The ''O 21'' class was a

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

of seven

submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...

s, built for the

Royal Netherlands Navy The Royal Netherlands Navy (, ) is the Navy, maritime service branch of the Netherlands Armed Forces. It traces its history to 8 January 1488, making it the List of navies, third-oldest navy in the world. During the 17th and early 18th centurie ...

.Raven (1988), p. 178. 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 The (, ) was the navy of Nazi Germany from 1935 to 1945. It superseded the Imperial German Navy of the German Empire (1871–1918) and the inter-war (1919–1935) of the Weimar Republic. The was one of three official military branch, branche ...

'' 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 Type VII U-boats were the most common type of German World War II U-boat. 704 boats were built by the end of the war. The type had several modifications. The Type VII was the most numerous U-boat type to be involved in the Battle of the Atlanti ...

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

(RNN) at the time.Jalhay (1982), p. 118.von Münching (1978), p. 38. They were similar to the submarines of the , except the boats had no

minelaying A minelayer is any warship, submarine, military aircraft or land vehicle deploying explosive mines. Since World War I the term "minelayer" refers specifically to a naval ship used for deploying naval mines. "Mine planting" was the term for ins ...

capability.van Willigenburg (2010), p. 68. The omission of the minelaying capability resulted in a smaller and more streamlined hull which made it possible to reach a higher surface speed.Gardiner and Chesneau (1980), p. 392. Visually the boats showed many similarities to the German

submarine.Bosscher and Busssemaker (2007), p. 26. The submarines of the ''O 21'' class had a length of , a

beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Radio beam *Particle beam, a stream of charged or neutral particles **Charged particle beam, a spatially lo ...

of and a draught of .Bosscher and Busssemaker (2007), p. 86. Above water they had a

displacement Displacement may refer to: Physical sciences Mathematics and physics *Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...

of 990 tons and submerged they had displacement of 1,205 tons.Jalhay (1982), p. 127. There was enough space aboard for a crew of 39 to 60 persons.Bosscher and Busssemaker (2007), p. 27.von Münching (1978), pp. 37-38. 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 tube A torpedo tube is a cylindrical device for launching torpedoes. There are two main types of torpedo tube: underwater tubes fitted to submarines and some surface ships, and deck-mounted units (also referred to as torpedo launchers) installed aboa ...

s; four were located at the bow, two at the stern and two external amidships. There was room for a total of fourteen

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, such ...

es, with eight being in the torpedo tubes and six for reloads.van Willigenburg (2010), p. 69. Besides the eight torpedo tubes the ''O 21'' class was also equipped with an

deck gun A deck gun is a type of naval artillery mounted on the deck of a submarine. Most submarine deck guns were open, with or without a shield; however, a few larger submarines placed these guns in a turret. The main deck gun was a dual-purpose w ...

, two single-mounted

anti-aircraft gun Anti-aircraft warfare (AAW) is the counter to aerial warfare and includes "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It encompasses surface-based, subsurface ( submarine-launched), and air-ba ...

s 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 Nazi Germany, officially known as the German Reich and later the Greater German Reich, was the German Reich, German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a Totalit ...

during the

Second World War World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

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 A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston, one up and one down, in one revolution of the crankshaft in contrast to a four-stroke engine which re ...

Sulzer 7 QD 42/50

diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

s that each could produce and drive the two

screws A screw is an externally helical threaded fastener capable of being tightened or released by a twisting force (torque) to the screw head, head. The most common uses of screws are to hold objects together and there are many forms for a variety ...

of the submarine to a maximum surface speed of .van den Pol (1990), p. 288. Besides the two diesel engines, the submarines also had two electric motors that each could produce and two banks of 96

cells Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a d ...

batteries with a capacity of 5,350 Ah.Jalhay (1982), p. 127. 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 Damen Naval is a Dutch shipyard, and a continuation of the Koninklijke Maatschappij De Schelde, responsible for a number of ships used by the Royal Netherlands Navy. It is owned by the Damen Group. Damen Naval is situated in Vlissingen. History ...

.Mark (1997), pp. 85-86. ''O 23'', ''O 24'', ''O 26'' and ''O 27'' by RDM and ''O 25'' at the

Wilton-Fijenoord Wilton-Fijenoord was a shipbuilding and repair company in Schiedam, Netherlands from 1929 to 1999. Presently, the shipyard of Wilton-Feijnoord is part of Damen Shiprepair Rotterdam. Merger of Wilton and Fijenoord (1929) Wilton-Fijenoord had two ...

shipyard.

Notes

Citations

References

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External links

Description of class
{{WWII Dutch ships Submarines of the Netherlands