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.

Design and description

L9 and its successors were enlarged to accommodate 21-inch (53.3 cm) torpedoes and more fuel. The submarine had a length of 238 ft overall, a beam of 23 ft and a mean draft of 13 ft. They displaced 914 LT on the surface and 1089 LT submerged. The L-class submarines had a crew of 38 officers and ratings. They had a diving depth of 150 ft. For surface running, the boats were powered by two 12-cylinder Vickers 1200 bhp diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600 hp electric motor. They could reach 17 kn on the surface and 10.5 kn underwater. On the surface, the L class had a range of 3800 nmi at 10 kn. The boats were armed with four 21-inch torpedo tubes in the bow and two 18-inch (45 cm) in broadside mounts. They carried four reload torpedoes for the 21-inch tubes for a grand total of ten torpedoes of all sizes. They were also armed with a 4 in deck gun.

Construction and career

HMS L21 was laid down on 15 September 1917 by Vickers at their Barrow-in-Furness shipyard, launched on 11 October 1918, and completed on 5 October 1920. L21 was sold in February 1939 and ran aground on the Isle of Arran, Bute whilst under tow to the breakers on 22 February 1939. She was refloated two days later on 24 February.