A further attempt to develop a fleet submarine similar to the K-boats, but with diesel engines for propulsion. These submarines, were by the standards of the day very comfortable, but lightly armed for their displacement.
The original intention was for a class of twenty boats, but a change of thinking on submarine construction, and the very high cost of these boats restricted the class to just three boats.
World War II
Thames
1939 - 2nd Submarine Flotilla based at Dundee - Scotland.
1940 - July 23rd struck a mine and sank of Norway.
Clyde and Severn
1939 - At Free Town, Siera Leone.
1940 - 2nd Submarine Flotilla based at Dundee - Scotland.
1941 - Mediterranean - Alexandria. Employed as cargo carriers for the besieged island of Malta (George Cross). September 1941, Clyde carried a total of 1,200 tons of supplies.
1944 - Sent to the Eastern Fleet.
All three submarines were badly underused during the Norwegian Campaign - they were to large.