HMS L21 was a L-class submarine built for the Royal Navy during World War I.
The boat was not completed before the end of the war and was sold for scrap in 1939.
[4] For surface running, the boats were powered by two 12-cylinder Vickers[5] 1,200-brake-horsepower (895 kW) diesel engines, each driving one propeller shaft.
On the surface, the L class had a range of 3,800 nautical miles (7,000 km; 4,400 mi) at 10 knots (19 km/h; 12 mph).
[4] The boats were armed with four 21-inch torpedo tubes in the bow and two 18-inch (45 cm) in broadside mounts.