| Navy | The Royal Navy |
| Type | Motor minesweeper |
| Class | MMS I |
| Pennant | J 521 |
| Built by | Herd & McKenzie Ltd. (Buckie, Scotland, U.K.) |
| Ordered | 17 Jan 1940 |
| Laid down | |
| Launched | |
| Commissioned | 11 Jun 1941 |
| End service | |
| History | Sold in 1948 |
Commands listed for HMS MMS 21 (J 521)
Please note that we're still working on this section
and that we only list Commanding Officers for the duration of the Second World War.
| Commander | From | To | |
| 1 | J I Hartnall, RNVR | 28 May 1941 | mid 1944 |
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