SM UB-21 was a German Type UB II submarine or U-boat in the German Imperial Navy () during World War I. The U-boat was ordered on 30 April 1915 and launched on 26 September 1915. She was commissioned into the German Imperial Navy on 20 February 1916 as SM UB-21. The submarine sank 33 ships in 26 patrols for a total of . Surrendered to Britain in accordance with the requirements of the Armistice with Germany, UB-21 was sunk as a target by HMS Terror in the Solent on 30 September 1920; the wreck was sold in 1970 and most had been cleared by 1998, although some remnants survive.
Design
A German Type UB II submarine, UB-21 had a displacement of when at the surface and while submerged. She had a total length of , a beam of , and a draught of . The submarine was powered by two Körting six-cylinder, four-stroke diesel engines producing a total of , two Siemens-Schuckert electric motors producing , and one propeller shaft. She was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, she could operate for at ; when surfaced, she could travel at . UB-21 was fitted with two torpedo tubes, four torpedoes, and one SK L/40 deck gun. She had a complement of twenty-one crew members and two officers and a 45-second dive time.