'SL' was the designation of Allied convoys (together with a numerical and sometimes a literal suffix) plying the route from Freetown, Sierra Leone, to Liverpool in the UK (September 1939/December 1944).

The series was suspended temporarily between October 1942 and March 1943, and from May 1943 travelled home from the region of Gibraltar with MKS convoys.

Ships carrying commodities from South America, Africa and the Indian Ocean areas steamed independently to Freetown, and here were grouped into convoys for the last leg of their voyage to the UK. The first of the 178 convoys of this series was SL.1 of 14 September/1 October 1939 with eight British merchant vessels (6,348-ton Barrister, 5,063-ton Cliftonhall, 5,138-ton Ettrickbank, 5,026-ton Henry Stanley, 5,505-ton Jose de Larringa, 5,017-ton Luciston, 4,982-ton Peebles and 6,196-ton Scientist) escorted by the light cruisers Capetown and Dauntless. The last of the series was SL.178 of 25 November/15 December 1944 with 19 merchant vessels and four escorts in the form of the frigate Ness and corvettes Cyclamen and Free French Lobélia and Commandant Drogou.

Another six convoys, suffixed G, joined the 'SL' convoys from Gibraltar.

One of the major battles associated with the convoys of the 'SL' series was that centred on SL.125. Some 37 merchant ships departed Freetown on 16 October 1942 and were joined at sea by five other ships to create a convoy comprising the 5,681-ton Norwegian Alaska, 5,322-ton Belgian Alexandre Andre, 4,457-ton Dutch Amstelkerk, 7,705-ton British Anglo Maersk, 3,942-ton British Baron Elgin, 3,355-ton British Baron Kinnaird, 3,642-ton British Baron Vernon, 2,871-ton Norwegian Belnor, 3,177-ton British Bornholm, 2,407-ton British Bothnia, 7,124-ton British British Ardour, 4,772-ton British Brittany, 7,519-ton British Bullmouth, 7,206-ton British Calgary, 5,953-ton British Clan Murray, 7,131-ton British Corinaldo, 6,244-ton British Debrett, 5,259-ton British Dundrum Castle, 5,758-ton British Empire Cougar, 5,691-ton British Empire Simba, 5,282-ton Norwegian Germa, 5,205-ton British Guinean, 5,026-ton British Henry Stanley, 5,178-ton British Hopecastle, 5,224-ton British King Edward, 1,961-ton British Lafonia, 5,029-ton British Lynton Grange, 1,418-ton British Mano, 8,979-ton British Marquesa, 5,283-ton British Nagpore, 7,591-ton British Pacific Star, 11,898-ton British Président Doumer, 4,933-ton Swedish San Francisco, 6,566-ton Dutch Sembilan, 6,373-ton British Silverwillow, 6,148-ton British Stentor, 6,405-ton British Tasmania, 3,168-ton British Tynemouth, 5,083-ton British Ville de Rouen, 4,974-ton British Welsh Trader, 5,620-ton US West Kebar and 4,871-ton British Zarian.

The German cryptanalysis branch of the B-Dienst decoded message traffic containing tactical information about the convoy, and the 'Streitaxt' wolfpack, comprising U-103, U-134, U-203, U-409, U-440, U-509, U-510, U-572, U-604 and U-659 was grouped on 23 October to intercept and attack the convoy in the Atlantic area to the west of the Canary islands group. The only US merchant vessel, and the escorting sloop Bridgewater, anti-submarine trawler Copinsay and Free French corvette Commandant Drogou, had been detached by the time U-203 located and reported the convoy on 25 October. U-203 was depth charged and damaged while attempting to attack the straggling tanker Anglo Maersk, which was then shadowed by U-134 and damaged by U-509.

On 27 October British armed merchant cruiser and troopship Esperance Bay was detached with the anti-submarine trawler Juliet, tug Salvonia, and New Zealand repair ship Kelantan when U-409 found and reported the main convoy, which now comprised 41 merchant ships escorted by the corvettes Petunia, Cowslip, Crocus and Woodruff. U-659 was depth charged and damaged while attempting to attack the convoy. After the moon had risen, Kapitänleutnant Horst Höltring’s U-604 sank the damaged Anglo Maersk, and Kapitänleutnant Werner Witte’s U-509 torpedoed the freighters Pacific Star and Stentor: the former went down with 5,037 tons of refrigerated meat and general cargo, but without suffering any personnel losses, and the latter sank with 6,000 tons of West African produce and 44 men.

On 28 October, following unsuccessful submerged daylight attacks., U-509 sank Nagpore, which went down with 1,500 tons of copper, 5,500 tons of general cargo and 19 men, and damaged Hopecastle after sunset. Kapitänleutnant Hermann Kottmann’s U-203 sank the damaged Hopecastle before dawn, and the ship went down with 5,500 tons of general cargo and five men.

On the night of 29/30 October U-509 sank Brittany during foul weather, the ship going down with 7,132 tons of general cargo and 14 men, The tanker Bullmouth, in ballast, was damaged by U-409 and sunk by Kapitänleutnant Hans Stock’s U-659. Corinaldo was damaged by both U-509 and U-659 before being sunk by Oberleutnant Hans Seidel’s U-203, going down with 5,141 tons of frozen meat and eight men.

An improvement in the weather on 30 October resulted in co-ordinated attacks on the night of 30/31 October. U-409 torpedoed Silverwillow, which went down with 9,000 tons of general cargo and five men, while Kapitänleutnant Horst Höltring’s U-604 torpedoed Président Doumer, which sank with a general cargo and 260 persons, and Baron Vernon, which sank with 5,500 tons of iron ore but no personnel losses as her crew was rescued by Baron Elgin. Tasmania was damaged by U-659 and sunk by Oberleutnant Gustav-Adolf Janssen’s U-103, going to the bottom with 8,500 tons of food and iron as well as two of her crew. Alaska was damaged by U-510 as she rescued survivors from Président Doumer and Tasmania, but reached England safely with the help of newly arriving escorts.

Long-range patrol bombers of RAF Coastal Command arrived over the convoy on 31 October, and Admiral Karl Dönitz cancelled the U-boat wolfpack’s operation on the morning of 1 November. The convoy was reinforced with 11 more escorts and reached Liverpool on 9 November. SL.125 had suffered the greatest loss of any of the convoys in the 'SL' series, but its timing had the unintentional but nonetheless invaluable benefit of focusing the available U-boat strength in the area on an area well away from 'Torch' convoys for the Allied invasion of North-West Africa on 8 November.

Also associated with the baseline 'SL' convoys were the 'SLF' fast convoys and 'SLS' slow convoys, in which the final letter was usually appended as a suffix to the numerical portion of the designation.