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 class and German Type VII class 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 O 19 class, 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 class submarine.[7] The submarines of the O 21 class had a length of 77,70 meters, beam of 6,80 meter and a draught of 3,95 meters.[8] Above water they had a displacement of 990 tons and underwater they had displacement of 1205 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 submarine.[12]
The primary armament of the O 21 class submarines consisted of eight 53.3 cm torpedo tubes; 4 were located at the bow, two at the stern and two external amidships.[13] There was room for a total of fourteen torpedoes, with 8 being in the torpedo tubes and six for reloads.[14] Besides the eight torpedo tubes the O 21 class was also equipped with a 8,8 cm deck gun, two (2x1) 40 mm anti-aircraft guns and a single 12.7 mm 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 7-cylinder two-stroke Sulzer 7 QD 42/50 diesel engines that each could produce 1,000 shaft horsepower and drive the two screws of the submarine to a maximum surface speed of 19.5 knots.[15] Besides the two diesel engines, the submarines also had two electric motors that each could produce 500 bhp and two banks of 96 cells batteries with a capacity of 5350 Ah.[16] This allowed the submarine to operate solely on electric power for 5 hours. The maximum underwater speed was 9 knots.
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 |