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 m.

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.

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 class submarine. 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. Above water they had a displacement of 990 tons and underwater they had displacement of 1205 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 deeper than the previous class, which was the O 19-class submarine.

Armament
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. There was room for a total of fourteen torpedoes, with 8 being in the torpedo tubes and six for reloads. 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.

Propulsion
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. 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. This allowed the submarine to operate solely on electric power for 5 hours. The maximum underwater speed was 9 knots.

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.