This was a U-boat wolfpack operation in the Atlantic against the MKS.33/SL.142, OS.62/KMS.36 and MKS.34/SL.143 paired convoys (18 December 1943/3 January 1944).
The wolfpack comprised U-107, U-231, U-270, U-275, U-305, U-377, U-382, U-415, U-541, U-618, U-641, U-645, U-667, U-758, U-801, U-953 and U-962, and for the loss of Oberleutnant Otto Ferro’s U-645 sank two destroyers, namely the British Hurricane and US Leary.
Knowing that the German blockade runner Osorno was approaching the mid-Atlantic on her return from the Far East with vital war supplies, and that the Germans were planning a U-boat offensive to distract them, the Allies planned 'Stonewall' to find and destroy the blockade runner, and also established two task groups to hunt the U-boats: Captain Arnold J. Isbell’s TG21.14 comprised the escort carrier Card and destroyers Decatur, Leary and Schenk, and Captain James R. Dudley’s TG21.15 comprised the escort carrier Core and destroyers Belknap, George E. Badger, Goldsborough and Green.
Grossadmiral Karl Dönitz, commander-in-chief of the German navy, had specifically ordered the creation of the 'Borkum' wolfpack for the interception of the MKS.33 and SL.142 convoy group and thus distract Allied attentions from Osorno, which was in fact located on 23 December by a Grumman Wildcat from Card despite the adversity of the weather, and later escorted into the Gironde estuary, downstream of Bordeaux, by German surface warships in 'Bernau'.
During the night of 23/24 December U-305 made the first sighting of the Card group, but was driven off by Schenck after being located by HF/DF as it tried to make a report. U-415 missed Card with a salvo of three pattern-running torpedoes and Decatur with a single conventional torpedo. Schenck avoided a torpedo from U-645 and then depth-charged and sank the boat. Oberleutnant Helmut Bork’s U-275 then secured a torpedo hit on Leary, which had been detached to support Schenk, and Oberleutnant Rudolf Zorn’s U-382 sank the destroyer.
The Core group was then forced to leave the area as its ships were running short of fuel.
During the evening of 24 December the OS.62 and KMS.36 convoys, heading south with the British Escort Group B1 and a support group centred on the escort carrier Striker, encountered the 'Borkum' wolfpack and lost the destroyer leader Hurricane to a torpedo from Kapitänleutnant Kurt Neide’s U-415. On 25 December and during the night of 25/26 December the boats of the 'Borkum' wolfpack attempted to continue their attacks on the OS.62 and KMS.36 convoys, but only U-305 and U-270 were able to launch torpedoes, albeit without any success, against the warships of the escort group which had remained in the boats' area.
On their way to a new concentration for 30 December against the expected MKS.34 and SL.143 convoys, some of the 'Borkum' wolfpack’s boats (U-270, U-801, U-571, U-305, U-275, U-382, U-758, U-641 and U-377) made limited contact on 29 and 30 December with the 6th Escort Group of British frigates and Canadian corvettes, which were attempting to rescue the survivors of the engagement on 27 December. U-275 and U-270 were attacked by the frigates and U-275 was also attacked from the air. The outbound U-629 and U-541, also searching for survivors, and U-421 and U-543 made several unsuccessful attacks on Allied escort vessels.
The MKS.34 and SL.143 convoys, supported by the British Escort Group B3, were located by German air reconnaissance on 30 and 31 December but passed through the U-boats' patrol line on 1/2 January. U-382, U-275 and U-305 all missed escort vessels in torpedo attacks. An attempt to intercept a southbound convoy on 2/3 January failed for lack of the air reconnaissance that might have located targets for the U-boats, and U-270, which had earlier been bombed, missed a destroyer with a single torpedo.
In an effort to make the Allied location of their patrol lines, and thus their avoidance, more difficult for the Allies, the 'Borkum' wolfpack was then divided into three smaller packs, namely 'Borkum I' with U-270, U-305 and U-382, 'Borkum II' with U-758 and U-641, and 'Borkum III' with U-377, U-953 and U-231. On 5, 6 and 7 January, at locations to the north-west of Spain, the boats came into frequent contact with Commander J. D. Birch’s British 5th Support Group (frigates Nene and Tweed, and Canadian corvettes Calgary, Snowberry, Edmundston and Camrose) searching for the blockade runners which were expected. On 5 January U-758 missed a corvette with a single torpedo. On 6 January U-270 shot down a Boeing Fortress long-range patrol bomber of the RAF’s No. 220 Squadron, but was itself damaged, and on 7 January Kapitänleutnant Rudolf Bahr’s U-305 sank Tweed with a single torpedo.
On 8 January one, on 9 January three, and on 10/11 January two Junkers Ju 290 long-range maritime patrol aircraft of Hauptmann Hermann Fischer’s Fernaufklärungsgruppe 5 were deployed against the expected MKS.35 and SL.144 convoys, but only on 9 January did one of its aircraft sight the convoy group to the west of Portugal. In the evening of 11 January U-305 sighted the convoy group. Using their 37-mm and 20-mm anti-aircraft cannon, the U-boats were able to fight off the aircraft of the US support group, which included the escort carrier Block Island, but were nonetheless unable to press home their attacks. U-953 missed the Canadian corvette Lunenburg of the escort group with a single torpedo. On 13 January the operation had to be broken off, and a Vickers Wellington medium-range patrol bomber of the RAF’s No. 172 Squadron, summoned to the scene by one of Block Island's aircraft, sank Kapitänleutnant Wolfgang Wenzel’s U-231.
From the middle of January the remaining boats of the 'Borkum' wolfpack (U-305, U-382, U-377 and U-641) and the newly arrived U-571, U-271, U-212, U-592, U-231 and U-238 work with the boats of the 'Rügen' wolfpack to the west of Ireland as single boats or in various concentrations.