When war broke out, only 57 U-Boats were available. He persuaded engineers that more boats were urgently required. Even so, the small, outgunned German fleet managed to strike painful blows to Great Britain by aiming directly for its soft underbelly. The Germans almost succeeded in cutting off Great Britain's shipping lanes, and thus its supply of fuel and raw materials. The German U-boat onslaught against British merchant ships during the autumn of 1940 was highly successful because the attacks were made on the surface at night and from such close range that a single torpedo would sink a ship. Soon, though, Allied technology was able to detect U-boats at night, and new convoy techniques, combined with powerfully-armed, fast modern aircraft searching the seas, meant that by 1941 it was clear that Germany was losing the war at sea.