Convoy HX 231 was the 231st of the numbered series of Second World War HX convoys of merchant ships from Halifax, Nova Scotia (HalifaX) to Liverpool. The ships departed New York City on 25 March 1943 and were met on 31 March by Escort Group B 7 of the Mid-Ocean Escort Force. The convoy was found by German U-boats on 4 April and attacked by eleven U-boats from the 1st, 3rd, 6th and 10th U-boat flotillas, from Brest, La Rochelle, St Nazaire and Lorient, respectively. The U-boats formed (Lionheart) and sank six ships before losing contact on 7 April, for the loss of and . The convoy reached Liverpool on 10 April.
In 1942, convoys west of 26ð West came under the command of the US Vice-Admiral, Arthur Bristol, along with the escort forces in the western North Atlantic as Task Force 24. The Canadian Western Local Escort Force (WLEF, also Task Group 24.18) was under the tactical command of the Commanding Officer Atlantic Coast, Rear-Admiral Leonard Murray, at Halifax, Nova Scotia, comprising 25 Canadian and 13 British destroyers and corvettes that escorted convoys from Halifax to the Western Ocean Meeting Point (WOMP) at 49ð West, when SC convoys began sailing from Halifax.
The Mid-Ocean Escort Force (MOEF) was based at St John's, Newfoundland, Derry in Ireland and Liverpool in Britain and became Task Group 24.1, with seven British, four Canadian and one US Navy escort groups. The MOEF escorted HX, SC, ON and ONS convoys to the Eastern Ocean Meeting Point (EASTOMP) at about 20ð West where the convoy was handed over to the Eastern Local Escort groups. Ships to Iceland, the base for Task Group 24.6, with two US escort groups, detached at about 25ð West (ICOMP). The mainly old coastguard ships escorting convoys on the Sydney to Greenland leg came under Task Force 24 as Task Group 24.9.
When the US established the Interlocking Convoy System along the east coast and the Caribbean, the northern termination was at New York and from September 1942, transatlantic convoys departed from there. The Western Atlantic escort forces were insufficient for the leg from New York to the WOMP and a new Halifax Ocean Meeting Point (HOMP) was created around 61ð West, where the eventual twelve WLEF groups (Task Group 24.18) were relieved. An attempt began in April 1942 to keep the Ocean Escort Groups together, to reap the benefits of teamwork and experience and in the summer of 1942 EASTOMP was moved westwards to accommodate British escorts being sent to the Caribbean. The British escort groups B1 to B7 became based permanently in Britain and the Canadian groups C1 to C4, with the US group A3, at St John's.
West of the Change of Operational Control (CHOP) line, west of 26ð west, control of convoys was taken over by the Commander in Chief of the United States Fleet, through the commander of Task force 24, Rear-Admiral M. K. Metcalf from 1 July 1942. To the east of the CHOP line, control transferred to the British Commander-in-Chief, Western Approaches in Liverpool. After the Arctic convoy Convoy RA 53 (1âÂÂ14 March), the run was suspended for the summer and destroyers on the Arctic run were transferred to Support Groups in the North Atlantic. The 20th Escort Group with eight aged destroyers and Hunt-class destroyers in the 22nd, 23rd and 34th Escort Groups, each of four corvettes, were scheduled for refurbishment. Convoy cycles in the North Atlantic were shortened in March, to accommodate the extra traffic, an HX and a SC convoy departing New York each week, that required 12 ocean escort groups, each of three destroyers, a frigate and six corvettes.
By the spring of 1943, the had expanded to the size that the (BdU, Commander of U-Boats) Rear-Admiral Karl Dönitz had advocated for in 1939. In March, 400 U-boats were in service, 47 were close to completion, 245 were under construction and seven U-boats were being converted. Of the 400 boats, 52 (13 per cent) were training submarines, 119 (29.8 per cent) were working up, seven (1.7 per cent) were prototypes and 222 (55.5 per cent) were operational front-line submarines. Eighteen boats were in the Arctic, 19 in the Mediterranean and three in the Black Sea, leaving 182 for operations in the Atlantic. Of the Atlantic boats, on 1 March 114 (62.6 per cent) were at sea and 68 (37.4 per cent) were in French harbours. Of the U-boats at sea, 44 were in transit (24.2 per cent) and 70 (38.4 per cent) were on patrol in operational areas, 45 in the North Atlantic, 13 in the Central Atlantic, five in the Western Atlantic and seven in the South Atlantic.
Convoy HX 231 departed New York on 25 March with 61 merchant ships in an -wide series of columns but the convoy lost cohesion in fog off the Grand Banks, an underwater plateau to the south-west of Newfoundland. The convoy rendezvoused with Escort Group B7 (Commander Peter Gretton) of the Mid-Ocean Escort Force at the MOMP to the south of Iceland, in the vicinity of 58ð North and 35ð West. The convoy had been sent northwards into cold and stormy weather. The wind in these high latitudes (closer to the North Pole) comes from the north and west, blowing freezing spray onto decks that has to be chipped off with picks and shovels. On the cargo-liner Shillong, the lifeboats along the port side got so icy that the davits bent and the lifeboats were disconnected and tied to the ship, to float free if the ship sank. Life rafts had been swept away along with the main mast that got so brittle in the cold that it snapped. The captain, J. H. Hollow, had to move out of position in the convoy to jettison the wreckage.