|Preceded by:||Gorgon class|
|Succeeded by:||M29 class|
|Length:||177 ft 3 in (54.03 m)|
|Beam:||31 ft (9.4 m)|
|Draught:||6 ft 9 in (2.06 m)|
|Speed:||11 knots (20 km/h; 13 mph)|
The ships of this class were ordered in March, 1915, as part of the Emergency War Programme of ship construction. They were designed to use the 9.2 inch Mk VI gun turrets removed from the Edgar class and the Mk X turrets held in stock for the Drake-class and Cressy-class cruisers. This resulted in the first four of the class, which were built by William Gray & Company of Hartlepool, receiving the Mk X mounting. The remaining ten ships, all built by Sir Raylton Dixon & Co., Middlesbrough, all received the Mk VI mounting.
During September 1915, the 9.2 inch guns of HMS M24, M25, M26 and M27 were removed for use as artillery. These were replaced by 7.5-inch guns. M24 and M25 received the spare guns reserved for the recently sunk pre-dreadnought battleship Triumph, M26 received one of Swiftsure's spare guns. M27 received 6-inch (M27) guns.
M21 and M23 also had their 9.2-inch gun removed in 1917, receiving 7.5-inch guns from the decommissioned pre-dreadnought Swiftsure.
The class used a mixture of propulsion methods. M21 and M22 were fitted with conventional triple-expansion steam engines, M24 was fitted with four-cylinder paraffin engines, and the remainder received Bolinder four-cylinder semi-diesel engines.
Ships of the class
- M15 - launched on 28 April 1915 and sunk by UC-38 on 11 November 1917.
- M16 - launched on 3 May 1915 and sold 29 January 1920.
- M17 - launched on 12 May 1915 and sold 12 May 1920.
- M18 - launched on 15 May 1915 and sold 29 January 1920.
- M19 - launched on 4 May 1915 and sold 12 May 1920.
- M20 - launched on 11 May 1915 and sold 29 January 1920.
- M21 - launched on 27 May 1915 and mined 20 October 1918.
- M22 - launched on 10 June 1915, renamed Medea 1925 and sold December 1938
- M23 - launched on 17 June 1915, renamed Claverhouse 1922 and sold 1959
- M24 - launched on 9 August 1915 and sold 29 January 1920.
- M25 - launched on 24 July 1915 and scuttled 16 September 1919
- M26 - launched on 24 August 1915 and sold 29 January 1920.
- M27 - launched on 8 September 1915 and scuttled 16 September 1919
- M28 - launched on 28 June 1915 and sunk during the Battle of Imbros 20 January 1918
- Colledge, J. J.; Warlow, Ben (2006) . Ships of the Royal Navy: The Complete Record of all Fighting Ships of the Royal Navy (Rev. ed.). London: Chatham Publishing. ISBN 978-1-86176-281-8.
- Jane's Fighting Ships of World War One (1919), Jane's Publishing Company
- Dittmar, F. J. & Colledge, J. J., British Warships 1914-1919, (Ian Allan, London, 1972), ISBN 0-7110-0380-7
- Gray, Randal (ed), Conway's All the World's Fighting Ships, 1906–1921, (Conway Maritime Press, London, 1985), ISBN 0-85177-245-5
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