U-boats exploited darkness by entering a convoy on the surface and attacking from among the merchant ships. Wargaming helped the Royal Navy infer how those attacks worked; Enigma and new sensors were not the cause. Placing a sloop or corvette inside the convoy meant a surfaced submarine could suddenly meet a better-armed escort at very short range. Damage that prevented the U-boat from diving could remove both its immediate escape and its future operational value, helping explain a rise in losses in late 1940 and early 1941.
Notable Point · 1:14:56-1:17:18
How wargaming exposed the U-boat night attack
Royal Navy wargaming inferred that U-boats were entering convoys on the surface after dark, allowing escorts placed inside the columns to turn the tactic into a trap.