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 and the
Mk X turrets held in stock for the and s. 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 ,
M26 received one of
Swiftsures 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 . 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. Admiral
Reginald Bacon, who had commanded several of the M15 class in the
Dover Patrol, wrote about his experiences with the ships of the patrol in 1919. While generally positive about the performance of the design, he noted that they had a tendency to
roll, using a specific occasion with
M25 as an example; :"The
M25, while in the mouth of the Thames at anchor, rolled 180 degrees in ten seconds - that is, she made two complete rolls of 45 degrees each way, each occupying only five seconds." == Service ==