'Westmark' was a U-boat wolfpack operation, together with 'Burggraf', 'Neuland' (ii), 'Ostmark' and 'Wildfang' (i), in the Atlantic against the SC.121 convoy (6/11 March 1943).

The wolfpack comprised U-228, U-230, U-332, U-359, U-405, U-409, U-432, U-448, U-523, U-526, U-527, U-566, U-591, U-616, U-634, U-659 and U-709, and for the loss of Kapitänleutnant Hermann Eckhardt’s U-432 to the depth charges and gun fire of the Free French corvette Aconit on 11 March sank 10 ships (36,518 tons) in attacks on the SC.121 and HX.228 convoys.

The SC.121 convoy of 59 ships, supported by Commander Paul R. Heineman’s US Escort Group A3 (US Coast Guard cutter Spencer, US destroyer Greer, Canadian corvettes Dauphin, Rosthern and Trillium, British corvette Dianthus, and British rescue ship Melrose Abbey) passed unobserved between the 'Wildfang' (i) and 'Burggraf' wolfpacks, but on 6 March U-405 sighted and reported the convoy, which as a result of a period of severe weather had become somewhat scattered and had several stragglers.

Grossadmiral Karl Dönitz, the 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, ordered the deployment of the 'Westmark' wolfpack against the convoy, and also the formation of the 'Ostmark' wolfpack to create a patrol line farther to the east from 8 March with U-190, U-229, U-439, U-447, U-530, U-618, U-641 and U-642 from the 'Neuland' (ii) wolfpack.

After U-405 had been driven off, Kapitänleutnant Paul Siegmann’s U-230 and U-566 re-established contact during the night of 6/7 March. Slipping in without being detected by the escorts, U-230 sank the 2,868-ton British Egyptian, and in the morning the 6,116-ton British Empire Impala, which had remained to recover survivors, was sunk by Kapitänleutnant Hans-Jürgen Zetzsche’s U-591. On 7 March U-228, U-230, U-409, U-526, U-591 and U-634 maintained contact in very bad weather including high wind, hail and snow, but could not attack.

During the morning of 8 March the wind abated, but the visibility remained very variable: U-527 missed the convoy but Oberleutnant Bernhard Müller’s U-633 sank the 3,921-ton British Guido, which had become separated. In the evening of the same day, Kapitänleutnant Max Wintermeyer’s U-190, Kapitänleutnant Herbert Uhlig’s U-527, Kapitänleutnant Hans-Jürgen Zetzsche’s U-591 and Kapitänleutnant Herbert Brünning’s U-642 sank the straggling 7,015-ton British Empire Lakeland, 5,242-ton British Fort Lamy (carrying the 291-ton LCT-2480 as cargo), 5,879-ton Yugoslav Vojvoda Putnik and 2,125-ton British Leadgate respectively.

On the following day the US Coast Guard cutters Bibb and Ingham and US destroyer Babbitt arrived from Iceland to strengthen the escort. Consolidated Liberator long-range patrol bombers of the RAF’s No. 120 Squadron also appeared overhead to provide air escort, and drove off the contact-keeping U-566.

Of the boats arriving by the fall of night, which included U-229, U-230, U-332, U-405, U-409, U-641 and U-665, two were bombed by the Liberator aircraft and four depth-charged by escorts including Babbitt and Rosthern. U-229 made an unsuccessful attack, but in the evening of 9 March Kapitänleutnant Kurt Lange’s U-530 sank the straggling 3,058-ton Swedish Milos. During the night Oberleutnant Hanns-Ferdinand Massmann’s U-409 and Korvettenkapitän Rolf-Heinrich Hopman’s U-405 made almost simultaneous attacks, the former sinking the 3,837-ton US Malantic and 5,989-ton British tanker Rosewood, and the latter sinking the 4,665-ton Norwegian Bonneville (carrying the 291-ton LST-2341 as cargo). Soon after this, Oberleutnant Robert Schetelig’s U-229 sank the 4,946-ton British Nailsea Court and damaged the 3,670-ton British Coulmore.

On 10 March the weather once again deteriorated, and only U-229 and U-616 made attacks, which failed, while U-523 and U-642 made brief but fruitless contact with independently routed ships. During the afternoon U-634, the last contact-keeper, was driven off.

Despite the fact that the defensive capability of the Allied escorts, reinforced during the day by the British corvettes Campion and Mallow, had been reduced by the partial malfunctioning of radar and radio equipment as a result of storm damage, early on 11 March the Germans broke off the operation.