Once the Allies had achieved air superiority in the Channel, U-boats were forced to travel under water during daylight at the start of their journey to the Atlantic. Close to the coast the water was shallow and when their batteries were exhausted, they had to surface anyway and aircraft located them by radar. Doenitz decided to build submarines which could fight it out on the surface (aircrat carried depth charges not bombs) so we have the VIIc Flak!