History
United Kingdom
Name	HMSL21
Ordered	December 1916[1]
Builder	Vickers Limited,Barrow-in-Furness
Laid down	15 September 1917
Launched	11 October 1919[1]
Fate	Sold for scrapping, February 1939
General characteristics
Class & type	L-class submarine
Displacement	914 long tons (929 t) surfaced 1,089 long tons (1,106 t) submerged
Length	238 ft 7 in (72.7 m)
Beam	23 ft 6 in (7.2 m)
Draught	13 ft 3 in (4.0 m)
Installed power	2,400 bhp (1,800 kW) (diesel) 1,600 hp (1,200 kW) (electric)
Propulsion	2 ×diesel engines 2 ×electric motors
Speed	17 kn (31 km/h; 20 mph) surfaced 10.5 kn (19.4 km/h; 12.1 mph) submerged
Range	3,800 nmi (7,000 km; 4,400 mi) at 10 kn (19 km/h; 12 mph) on the surface
Test depth	150 feet (45.7 m)
Complement	38
Armament	4 × bow21 in (533 mm)torpedo tubes 2 × beam18 in (457 mm) torpedo tubes 1 ×4-inchdeck gun

HMSL21 was aL-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 forscrap in 1939.

L9 and its successors were enlarged to accommodate21-inch (53.3 cm) torpedoes and more fuel. The submarine had a length of 238 feet 7 inches (72.7 m)overall, abeam of 23 feet 6 inches (7.2 m) and a meandraft of 13 feet 3 inches (4.0 m).^[2] Theydisplaced 914 long tons (929 t) on the surface and 1,089 long tons (1,106 t) submerged. The L-class submarines had a crew of 38 officers andratings.^[3] They had a diving depth of 150 feet (45.7 m).^[4]

For surface running, the boats were powered by two 12-cylinderVickers^[5] 1,200-brake-horsepower (895 kW)diesel engines, each driving onepropeller shaft. When submerged each propeller was driven by a 600-horsepower (447 kW)electric motor.^[2] They could reach 17knots (31 km/h; 20 mph) on the surface and 10.5 knots (19.4 km/h; 12.1 mph) underwater. On the surface, the L class had a range of 3,800nautical miles (7,000 km; 4,400 mi) at 10 knots (19 km/h; 12 mph).^[4]

The boats were armed with four 21-inchtorpedo tubes in the bow and two18-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.^[6] They were also armed with a 4-inch (102 mm)deck gun.^[3]

HMSL21 waslaid down on 15 September 1917 byVickers at theirBarrow-in-Furnessshipyard,launched on 11 October 1918, and completed on 5 October 1920.L21 was sold in February 1939 and ran aground on theIsle of Arran,Bute whilst under tow to the breakers on 22 February 1939. She was refloated two days later on 24 February.^[7]

• Akermann, Paul (2002).Encyclopaedia of British Submarines 1901–1955 (reprint of the 1989 ed.). Penzance, Cornwall: Periscope Publishing.ISBN 1-904381-05-7.
• Colledge, J. J.; Warlow, Ben (2006) [1969].Ships of the Royal Navy: The Complete Record of All Fighting Ships of the Royal Navy from the 15th Century to the Present (Rev. ed.). London: Chatham Publishing.ISBN 978-1-86176-281-8.OCLC 67375475.
• Gardiner, Robert & Gray, Randal, eds. (1985).Conway's All the World's Fighting Ships 1906–1921. Annapolis, Maryland: Naval Institute Press.ISBN 0-85177-245-5.
• Harrison, A. N. (January 1979)."The Development of HM Submarines From Holland No. 1 (1901) to Porpoise (1930) (BR3043)". RN Subs. Retrieved27 September 2022.

• British L-class submarines
• Ships built in Barrow-in-Furness
• 1919 ships
• World War I submarines of the United Kingdom
• Royal Navy ship names
• Maritime incidents in 1939

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