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O 21-class submarine
Dutch submarine class (1940–1959) From Wikipedia, the free encyclopedia
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The O 21 class was a class of seven submarines, built for the Royal Netherlands Navy.[2] 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.[4]
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Design
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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.[5][6] They were similar to the submarines of the O 19 class, except the boats had no minelaying capability.[7] The omission of the minelaying capability resulted in a smaller and more streamlined hull which made it possible to reach a higher surface speed.[8] Visually the boats showed many similarities to the German Type VII submarine.[9] 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).[10] Above water they had a displacement of 990 tons and submerged they had displacement of 1,205 tons.[11] There was enough space aboard for a crew of 39 to 60 persons.[12][13]
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.[14]
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.[15] There was room for a total of fourteen torpedoes, with eight being in the torpedo tubes and six for reloads.[16] 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.[8] 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.[6]
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).[17][8] 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.[11] This allowed the submarine to operate solely on electric power for five hours.[3] The maximum underwater speed was 9 knots (17 km/h; 10 mph).[13]
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Boats
The boats were built by three different shipyards. O 21 and O 22 were built by the Koninklijke Maatschappij De Schelde.[18] O 23, O 24, O 26 and O 27 by RDM and O 25 at the Wilton-Fijenoord shipyard.[2]
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