The O 21 class was a class of seven submarines, built for the Royal Netherlands Navy.[1] The boats 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 boats and they entered German service as UD-3, UD-4 and UD-5. The submarines' diving depth was 100m (300feet).
At the start of the Second World War the O 21 class was together with the British U, S and T classes and German Type VII one of the most advanced submarine classes in service at the time.[2]
The O 21 class submarines were designed by the Dutch engineer G. de Rooy, chief engineer of the Royal Netherlands Navy (RNN) at the time.[3] [4] They were similar to the submarines of the, except the boats had no minelaying capability.[5] The omission of the minelaying capability resulted in a smaller and more streamlined hull which made it possible to reach a higher surface speed.[6] Visually the boats showed many similarities to the German Type VII submarine.[7] The submarines of the O 21 class had a length of 77.7m (254.9feet), a beam of 6.8m (22.3feet) and a draught of 3.95m (12.96feet).[8] Above water they had a displacement of 990 tons and submerged they had displacement of 1,205 tons.[9] There was enough space aboard for a crew of 39 to 60 persons.[10] [11]
One of the requirements the RNN set for the O 21 class was that it had to be able to dive 20m (70feet) deeper than the previous class, which was the O 19 class.[12]
The primary armament of the O 21-class submarines consisted of eight 53.3cm (21inches) torpedo tubes; four were located at the bow, two at the stern and two external amidships.[13] There was room for a total of fourteen torpedoes, with eight being in the torpedo tubes and six for reloads.[14] Besides the eight torpedo tubes the O 21 class was also equipped with an deck gun, two single-mounted anti-aircraft guns and a single machine gun. The three unfinished boats (O 25, O 26 and O 27) that were captured and later completed by Nazi Germany during the Second World War had besides the eight torpedo tubes, a single 8.8 cm deck gun, and two 20 mm guns.
The O 21-class submarines were equipped with two seven-cylinder two-stroke Sulzer 7 QD 42/50 diesel engines that each could produce 1000shp and drive the two screws of the submarine to a maximum surface speed of 19.5kn.[15] Besides the two diesel engines, the submarines also had two electric motors that each could produce 500bhp and two banks of 96 cells batteries with a capacity of 5,350 Ah.[16] This allowed the submarine to operate solely on electric power for five hours. The maximum underwater speed was .
The boats were built by three different shipyards. O 21 and O 22 were built by the Koninklijke Maatschappij De Schelde.[17] O 23, O 24, O 26 and O 27 by RDM and O 25 at the Wilton-Fijenoord shipyard.
| Name | Laid down | Launched | Commissioned | Decommissioned |
|---|---|---|---|---|
| 13 July 1937 | 21 October 1939 | 10 May 1940 (uncompleted) | 2 November 1957 | |
| 15 September 1938 | 20 January 1940 | 10 May 1940 (uncompleted) | 8 November 1940 | |
| 12 October 1937 | 5 December 1939 | 13 May 1940 (uncompleted) | 1 December 1948 | |
| 12 November 1937 | 18 March 1940 | 13 May 1940 (uncompleted) | 22 February 1954 | |
Commissioned as: UD-3 | 10 April 1939 | 1 May 1940 | 1 March 1942 | 13 October 1944 3 May 1945 (scuttled) |
Commissioned as: UD-4 | 20 April 1939 | 23 November 1940 | 1 March 1941 | 3 May 1945 (scuttled) |
Commissioned as: UD-5 | 3 August 1939 | 26 September 1941 | 1 November 1941 13 July 1945 (Dutch navy) | 14 November 1959 |