Convoy zigzagging and the inner escort

Convoy zigzagging made long-range torpedo solutions unreliable, so a U-boat had to close the range and expose itself to detection. Early in the war, some U-boats exploited the escort screen by entering the merchant columns on the surface while escorts watched the approaches. Wrens analysing attack patterns at Liverpool concluded that the attackers must already be inside the convoy. The Royal Navy responded by placing an escort among the merchant ships. A surfaced submarine there faced a gunfight with a ship designed for surface combat, which made the infiltration tactic far more dangerous. The attacks on U-99 and U-100 followed a different sequence: depth charges forced the boats up before they were destroyed on the surface.