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 effectively search for convoys with the advantage of intelligence gained through B-Dienst decryption of British Naval Cypher Number 3.[5] However, of the 180 trans-Atlantic convoys sailing from the end of July 1942 until the end of April 1943, only 20 percent lost ships to U-boat attack.
The Norwegian-manned corvettes of escort group B-6 fought three of these convoy battles in sequential voyages with convoys SC 104, ON 144, and HX 217.[6]
15 November
After rendezvousing with convoys to and from Iceland, Convoy ON 144 was discovered and shadowed by U-521.[7]
16 November
When initial attempts to summon additional U-boats to the convoy were unsuccessful, U-521 was granted permission to attack, and missed with a salvo of six torpedoes.[7]Rose unsuccessfully counterattacked with depth charges.[1]
17 November
U-184, U-262 and U-264 found the convoy and launched a simultaneous attack after sunset. U-262 missed with three torpedoes.[7]U-264 sank the 6,696-ton Greek freighter Mount Taurus, and U-184 sank the 3,192-ton British freighter Widestone.[2]
18 November
U-624, U-522, U-521, U-224, U-383, U-454 and U-753 launched torpedoes in the pre-dawn hours.[7]U-624 sank the 5,344-ton British tanker President Sergent and the 4,732-ton American freighter Parismina and damaged the 5,432-ton American freighter Yaka which was later sunk by U-522.[2] The Type 271 centimeter-wavelength radar-equipped corvettes counterattacked, and U-184 was sunk by one of those attacks. Montbretia was torpedoed by U-262 and sank while still moving forward.[7]
19 November
The U-boats had broken off the engagement by the time the four surviving corvettes were reinforced by the destroyers HMS Firedrake and USS Badger, and the Western Local Escort Force assumed responsibility for the convoy on 20 November.[7]
Blair, Clay (1998). Hitler's U-Boat War: The Hunted 1942–1945. Random House. ISBN0-679-45742-9.
Hague, Arnold (2000). The Allied Convoy System 1939–1945. Naval Institute Press. ISBN1-55750-019-3.
Milner, Marc (1985). North Atlantic Run. Naval Institute Press. ISBN0-87021-450-0.
Morison, Samuel Eliot (1975). History of United States Naval Operations in World War II, Volume I The Battle of the Atlantic 1939–1943. Little, Brown and Company.
Rohwer, J.; Hummelchen, G. (1992). Chronology of the War at Sea 1939–1945. Naval Institute Press. ISBN1-55750-105-X.
Tarrant, V.E. (1989). The U-Boat Offensive 1914–1945. Arms and Armour. ISBN1-85409-520-X.
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