M15-class monitor

The M15 class comprised fourteen monitors of the Royal Navy, all built and launched during 1915.

Design
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 HMS Triumph (1903), 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 HMS Swiftsure (1903).

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.

Service
M25, M26, M27 and M28 served in the Dover Patrol from 1915 to 1918. The remainder served in the Mediterranean from 1915, with M23 joining the Dover Patrol in June 1917 and M21 in October 1917.

As part of the intervention into the Russian civil war M23, M24, M25 and M27 served in support of British and White Russian forces in the White Sea in May to September 1919.

M22 was converted to a minelayer in 1920, whilst M23 became a drill ship, surviving until 1959.

Ships of the class

 * M15 - launched on April 28, 1915 and sunk by SMU UC-38 on November 11, 1917.
 * M16 - launched on May 3, 1915 and sold January 29, 1920.
 * M17 - launched on May 12, 1915 and sold May 12, 1920.
 * M18 - launched on May 15, 1915 and sold January 29, 1920.
 * M19 - launched on May 4, 1915 and sold May 12, 1920.
 * M20 - launched on May 11, 1915 and sold January 29, 1920.
 * M21 - launched on May 27, 1915 and mined October 20, 1918.
 * M22 - launched on June 10, 1915, renamed Medea 1925 and sold December 1938
 * M23 - launched on June 17, 1915, renamed Claverhouse 1922 and sold 1959
 * M24 - launched on August 9, 1915 and sold January 29, 1920.
 * M25 - launched on July 24, 1915 and scuttled September 16, 1919
 * M26 - launched on August 24, 1915 and sold January 29, 1920.
 * M27 - launched on September 8, 1915 and scuttled September 16, 1919
 * M28 - launched on June 28, 1915 and sunk during the Battle of Imbros January 20, 1918