Timm won the Iron Cross during the Norwegian invasion in May 1940, and then he transferred into the U-boat fleet. After U-boat commander's training at Pillau, in September 1941 Timm took command of the new Type VIIC U-boat . After training missions in the Baltic Sea, the "U-251" was assigned to the "11th U-boat Flotilla", which was based at Bergen, Norway, in April 1942. Timm next commanded nine war patrols into the Arctic Sea, on the prowl against the arctic convoys of World War II to the northern seaports of the Soviet Union. There, the "U-251" sank two merchant ships: The first one, on 3 May 1942, was the 6,135-ton British merchant ship SS "Jutland" of Convoy PQ-15, while the second, in July 1942, was the American cargo ship from the ill-fated Convoy PQ 17. Timm also took part in "Operation Wunderland" in the Kara Sea, surfacing close to Uyedineniya Island and destroying a Soviet weather station with cannon fire from his deck gun.