O 21-class submarine
The O 21 class was a class of seven submarines, built for the Royal Netherlands Navy. The ships were still incomplete at the start of the German invasion of the Netherlands O 21, O 22, O 23 and O 24 were hastily launched and escaped to the United Kingdom. O 25, O 26 and O 27 were not able to escape and were captured by the German forces. The Kriegsmarine ordered the completion of the ships and they entered German service as UD-3, UD-4 and UD-5. The submarines diving depth was.
Design
One of the requirements the Royal Netherlands Navy set for the O 21-class was that it had to be able to dive 20 meters deeper than the previous class, which was the O 19-class submarine.Construction
The ships were built by three different shipyards. O 21 and O 22 were built by the Koninklijke Maatschappij.O 23, O 24, O 26 and O 27 by R.D.M, Rotterdam and O 25 also in Rotterdam at Fijenoord shipyard.
| Name | Laid down | Launched | Commissioned | Decommissioned |
| 20 November 1937 | 21 October 1939 | 10 May 1940 | 2 November 1957 | |
| 20 November 1937 | 20 January 1940 | 10 May 1940 | 8 November 1940 | |
| 12 October 1937 | 5 December 1939 | 13 May 1940 | 1 December 1948 | |
| 12 November 1937 | 18 March 1940 | 13 May 1940 | 22 February 1954 | |
Commissioned as: UD-3 | 10 April 1939 | 1 May 1940 | 8 June 1941 | 13 October 1944 3 May 1945 |
Commissioned as: UD-4 | 20 April 1939 | 23 November 1940 | 28 January 1941 | 3 May 1945 |
Commissioned as: UD-5 | 3 August 1939 | 1941 | 30 January 1942 13 July 1945 | 14 November 1959 |