O 21-class submarine

The O 21 class was a class of seven submarines, built for the Royal Netherlands Navy. 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.
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.

Design

The O 21 class submarines were designed by the Dutch engineer G. de Rooy, chief engineer of the Royal Netherlands Navy at the time. They were similar to the submarines of the, except the boats had no minelaying capability. The omission of the minelaying capability resulted in a smaller and more streamlined hull which made it possible to reach a higher surface speed. Visually the boats showed many similarities to the German Type VII submarine. The submarines of the O 21 class had a length of, a beam of and a draught of. Above water they had a displacement of 990 tons and submerged they had displacement of 1,205 tons. There was enough space aboard for a crew of 39 to 60 persons.
One of the requirements the RNN set for the O 21 class was that it had to be able to dive deeper than the previous class, which was the O 19 class.

Armament

The primary armament of the O 21-class submarines consisted of eight torpedo tubes; four were located at the bow, two at the stern and two external amidships. There was room for a total of fourteen torpedoes, with eight being in the torpedo tubes and six for reloads. 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 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.

Propulsion

The O 21-class submarines were equipped with two seven-cylinder two-stroke Sulzer 7 QD 42/50 diesel engines that each could produce and drive the two screws of the submarine to a maximum surface speed of. Besides the two diesel engines, the submarines also had two electric motors that each could produce and two banks of 96 cells batteries with a capacity of 5,350 Ah. This allowed the submarine to operate solely on electric power for five hours. The maximum underwater speed was.

Boats

The boats were built by three different shipyards. O 21 and O 22 were built by the Koninklijke Maatschappij De Schelde.
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	2 November 1957
	15 September 1938	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	1 March 1942	13 October 1944 3 May 1945
Commissioned as: UD-4	20 April 1939	23 November 1940	1 March 1941	3 May 1945
Commissioned as: UD-5	3 August 1939	26 September 1941	1 November 1941 13 July 1945	14 November 1959