This was the designation of British coastal convoys (together with a numerical suffix) plying the route from Methil on the Firth of Forth or the estuary of the Tyne river to Southend on the Thames river estuary, and as such reciprocals of the 'FN' series (September 1939/May 1945).
The route was protected by defensive mine fields but was vulnerable to attack by U-boats, S-boote, aircraft and mines. The FS convoys were numbered from 1 to 100 in 19 repeating phases. There were 1,778 such convoys, and these comprised 18,448 ship movements.
The first of this 'Forth South' series was FS.1 of 6/8 September 1939 from Methil to Southend with the 3,713-ton French Amienois, 3,317-ton French Cap Blanc, 3,433-ton British Holmside and 2,955-ton French Saumur, escorted by the British destroyer Matabele and French destroyers Boulonnais, Brestois, Cyclone and Mistral.
The last of the series was FS.18 of 27/28 May 1945 from Methil to Southend with the 764-ton Belgian Anna, 4,683-ton Greek Armathia, 3,942-ton British Baron Elgin, 4,714 ton Belgian Belgian Fisherman, 2,878-ton British Confederation Park, 7,136-ton British Fort Chipewyan, 1,333-ton Norwegian Harpefjell and probably 7,219-ton British Samaye.
Additional 'FS' series comprised British convoys from Scapa Flow to Norway and four French convoys from Best to Namsos in Norway, both these series taking place in April and May 1945.