This was a U-boat patrol operation along the west coast of Norway to screen the German navy’s surface forces involved in ‘Weserübung’ from attack by Allied surface units (April 1940).
The U-boat groups, in each of which the boats came under the direct command of Konteradmiral Karl Dönitz, the Befehlshaber der Unterseeboote, were Gruppe 1 in the Vestfjord area with U-25, U-46, U-51, U-64 and U-65; Gruppe 2 in the Trondheim area with U-30 and U-34; Gruppe 3 in the Bergen area with U-9, U-14, U-56, U-60 and U-62; Gruppe 4 in the Stavanger area with U-1 and U-4; Gruppe 5 in the area to the east of the Shetland islands group with U-47, U-48, U-49, U-50, U-52 and later U-37; Gruppe 6 in the Pentland Firth area with U-13, U-19, U-57, U-58 and U-59; Gruppe 8 in the Tindesnes area with U-2, U-3, U-5 and U-6; and Gruppe 9 in the area of the Shetland and Orkney island groups with U-7 and U-10.
'Hartmut' (i) was essentially unsuccessful, despite the fact that many of the boats managed to achieve good firing positions, because of defects in their torpedoes' depth-keeping mechanisms and magnetic fusing. The only successes were registered by U-4, which on 10 April sank the British submarine Thistle and U-13, which sank two transports totalling 9,875 tons (4,935 ton British Swainby on 17 April and 1,281-ton Danish Lily on 26 April) and torpedoed and damaged the 6,999-ton British tanker Scottish American on 28 April. In addition, U-59 sank the 2,118-ton Norwegian Navarra on the boat’s outward voyage, and U-37 sank three ships totalling 18,715 tons (9,076-ton Swedish Sveaborg on 10 April, 5,128-ton Norwegian Tosca on 10 April and 4,511-ton British Stancliffe on 12 April) while returning from a special operation.
Four boats were lost: Kapitänleutnant Max-Hermann Baurer’s U-50 and Kovettenkapitän Jürgen Deecke’s U-1 on about 6 April in the British Minefield No. 7 (laid by the destroyers Esk, Express, Icarus and Ivanhoe) on the way out through the German 'Westwall' minefield, Kapitänleutnant Georg-Wilhelm Schulz’s U-64 off Narvik on 13 April to a Fairey Swordfish aeroplane from the battleship Warspite, and OberleutnantKurt von Gossler’s U-49 on 15 April to the destroyers Brazen and Fearless off the Vaagsfjord, when important documents and a grid map were captured by the British.