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On This Day - March 21
| 1917 | Platypus (I 56) 1916 - 1957 | Commissioned |
| 1940 | Ursula (N 59) | HMS Ursula torpedoes and sinks the German merchant Heddernheim in the Skaggerak about 10 nautical miles east-north-east of Skagen, Denmark. |
| 1940 | Upright (N 89) | Launched |
| 1941 | Talent (P 337) | Laid Down |
| 1941 | Tasman (P 337) | Laid Down |
| 1943 | Splendid (P 228) | HMS Splendid torpedoes and sinks the Italian tanker Giorgio about 8 nautical miles east-north-east of Cefal, Sicily, Italy. |
| 1943 | X 6 (Piker II) | Completed |
| 1981 | Splendid (S 106) | Completed |
