SL convoys
SL convoys were a group of North Atlantic trade convoys in World War II. Merchant ships carrying goods to the British Isles from South America, Africa, and the Indian Ocean sailed to Freetown, Sierra Leone, to be joined into a convoy for the final leg to Liverpool.
They were one of four main trade routes at the start of the Battle of the Atlantic. The others were HX (from North America to the U.K.), HG (from the Mediterranean), and a short-lived series called HN (from Norway).
The first SL convoy, SL 1, sailed on 14 September 1939 with eight ships. A faster section, SL 1(F), followed six days later with three ships. The slower section could be marked with an S. Early SL convoys often had armed escorts, but a full anti-submarine screen arrived only when the slower and faster parts met with an Escort Group in the Southwest Approaches.
Freetown was mainly a protected anchorage with little in the way of shore facilities. The town had been a resettlement area for freed slaves and European development was limited. The SL operation was controlled from aboard the old liner Edinburgh Castle, which also carried a hospital ship offshore to avoid the unhealthy conditions ashore. Tropical diseases were common, and fresh water supplies were often polluted. Fuel and coal were kept aboard detained merchant ships to supply the convoy.
Shore facilities could not support anti-submarine escorts until January 1941. Air cover came from bases in Cornwall, Gibraltar, and Freetown when conditions allowed, but there were gaps where U-boats and raiders could patrol unseen—one in the north near the Azores and one in the south near the Canaries. The northern air gap closed in October 1943 when patrols began from the Azores.
In 1942–43, changes limited sailings from Freetown. Convoy SL 125 left on 16 October 1942, and after Operation Torch in North Africa, sailings from Freetown stopped. Ships from the east coast of the Americas began to go to Halifax until SL 126 left Freetown on 12 March 1943. In April 1943, SL 128 merged with MKS 12 off Gibraltar to form SL 128/MKS 12, and later SL convoys used similar joint designations.
As Allied forces captured French Atlantic ports in 1944, U-boat range decreased. The Atlantic coast threat eased enough that ships could sail independently to Gibraltar after SL 178/MKS 69 left Freetown on 25 November 1944.
Outbound convoys from Liverpool had started as OB convoys in 1939, moving south through St George’s Channel into the Atlantic and dispersing after a few days. Because dispersal left ships vulnerable, OB convoys were replaced by ON and OS convoys. OS convoys ran from Liverpool to Freetown, with escort groups screening the southbound leg and then bringing the escort back north to meet SL convoys. OS 1 left Liverpool on 24 July 1941 and arrived at Freetown on 10 August. OS 40 reached Freetown on 27 September 1942, but OS 41 and OS 42 dispersed at sea; OS convoys were paused during Operation Torch until OS 43 left Liverpool on 14 February 1943. OS 46 joined with KMS 13 as OS 46/MKS 13. The practice of joint OS/KMS designations continued, ending with OS 130/KMS 105 on 27 May 1945. The Mediterranean route became safe for Indian Ocean destinations, and OS 92/KMS 66 was the last to reach Freetown on 4 November 1944.
This page was last edited on 3 February 2026, at 13:52 (CET).