Convoy SC 121 was the 121st of the numbered series of World War II Slow Convoys of merchant ships from Sydney, Cape Breton Island to Liverpool.[2] The ships departed New York City 23 February 1943;[3] and were met by the Mid-Ocean Escort Force Group A-3 consisting of the United States Coast Guard (USCG) Treasury-class cutter USCGC Spencer, the American Wickes-class destroyer USS Greer, the British and Canadian Flower-class corvettes HMS Dianthus, HMCS Rosthern, HMCS Trillium and HMCS Dauphin and the convoy rescue ship Melrose Abbey.[4][5] Three of the escorts had defective sonar and three had unserviceable radar.[6]
As western Atlantic coastal convoys brought an end to the second happy time, Admiral Karl Dönitz, the Befehlshaber der U-Boote (BdU) or commander in chief of U-Boats, shifted focus to the mid-Atlantic to avoid aircraft patrols. Although convoy routing was less predictable in the mid-ocean, Dönitz anticipated that the increased numbers of U-boats being produced would be able to find convoys with the advantage of intelligence gained through B-Dienst decryption of British Naval Cypher Number 3.[7] Only 20 percent of the 180 trans-Atlantic convoys, from the end of July 1942 until the end of April 1943, lost ships to U-boat attack.[8]
On 6 March U-405 sighted the convoy,[5] which had been scattered by nine consecutive days of northwesterly Force 10 gales and snow squalls.[9] The storm damaged the radio communication system aboard the escort commander's ship Spencer and Dauphin had to leave the convoy with damaged steering gear.[6] U-230 torpedoed the British freighter Egyptian on the night of 6–7 March.[10] The British freighter Empire Impala stopped to rescue survivors and was torpedoed after dawn by U-591.[10]
U-190 torpedoed the British freighter Empire Lakeland when the gale subsided on 8 March and four more stragglers were sunk by U-526, U-527, U-591, and U-642.[5] On 9 March the convoy escort was reinforced by No. 120 Squadron RAF B-24 Liberators from Northern Ireland and by the Wickes-class destroyer USS Babbitt and the Treasury-class cutters USCGC Bibb and USCGC Ingham from Iceland.[5]
U-530 torpedoed straggling Swedish freighter Milos on the evening of 9 March. That night U-405 torpedoed the Norwegian freighter Bonneville while U-229 torpedoed the British freighter Nailsea Court and U-409 torpedoed the British escort oiler Rosewood and American ammunition ship Malantic.[11]
The Flower-class corvettes HMS Campion and Mallow reinforced the convoy escort on 10 March,[5] and the convoy reached Liverpool on 14 March.[10] Only 76 of the 275 crewmen of the sunken ships were rescued.[6]