'Thor' was a U-boat wolfpack operation in the Norwegian Sea against the JW.58 convoy (10 March/5 April 1944).
The wolfpack comprised U-278, U-307, U-312, U-313, U-361 and U-959, and neither lost any of its own number nor sank any ship.
On 27 March the JW.58 convoy of 47 laden ships departed Loch Ewe on the north-west coast of Scotland together with the US cruiser light Milwaukee, which was being transferred to the Soviet Northern Fleet. Under the command of Rear Admiral F. H. G. Dalrymple-Hamilton, the main strength of the escort was provided the escort carriers Activity and Tracker and the light anti-aircraft cruiser Diadem within the convoy. The local escort from 27 to 29 March comprised the minesweepers Rattlesnake, Onyx and Orestes, and the corvettes Rhododendron and Stalwart. Close anti-submarine escort was provided by the destroyers Westcott, Whitehall and Wrestler, and the corvettes Bluebell, Honeysuckle and Lotus from 27 March to 4 April.
From Iceland three ships joined the convoy, though one of these later returned, escorted by the frigate Fitzroy and minesweepers Chamois and Chance.
On 29 March the main body of the escort joined, this comprising Diadem and the destroyers Onslow, Obedient, Offa, Opportune, Oribi, Orwell, Impulsive, Inconstant, Saumarez, Serapis, Scorpion, Venus and Free Norwegian Stord. In addition, the destroyers Beagle, Boadicea, Keppel and Walker joined, as did Captain F. J. Walker’s 2nd Support Group with the sloops Starling, Magpie, Whimbrel, Wild Goose and Wren, which remained with the convoy until 4 April.
On 29 March Oberleutnant Klaus Fischer’s U-961, in transit to the Atlantic, was located by Starling and sunk, and on the following day Admiral Sir Bruce Fraser, commanding the Home Fleet, departed Scapa Flow with a distant covering force comprising the battleships Anson and Duke of York, fleet carriers Furious and Victorious, escort carriers Emperor, Fencer, Pursuer and Searcher, light cruisers Belfast, Jamaica and Sheffield, light anti-aircraft cruiser Royalist, and destroyers Milne, Meteor, Onslaught, Undaunted, Ursa, Verulam, Vigilant, Virago, Wakeful and Canadian Algonquin and Sioux.
On 30 March the JW.58 convoy was located and reported by German air reconnaissance, but the Grumman Wildcat fighters launched by Activity and Tracker shot down two Junkers Ju 88, three Focke-Wulf Fw 200 Condor, and one Blohm und Voss Bv 138 aircraft on 31 March and 1 April. Most of the U-boats from the 'Thor', 'Blitz' (iv) and 'Hammer' (iv) wolfpacks , together with the outbound U-360, U-361, U-716, U-739 and U-990, established contact with the escort shortly after 24.00 on 1 April and maintained it until the evening of 3 April, and in some cases made attacks, all of which were unsuccessful: U-968 attacked twice, U-674 twice, U-278 twice, U-313 once, U-711 twice, U-354 once, U-288 once, U-990 once, U-739 twice on the destroyer Ashanti, U-277 twice, U-315 once and U-312 once.
A rocket attack by a Grumman Avenger anti-submarine aeroplane of the FAA’s No. 846 Squadron from Tracker on 1 April damaged U-673, which was wrongly claimed as sunk by the destroyer Beagle. Korvettenkapitän Günter La Baume’s U-355 was lost to unknown causes between 1 and 4 April. On 2 April Keppel sank Kapitänleutnant Klaus-Helmuth Becker’s U-360 with 'Hedgehog' projectiles, and on 3 April an Avenger and a Grumman Wildcat of the FAA’s No. 846 Squadron from Tracker and a Fairey Swordfish anti-submarine aeroplane of the FAA’s No. 819 Squadron from Activity combined to sink Oberleutnant Willy Meyer’s U-288 with rockets and depth charges after the latter had shot down a Swordfish with anti-aircraft fire.
The Soviet destroyers Gremyashchiy, Kuybyshev, Razumnyi and Razyaryonnyi, four 'AM' type minesweepers and four 'BO' type submarine chasers departed the Kola inlet on 3 April, and on the following day escorted the convoy into the Kola inlet, continuing on, from 8 April, with the KB.3 convoy (nine ships escorted by Kuybyshev, Razumnyi and Razyaryonnyi, four minesweepers and three submarine chasers) into the White Sea.