|Builder:||Vickers Limited, Barrow-in-Furness|
|Laid down:||19 April 1917|
|Commissioned:||21 February 1919|
|Fate:||Sunk, 24 December 1941|
|Class and type:||H class submarine|
|Length:||171 ft 0 in (52.12 m)|
|Beam:||15 ft 4 in (4.67 m)|
HMS H31 survived until World War II . During the war, she took part in the operation to keep the German battleship Scharnhorst in Brest, France in November 1941 before the "Channel Dash" to German homeports in the company of Gneisenau and Prinz Eugen in February 1942. During the operation, H31 was sunk by unknown causes, but most believe she was mined in the Bay of Biscay on 24 December 1941.
Like all post-H20 British H-class submarines, H31 had a displacement of 440 tonnes (490 short tons) at the surface and 500 tonnes (550 short tons) while submerged. It had a total length of 171 feet (52 m), a beam length of 15 feet 4 inches (4.67 m), and a draught length of 12 metres (39 ft). It contained a diesel engines providing a total power of 480 horsepower (360 kW) and two electric motors each providing 320 horsepower (240 kW) power. The use of its electric motors made the submarine travel at 11 knots (20 km/h; 13 mph). It would normally carry 16.4 tonnes (18.1 short tons) of fuel and had a maximum capacity of 18 tonnes (20 short tons).
The submarine had a maximum surface speed of 13 knots (24 km/h; 15 mph) and a submerged speed of 10.5 knots (19.4 km/h; 12.1 mph). Post-H20 British H-class submarines had ranges of 2,985 nautical miles (5,528 km; 3,435 mi) at speeds of 7.5 knots (13.9 km/h; 8.6 mph) when surfaced. H31 was fitted with an anti-aircraft gun and four 21 inches (530 mm) torpedo tubes. Its torpedo tubes were fitted to the bows and the submarine was loaded with eight 21 inches (530 mm) torpedoes. It is a Holland 602 type submarine but was designed to meet Royal Navy specifications. Its complement was twenty-two crew members.
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