Events
On This Day - March 21
| 1917 | 1916 - 1957: Platypus | Commissioned |
| 1940 | Ursula (N59) | HMS Ursula torpedoes and sinks the German merchant Heddernheim in the Skaggerak about 10 nautical miles east-north-east of Skagen, Denmark. |
| 1940 | Upright (N89) | Launched |
| 1941 | Talent (P337) | Laid Down |
| 1941 | Tasman (P337) | Laid Down |
| 1943 | Splendid (P228) | HMS Splendid torpedoes and sinks the Italian tanker Giorgio about 8 nautical miles east-north-east of Cefal, Sicily, Italy. |
| 1943 | X6 | Completed |
| 1981 | Splendid (S106) | Completed |
