HNLMS O 21
Class overview
Name	O 21 class
Builders	Rotterdamsche Droogdok Maatschappij (4) Koninklijke Maatschappij De Schelde (2) Wilton-Fijenoord (1)
Operators	Royal Netherlands Navy  Kriegsmarine
Preceded by	O 19 class
Succeeded by	Dolfijn class
Built	1937–1940
Completed	7
Lost	2
Scrapped	5
General characteristics
Type	Submarine
Displacement	990 tons surfaced 1205 tons submerged
Length	77.7 m (255 ft)
Beam	6.8 m (22 ft)
Draught	3.95 m (13 ft)
Propulsion	2 × Sulzer 7QD42 7-cylinder diesel engines, 2,500 bhp (1,864 kW) each 2 × 500 bhp (373 kW) electric motors
Speed	19.5 kn (36.1 km/h; 22.4 mph) surfaced 9 kn (17 km/h; 10 mph) submerged
Range	10,000 nmi (19,000 km; 12,000 mi) at 12 kn (22 km/h; 14 mph) surfaced 28 nmi (52 km; 32 mi) at 8.5 kn (15.7 km/h; 9.8 mph) submerged
Complement	39
Armament	8 × 21 in (530 mm) torpedo tubes (4 bow, 2 stern, 2 traversing amidships )

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).

Design

One of the requirements the Royal Netherlands Navy 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 submarine.^[1]

Construction

The ships were built by three different shipyards. O 21 and O 22 were built by the Koninklijke Maatschappij. O 23, O 24, O 26 and O 27 by R.D.M, Rotterdam and O 25 also in Rotterdam at Fijenoord shipyard.