Even more dangerous to British ships were German submarines, the U-boats (Unterseeboten) which could hunt ships unseen beneath the waves and destroy them with torpedoes.

German U-boats had shown how dangerous they were from the very start of the war. A single submarine penetrated the Royal Navy's harbour at Scapa Flow and sank the battleship Royal Oak while it was at anchor in October 1939. The menace of the U-boats increased after the fall of France in July 1940, for they could now use French ports as bases for attacks on ships far out in the Atlantic.

As protection against U-boat attacks, ships travelled in convoys, guarded by Royal Navy destroyers and corvettes. The Germans, however, took to hunting in 'wolf packs' of fifteen to twenty U-boats, which waited in a line across likely convoy routes, ready to attack. By early 1941 so many British ships were being sunk by the wolf packs that Churchill said a 'Battle of the Atlantic' was taking place.