O 21-class submarine

HNLMS O 21
Class overview
Name:	O 21 class
Builders:	R.D.M, Rotterdam (4) Koninklijke Maatschappij De Schelde, Flushing (2) Fijenoord, Rotterdam (1)
Operators:	Royal Netherlands Navy  Kriegsmarine
Preceded by:	O 19 class
Succeeded by:	HNLMS Dolfijn
Built:	1937–1940
Completed:	7
General characteristics
Type:	Submarine
Displacement:	990 tons surfaced 1205 tons submerged
Length:	77.7 m (254 ft 11 in)
Beam:	6.8 m (22 ft 4 in)
Draught:	3.95 m (13 ft 0 in)
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 ships 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 ships and they entered German service as U-D3, U-D4 and U-D5. The submarines diving depth was 100 metres (330 ft).

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.

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