[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.

[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.

[14] 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.

[6] 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.