| |
| History | |
|---|---|
 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) |
| Propulsion |
|
| Speed | 11 knots |
| Complement | 69 |
| Armament |
|
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.