On the basis of World War I experience, SL convoys were one of four trade convoy routes organized at the beginning of the Battle of the Atlantic. The other routes were HX convoys from North America, HG convoys from the Mediterranean, and a short-lived series of HN convoys from Norway. Eight ships sailed as convoy SL 1 on 14 September 1939 and three faster ships sailed six days later as a faster section -- sometimes designated SL(F) 1 or SL 1(F). The slower convoy was sometimes similarly suffixed with an (S). Early convoys were usually accompanied by an armed merchant cruiser or one of the South Atlantic Stationcruisers based at Freetown; but no anti-submarine screen was provided until the slower and faster sections rendezvoused with a single Escort Group in the Southwest Approaches.[1]
Freetown was little more than a protected anchorage with no shore facilities. The town had been established as a resettlement area for freed slaves, with negligible European development. Convoy operations were coordinated by a naval staff aboard the elderly Union-Castle LinerEdinburgh Castle.Edinburgh Castle and a hospital ship anchored as far offshore as practicable to avoid the unhealthy conditions ashore. Tropical diseases were endemic in the oppressive heat and humidity. Local fresh water supplies were polluted. Refueling coal from the United Kingdom and oil from the West Indies was held and distributed afloat in detained merchant ships. Shore facilities were inadequate to support anti-submarine escorts for convoys until January 1941.[2] Air cover was flown from Cornwall, Gibraltar, and Freetown when conditions allowed; but a northern Azores air gap and a southern Canaries air gap remained where U-boats and surface raiders could patrol the convoy routes unobserved.[3] The northern gap was closed when air patrols began flying from the Azores in October 1943.[4]
Convoy SL 125 sailed on 16 October 1942 before Operation Torch discontinued sailings from Freetown. Shipping was routed along the east coast of the Americas to Halifax until convoy SL 126 sailed from Freetown on 12 March 1943. Convoy SL 128 merged with convoy MKS 12 off Gibraltar in April 1943 to be designated SL 128/MKS 12 and all subsequent SL convoys had a similar joint designation format. The effective range of U-boats was decreased by Allied capture of French Atlantic seaports in 1944. The reduced threat of submarine attack off the African Atlantic coast allowed merchant ships to sail independently to Gibraltar after convoy SL 178/MKS 69 left Freetown on 25 November 1944.[2]
OS convoys
From 7 September 1939, OutBound OB convoys had sailed from Liverpool south through St George's Channel to the open Atlantic. OB convoys were escorted for about four days from Land's End before the convoy would disperse and individual ships proceed independently to their destinations. As U-boats found and sank increasing numbers of ships dispersed from OB convoys, OB convoys were replaced by ON convoys and by OS convoys formed of ships Outbound to the South Atlantic and escorted all the way to Freetown. Convoy OS 1 sailed from Liverpool on 24 July 1941, and reached Freetown on 10 August. An escort group would screen a southbound OS convoy and return with a northbound SL convoy. Convoy OS 40 reached Freetown on 27 September 1942, but following convoys OS 41 and OS 42 dispersed at sea; and OS convoys were suspended by Operation Torch until convoy OS 43 left Liverpool on 14 February 1943. Convoy OS 46 was combined with convoy KMS 13 of ships detaching off Gibraltar with the joint designation OS 46/MKS 13. Sailings from Liverpool continued under the joint designations until convoy OS 130/KMS 105 on 27 May 1945; but, as the Mediterranean route became safe for Indian Ocean destinations, convoy OS 92/KMS 66 was the last to proceed as far as Freetown on 4 November 1944.[5]
Convoy battles
SL 7 lost Arlington Court torpedoed by U-43 on 16 November 1939.[6]
SL 8 lost Royston Grange torpedoed by U-28 on 25 November 1939.[7]
SL 34 lost two ships torpedoed by U-46 on 12 June 1940.[7]
SL 36 lost two ships torpedoed by U-43 on 30 June 1940 and by U-30 on 1 July.[7]
SL 109 lost Denpark torpedoed by U-128 on 12 May 1942.[14]
OS 28 lost two ships torpedoed by U-159 on 21 May 1942.[13]
OS 33 lost seven ships torpedoed by three U-boats, and escort sank U-136 on 12 July 1942.[21]
SL 115 escort sank the Italian submarine Pietro Calvi on 14 July 1942.[22]
OS 34 lost two ships torpedoed by U-564 on 19 July 1942.[13]
SL 118 lost four ships torpedoed by three U-boats in August 1942.[14]
SL 119 lost two ships torpedoed by U-566 on 28 August 1942.[14]
SL 125 was used as a tactical diversion to clear U-boats from the path of troopship convoys for Operation Torch. Twelve ships sunk with 426 dead in the final days of October, 1942, constituted the largest loss from any SL convoy.[23]
OS 44 lost four ships torpedoed by U-107 on 13 March 1943.[13]
SL 126 lost four ships torpedoed by U-404 and U-662 in March 1943.[14]
OS 45 lost two ships torpedoed by U-124 on 2 April 1943.[13]
SL 128/MKS 12 lost Laconikos torpedoed by U-89 on 7 May 1943.[14]
SL 129/MKS 13 lost Alpera bombed by aircraft on 22 May 1943.[14]
SL 131/MKS 15 lost two ships bombed by aircraft on 23 June 1943.[14]
OS 52/KMS 21 lost two ships bombed by aircraft on 26 and 27 July 1943.[13]
OS 58/KMS 32 lost Warfield bombed by aircraft on 15 August 1943.[13]
SL 135/MKS 22 escort sank U-634 on 30 August 1943.[24]