'Blitz' (iv) was a U-boat wolfpack operation, in tandem with 'Hammer' (iv) and 'Thor', in the Arctic Ocean against the JW.58 convoy during its 'FY' passage to the northern USSR (24 March/5 April 1944).
The wolfpack comprised U-277, U-288, U-315, U-355, U-360, U-361, U-711, U-956 and U-990, and for the loss of Korvettenkapitän Günter La Baume’s U-355 and Kapitänleutnant Klaus-Helmuth Becker’s U-360 sank no ship.
The JW.58 convoy of 47 ships departed Loch Ewe on 27 March. With the convoy was the elderly US light cruiser Milwaukee, which was being transferred to the Soviet navy, and as close support under the command of Vice Admiral F. H. G. Dalrymple-Hamilton the anti-aircraft cruiser Diadem and escort carriers Tracker and Activity. The local escort on 27/29 March comprised the corvettes Rhododendron and Stalwart, and the minesweepers Rattlesnake, Onyx and Orestes. The close escort between 27 March and 4 April comprised the destroyers Westcott, Whitehall and Wrestler, and the corvettes Bluebell, Honeysuckle and Lotus.
The convoy was supplemented from Iceland by three more ships, of which one soon turned back, escorted by the frigate Fitzroy and minesweepers Chamois and Chance. On 29 March the convoy was joined by the main body of the escort, comprising Diadem and the destroyers Onslow, Obedient, Offa, Opportune, Oribi, Orwell, Impulsive, Inconstant, Saumarez, Serapis, Scorpion, Venus and Norwegian Stord. In addition, the destroyers Keppel, Beagle, Boadicea and Walker joined the British force, as did Captain F. J. Walker’s 2nd Support Group (sloops Starling, Magpie, Wild Goose, Wren and Whimbrel), which remained until 4 April.
On 29 March U-961, on passage to the Atlantic, was located and sunk by Starling.
On 30 March Admiral Sir Bruce Fraser, commanding the Home Fleet, departed Scapa Flow as a covering force with the battleships Anson and Duke of York, the fleet carriers Furious and Victorious, the escort carriers Emperor, Searcher, Pursuer and Fencer, the light cruisers Belfast, Jamaica and Sheffield, the anti-aircraft cruiser Royalist, and the destroyers Milne, Meteor, Onslaught, Undaunted, Ursa, Verulam, Vigilant, Virago, Wakeful and Canadian Algonquin and Sioux.
On the same day the convoy was spotted by German aerial reconnaissance but the Grumman Wildcat fighters of Tracker and Activity shot down one Junkers Ju 88 of the 1./FAGr 22, three Focke-Wulf Fw 200 Condor aircraft of the 3./KG 40 on 31 March, one Blohm und Voss Bv 138 of the 1./SAGr 130 on 1 April, and one Ju 88 of the 1./FAGr 124 on 2 April.
Most of the 'Thor' boats (U-278, U-312, U-313 and U-674), 'Blitz' (iv) boats (U-277, U-355, U-711 and U-956) and 'Hammer' (iv) boats (U-288, U-315, U-354 and U-968) and the additional outbound U-716, U-739, U-360, U-361 and U-990 made contact with the escort shortly after midnight on 1 April and kept in touch until the evening of 3 April. Some of the boats managed to make attacks, but were all 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 of No. 846 Squadron from Tracker on 1 April damaged a U-boat that was claimed as sunk by the destroyer Beagle, but U-673 was in fact only damaged. U-355 was lost to unknown causes between 1 and 4 April. On 2 April Keppel sank U-360 in a 'Hedgehog' attack, and on 3 April an Avenger and a Grumman Wildcat of No. 846 Squadron from Tracker and a Fairey Swordfish of No. 819 Squadron from Activity sank U-288 with rockets and depth charges after the boat had shot down another Swordfish with cannon fire.
On 3 April a Soviet force left the Kola Inlet with the destroyers Razyarenny, Gremyashchy, Razumny and Kuybyshev, four minesweepers and four submarine chasers, and on the following day escorted the convoy into the Kola Inlet, continuing on, from 8 April, with the KB.3 convoy of nine ships with Razumny, Razyarenny and Kuybyshev, four minesweepers and three submarine chasers to the White Sea.