relying on electric motors for propulsion while underwater. This design had important tactical implications. The electric motors were far less powerful than the diesel engines and had a short battery life. When submerged, most submarines were capable of about 10 knots, little more than the slowest merchantman. So a submerged submarine was not only much slower than when on the surface, but also unable to proceed at its maximum submerged speed for any length of time. Submarines of the Second World War were more submersibles than true submarines. Under Admiral Karl Dönitz the U-boats further developed the tactics of surface night attacks that had first been used in the First World War and then refined in exercises in the Baltic before the Second World War. Rather than attacking in daylight while submerged, the German U-boat commanders developed a tactic for operating more like torpedo boats than submarines, attacking on the surface at night where they could use their higher surfaced speed. Approaching the convoys at night on the surface, they found that they could pass quite close to the escorts and yet remain undetected. Look outs high up on the bridges of the escort ships found it all but impossible to spot the low shape of the U-boat with the tiny silhouette of the boat's conning tower against the darkness of the water. But for the U-boats, the escorts and the merchantmen stood out starkly against the lightness of the sky. The convoy battles of the First World War had taught the Germans that a single submarine had little prospect of success against a well defended convoy. Instead of attacking the Allied convoys singly, the German U-boats now began to work in packs co-ordinated centrally by radio. The boats spread out into a long patrol line that crossed the path of the Allied convoy routes. Once in position the boats used hydrophones to pick up the propeller noises of the convoys, or used binoculars to try to spot the tell-tale smoke of a convoy on the horizon. When one boat sighted a convoy, it followed it, broadcasting the convoy's position and waiting for other boats to come up before attacking. So instead of being faced by a single submarine, the convoy escorts had to cope with a group of U-boats attacking in a single night. The most daring commanders, like Otto Kretschmer, not only penetrated the convoy's screen but attacked