'Knappen' was a U-boat wolfpack operation, in tandem with 'Ritter', in the Atlantic against the ON.166 convoy (19/25 February 1943).
The wolfpack comprised U-91, U-92, U-600 and U-604, and for the loss of none of its own number sank two ships (11,673 tons) and damaged another two ships in attacks on the ON.166 convoy.
From 14 February the 'Ritter' pack had been created in the central Atlantic to attack the HX.226 convoy of 49 ships bound from New York to Liverpool and supported by Commander K. F. Adams’s Canadian Escort Group C3 (destroyers Assiniboine and British Burnham, British frigate Jed, and corvettes Malbaie and British Bittersweet and Eyebright). Warning of this convoy’s passage had been provided by the B-Dienst, the German naval signals intelligence branch, and this allowed Grossadmiral Karl Dönitz, commander-in-chief of the German navy but still in day-to-day command of the U-boat arm via Konteradmiral Eberhard Godt, the service’s operations chief, to concentrate U-225, U-332, U-377, U-454, U-468, U-529, U-603, U-623, U-628, U-653 and U-753).
On 15/16 February the SC.119 convoy of 60 ships, supported by Commander E. C. Bayldon’s British Escort Group B1 (destroyers Hurricane, Rockingham and Watchman frigate Kale, and corvettes Dahlia, Meadowsweet, Monkshood and Wallflower) was re-routed to the north and passed 'Ritter' without being spotted. An escorting Consolidated Liberator long-range patrol bomber of the RAF’s No. 120 Squadron sank Oberleutnant Wolfgang Leimkühler’s U-225. Following the receipt of 'Ultra' information, the HX.226 convoy was re-routed onto a wide detour to the north and passed the U-boat patrol line on 18 February, again without being spotted.
On 17 February the ON.166 convoy and the following ONS.167 convoy, supported by the British Escort Group B3, again benefited from 'Ultra' intelligence and were re-routed to the south of the 'Ritter' wolfpack as this and the new 'Neptun' (i) wolfpack formed a long patrol line designed to ensure the interception of the ON.166 convoy along its original route, which had been discovered by the B-Dienst. On 18/19 February, decoded reports from escorting aircraft revealed the new British routes, and the outbound U-91, U-92, U-600 and U-604 were grouped as the 'Knappen' wolfpack in an area to the south-east of that in which 'Ritter' was operating.
Toward 12.00 on 20 February, U-604 spotted the ON.166 convoy of 40 grouped ships and nine stragglers, supported by Commander Paul R. Heineman’s US Escort Group A3 (US Coast Guard cutters Spencer and Campbell, Canadian corvettes Chilliwack, Dauphin, Rosthern and Trillium, and British corvette Dianthus). During the night of 20/21 February U-604 was located on radar by Spencer and depth-charged before the wolfpacks directed to the scene could arrive. During the morning of 21 February U-753 located the straggling 5,964-ton Norwegian Stigstad, which was later sunk by Oberleutnant Hans-Joachim Bertelsmann’s U-603 and Oberleutnant Eberhard Hüttemann’s U-332. Oberleutnant Hermann Schrüder’s U-623 was sunk by a Liberator of No. 120 Squadron. In the afternoon U-91 was bombed by a flying boat, which also drove off U-332, U-454 and U-753 in co-operation with Campbell, Dianthus and Dauphin.
During the night of 21/22 February U-92 reached the convoy and in two approaches torpedoed the 9,990-ton British Empire Trader and 9,348-ton Norwegian N. T. Nielsen-Alonso. The first was detached with Dauphin but later had to be sunk by the corvette, and the second was sunk by the Free Polish destroyer Burza, which joined the escort from the ONS.167 convoy, after U-92 and U-753 had attempted to finish off the damaged ship with torpedoes.
At this time Spencer possibly or a Liberator of the RAF’s No. 120 Squadron probably sank Kapitänleutnant Georg-Werner Fraatz’s U-529, whose loss became known only in March.
During daylight on 22 February U-606, U-603 and U-628 took turns in maintaining contact with the convoy, and by the fall of nightl had brought up U-92, U-358, U-223, U-186 and U-753, though most of these boats were then driven off. U-753, which was depth-charged by Rosthern and Trillium, and Oberleutnant Hans-Heinrich Döhler’s U-606 attacked almost simultaneously. U-606 torpedoed the 5,687-ton US Chattanooga City, 6,615 British Empire Redshank and 4,959-ton US Expositor: the first two sank, while the third went down only when finished by Kapitänleutnant Karl-Franz Heine’s U-303. Immediately after its own attack run, U-606 was depth-charged by Chilliwack and Burza, soon afterward collided with Campbell, and sank. Campbell started to take water into her engine room, became unmanoeuvrable and was taken in tow by Burza.
During the night Kapitänleutnant Horst Höltring’s U-604 sank the 1,683-ton British rescue ship Stockport coming up from astern. Then in the morning of 23 February Kapitänleutnant Heinrich Hasenschar’s U-628 and Kapitänleutnant Siegfried Hesemann’s U-186 attacked almost simultaneously. Hasenschar’s U-628 torpedoed the 6,907-ton Panamanian Winkler and 6,409-ton Norwegian Glittre, which were later finished by Kapitänleutnant Karl-Jürg Wächter’s U-223 and Bertelsmann’s U-603. Hesemann’s U-186 sank the 6,207-ton British Eulima and 5,401-ton US Hastings in attacks facilitated by the fact that the convoy was by now escorted only by Spencer, Rosthern, Chilliwack and Dianthus, the last remaining behind with the disabled ships.
On 23 February U-628 and U-707 kept contact, and by the evening had brought up U-621, U-358, U-653, U-468, U-92 and U-600. After the fall of night and now escorted by only three warships, the convoy used sharp changes of course dark to shake off the U-boats, so there were no attacks during the night of 23/24 February. Nevertheless, two stragglers were sunk or damaged: Oberleutnant Günter Gretschel’s U-707 sank the 7,176-ton US Jonathan Sturges, and Kapitänleutnant Gerhard Feiler’s U-653 damaged the 9,382-ton Dutch Madoera. During the morning of 24 February Kapitänleutnant Bernhard Zurmühlen’s U-600 torpedoed and damaged the 4,391-ton Norwegian Ingria from the convoy, which was now escorted by Spencer, Chilliwack, Rosthern and Trillium; Hasenschar’s U-628 later sank Ingria.
On 24 February U-628 and U-603 kept contact with the convoy and brought up U-621, U-600 and U-604, of which the last was depth-charged and damaged. Aircraft from Newfoundland arrived overhead and drove off the U-boats, though U-621 missed Spencer during the evening. The escort was then reinforced by the British destroyers Montgomery and Witherington. Toward morning on 25 February U-628, U-92 and U-600 attacked almost simultaneously: the first sank the 7,264-ton British Manchester Merchant, and the other two boats missed Spencer.
On 25 February U-468 maintained contact, but as the area began to be covered in mist only U-600 and U-621 came up, albeit only briefly. Contact was lost in the evening and the Germans broke off the operation during the morning of 26 February.