HMS H21

HMS H21 was a British H class submarine built by Vickers Limited, Barrow-in-Furness, as the first of the Batch 3 H class submarines. She was launched on 20 October 1917 and was commissioned on 28 January 1918. H21 was an improved design of the H class which produced a larger displacement and the latest 21 inch torpedo tubes replacing the old 18 inch torpedo tubes. This would be incorporated into all batch 3 H class submarines. HMS H21 was sold to John Cashmore Ltd on 13 July 1926 for scrapping at Newport.

Design
Like all post-H20 British H-class submarines, H21 had a displacement of 423 LT at the surface and 510 LT while submerged. It had a total length of 171 ft, a beam of 15 ft, and a draught of 12 m. It contained a diesel engines providing a total power of 480 hp and two electric motors each providing 320 hp power. The use of its electric motors made the submarine travel at 11 kn. It would normally carry 16.4 LT of fuel and had a maximum capacity of 18 LT.

The submarine had a maximum surface speed of 13 kn and a submerged speed of 10.5 kn. Post-H20 British H-class submarines had ranges of 2985 nmi at speeds of 7.5 kn when surfaced. H21 was fitted with an anti-aircraft gun and four 21 in torpedo tubes. Its torpedo tubes were fitted to the bows and the submarine was loaded with eight 21 in torpedoes. It is a Holland 602 type submarine but was designed to meet Royal Navy specifications. Its complement was twenty-two crew members.