. When they increased the detonation depth to 50' they achieved more kills, as the DC's went off below a U-boat which had just commenced its emergency dive. Probably the best ASW weapon deployed from ships was the Hedgehog, as it could often be deployed while the pinging escort still had ASDIC contact. Hedgehog attacks were four times as successful as DC attacks. If I recall correctly, DC's had a three percent success rate whereas Hedgehog had a twelve percent success rate; I believe these figures were cited in Clay Blair's two volume set.
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