| 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.
| Commander | From | To | |
| 1 | J I Hartnall, RNVR | 28 May 1941 | mid 1944 |
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