This was an Allied programme of patrols by cruisers, destroyers and aircraft in the ‘narrows’ part of the Atlantic to interdict U-boats and blockade runners (7/13 December 1943).
On 25 November the U-boat command in Germany had sent the signal 'Kammerarrest' to U-boats operating in the South Atlantic. Forbidding attacks on single merchantmen from 1 December, this signal was intercepted and decoded by the US Op 20G on the following day, and confirmed the imminent arrival of the first of the expected five blockade runners known to be coming from Japan according to indications from ‘Purple’ and other decrypts. This resulted in the activation of ‘Barrier’ patrols involving Rear Admiral Albert C. Read’s Task Force 41, comprising five task groups each comprising one cruiser and one destroyer, US Navy and Brazilian air squadrons from Natal and Recife, and air patrols by US patrol bombers from Ascension island, beginning on 1 December.
In addition ‘Freecar’ was launched a few days later with a number of British, French and Italian co-belligerent vessels including two British armed merchant cruisers (14,170-ton Corfu and 11,137-ton Cilicia), French heavy cruiser Suffren, and Italian light cruisers Luigi di Savoia Duca degli Abruzzi and Emanuele Filiberto Duca d’Aosta operating from Freetown, Sierra Leone, and Dakar, French West Africa.
The first of the blockade runners, the 6,951-ton Osorno, had left Kobe on 2 October and rounded the Cape of Good Hope on 15 November, and this vessel was sighted on 8 December by a Consolidated Liberator of the US Navy’s VPB-107 squadron based on Ascension island, but the searching TG41.4 (cruiser Marblehead and destroyer Winslow) managed to intercept only an Allied freighter while Osorno, disguised as the British Prome, escaped.
A search to the north-west yielded no result, but U-510, on passage to the Indian Ocean and having sighted Osorno, was reported by a Liberator from Ascension and hunted on 11/13 December by TG41.3 (cruiser Memphis and destroyer Somers reinforced by the destroyer Jouett from TG41.1). On 13 December the Allies decoded U-510’s signal about the sighting of Osorno, and decided that the blockade runner has already broken past the ‘Barrier’ and ‘Freecar’ patrols, which were therefore suspended. This helped the following blockade runner, the 2,170-ton Alsterufer, to pass through the narrows without being detected.
On 12 December the British had begun the follow-on ‘Stonewall’ operation in the North Atlantic.