HMS M21 was a First World War Royal Navy M15-class monitor. After service in the Mediterranean and the Dover Patrol, she struck a mine off Ostend in January 1918 and sank off Dover.

United Kingdom
Name: HMS M21
Builder: Sir Raylton Dixon & Co.
Laid down: 1 March 1915
Launched: 27 May 1915
Fate: Sunk 20 October 1918 off Dover
General characteristics
Class and type: M15 class monitor
Displacement: 540 tons
Length: 177 ft 3 in (54.03 m)
Beam: 31 ft (9.4 m)
Draught: 6 ft 9 in (2.06 m)
  • 2 shaft
  • Triple Expansion steam engines
  • 600 ihp
Speed: 11 knots
Complement: 69


Intended as a shore bombardment vessel, M21's primary armament was a single 9.2 inch Mk VI gun removed from the Edgar-class cruiser HMS Theseus.[1] In addition to her 9.2 inch gun,she also possessed one 12 pounder and one six pound anti-aircraft gun. Due to the shortage of Bolinder diesel engines that equipped her sisters, she was fitted with 2 shaft triple expansion steam engines that allowed a top speed of eleven knots. The monitor's crew consisted of sixty nine officers and men.


HMS M21 ordered in March, 1915, as part of the War Emergency Programme of ship construction. She was laid down at the Sir Raylton Dixon & Co. Ltd shipyard at Govan in March 1915, launched on 27 May 1915, and completed in July 1915.

World War I

M21 served initially in the Mediterranean from September 1915. On her return from the Mediterranean in September 1917, M21 had her main 9.2in gun removed, as it was required for artillery use on the Western Front, and a BL 7.5-inch (190.5 mm) Mk III 50-caliber gun was fitted in lieu.

M21 then served with the Dover Patrol from October 1917.


M21 struck a mine off Ostend on 20 October 1918. She was taken in tow to Dover, but sank off West Pier.


  1. Randal Gray (ed). Conway's All the World's Fighting Ships, 1906-1921. Conway Maritime Press. p. 48. ISBN 0-85177-245-5.CS1 maint: extra text: authors list (link)

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