The Group 3 boats had two QF 4-inch guns fore and aft of the lengthened conning tower.

Also, 76 tons of fuel oil was carried in external wing tanks for the first time in British submarines.

In the Group 2 boats, L14, L17 and L24 to L27 were built as minelayers carrying 16 mines but without the two beam torpedo tubes.

The introduction of the L class came too late to contribute significantly in World War I. L2 was accidentally depth-charged by three American destroyers in early 1918.

L24 was sunk with all hands lost in a collision with the battleship Resolution during an exercise off Portland Bill in the English Channel on 10 January 1924.

The L class served throughout the 1920s and the majority were scrapped in the 1930s but three remained operational as training boats during World War II.

[3] All three sub-classes had the same propulsion system: two diesel engines for use while surfaced and two corresponding electric motors for use submerged.

While running on the surface at 10 knots (19 km/h; 12 mph), the submarines could cruise for a range of 3,800 nautical miles (7,000 km; 4,400 mi); range figures for the Type-III boats were instead 4,500 nautical miles (8,300 km; 5,200 mi) at 8 knots (15 km/h; 9.2 mph).