# 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 100 meters (330 ft).
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.

## Design

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. They were similar to the submarines of the O 19 class, 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 77.70 metres (254 ft 11 in), a beam of 6.80 metres (22 ft 4 in) and a draught of 3.95 metres (13 ft 0 in). 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 20 m (66 ft) deeper than the previous class, which was the O 19 class.

### Armament

The primary armament of the O 21-class submarines consisted of eight 53.3-centimetre (21.0 in) 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 8.8 cm (3.5 in) deck gun, two single-mounted 40 mm (1.6 in) anti-aircraft guns and a single 12.7 mm (0.50 in) 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.

### 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 1,000 shaft horsepower (750 kW) and drive the two screws of the submarine to a maximum surface speed of 19.5 knots (36.1 km/h; 22.4 mph). Besides the two diesel engines, the submarines also had two electric motors that each could produce 500 brake horsepower (500 bhp) 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 9 knots (17 km/h; 10 mph).

## 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.
O 21 class construction data :
| Name | Laid down | Launched | Commissioned | Decommissioned |
| O 21 | 13 July 1937 | 21 October 1939 | 10 May 1940 (uncompleted) | 2 November 1957 |
| O 22 | 15 September 1938 | 20 January 1940 | 10 May 1940 (uncompleted) | 8 November 1940 |
| O 23 | 12 October 1937 | 5 December 1939 | 13 May 1940 (uncompleted) | 1 December 1948 |
| O 24 | 12 November 1937 | 18 March 1940 | 13 May 1940 (uncompleted) | 22 February 1954 |
| O 25 Commissioned as: UD-3 | 10 April 1939 | 1 May 1940 | 1 March 1942 | 13 October 1944 3 May 1945 (scuttled) |
| O 26 Commissioned as: UD-4 | 20 April 1939 | 23 November 1940 | 1 March 1941 | 3 May 1945 (scuttled) |
| O 27 Commissioned as: UD-5 | 3 August 1939 | 26 September 1941 | 1 November 1941 13 July 1945 (Dutch navy) | 14 November 1959 |