'Schlei' was a U-boat wolfpack operation in the Norwegian Sea and UK/Iceland gap (14/25 January 1942).
The wolfpack comprised U-136, U-213, U-404, U-578, U-591, U-656 and U-753, and for the loss of none of its own number sank three ships (5,878 tons) of the ON.63 and SC.67 convoys. Two of the three ships were corvettes (British Arbutus and Canadian Spikenard), and the third the 4,028-ton Norwegian transport Heina, all three ships falling victim to Kapitänleutnant Heinrich Zimmermann’s U-136.
The boats were initially part of a force of 12 'Type VII' class boats on passage from Germany to new bases in German-occupied France, and were halted, on the specific order of Adolf Hitler, in an area to the west of the Færoe islands group and the Hebrides island groups in case they were needed to repel the Allied invasion of Norway which the German leader believed was imminent. After their Atlantic pause, the boats continued on their way to France on 24 January.