|Laid down:||29 July 1915|
|Commissioned:||1 October 1915|
|Fate:||Sold, 14 December 1921|
|Class and type:||E class submarine|
|Length:||181 ft (55 m)|
|Beam:||15 ft (4.6 m)|
Like all post-E8 British E-class submarines, E21 had a displacement of 622 tonnes (686 short tons) at the surface and 807 tonnes (890 short tons) while submerged. It had a total length of 180 feet (55 m) and a beam length of 22 feet 8.5 inches (6.922 m). It contained two diesel engines each providing a power of 1,600 horsepower (1,200 kW) and two electric motors each providing 840 horsepower (630 kW) power. Its complement was thirty-one crew members.
The submarine had a maximum surface speed of 16 knots (30 km/h; 18 mph) and a submerged speed of 10 knots (19 km/h; 12 mph). British E-class submarines had fuel capacities of 50 tonnes (55 short tons) of diesel and ranges of 3,255 miles (5,238 km; 2,829 nmi) when travelling at 10 knots (19 km/h; 12 mph). E21 was capable of operating submerged for five hours when travelling at 5 knots (9.3 km/h; 5.8 mph). It was fitted with a 6 pounds (2.7 kg) 6-pounder gun, five 18 inches (460 mm) torpedo tubes, and one spare torpedo tube. Its torpedo tubes were fitted at the front and the aft; unlike pre-E9 submarines, the two midship section torpedo tubes were not included.
E-Class submarines contained wireless systems with 1 kilowatt (1.3 hp) power ratings; in some submarines, these were later upgraded to 3 kilowatts (4.0 hp) systems by removing a midship torpedo tube. Its claimed highest dive depth was 100 feet (30 m) although it was capable of reaching depths of below 200 feet (61 m). Some submarines contained Fessenden oscillator systems.
- Hutchinson, Robert (2001). Jane's Submarines: War Beneath the Waves from 1776 to the Present Say. London: HarperCollins. ISBN 978-0-00-710558-8. OCLC 53783010.