SM UB-21 was a German Type UB II submarine or U-boat in the German Imperial Navy (German: Kaiserliche Marine) 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.[1]
A Type UB II submarine, UB-21 had a displacement of 263t when at the surface and 292t while submerged. She had a total length of 36.13m (118.54feet), a beam of 4.36m (14.3feet), and a draught of 3.7m (12.1feet). The submarine was powered by two Körting six-cylinder, four-stroke diesel engines producing a total of 280PS, two Siemens-Schuckert electric motors producing 280PS, 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 5sp=usNaNsp=us SK L/40 deck gun. She had a complement of twenty-one crew members and two officers and a 45-second dive time.
| Date | Name | Nationality | Tonnage[2] | Fate[3] | |
|---|---|---|---|---|---|
| 5 May 1916 | Harald |  Sweden | 275 | Sunk | |
| 20 October 1916 | Lekna | Sweden | 204 | Sunk | |
| 20 October 1916 | Randi |  Norway | 467 | Sunk | |
| 20 October 1916 | Svartvik | Sweden | 322 | Sunk | |
| 21 October 1916 | Fritzöe | Norway | 641 | Captured as prize | |
| 21 October 1916 | Grönhaug | Norway | 667 | Sunk | |
| 22 October 1916 | London |  Denmark | 184 | Sunk | |
| 22 October 1916 | Thor | Norway | 372 | Sunk | |
| 3 November 1916 | Pluto | Norway | 1,148 | Captured as prize | |
| 16 February 1917 | Lady Ann |  United Kingdom | 1,016 | Sunk | |
| 17 February 1917 | Excel | United Kingdom | 157 | Sunk | |
| 22 February 1917 | John Miles | United Kingdom | 687 | Sunk | |
| 29 March 1917 | Bywell | United Kingdom | 1,522 | Sunk | |
| 31 March 1917 | Norden | Norway | 776 | Captured as prize | |
| 29 April 1917 | Victoria | United Kingdom | 1,620 | Sunk | |
| 2 May 1917 | Rikard Noordrak | Norway | 1,123 | Sunk | |
| 5 May 1917 | Edith Cavell | United Kingdom | 20 | Sunk | |
| 6 May 1917 | Harold | Sweden | 1,679 | Sunk | |
| 8 May 1917 | Batavier II |  Netherlands | 157 | Captured as prize | |
| 6 June 1917 | S.N.A. 2 |  France | 2,294 | Sunk | |
| 7 June 1917 | Sir Francis | United Kingdom | 1,991 | Sunk | |
| 21 July 1917 | Trelyon | United Kingdom | 3,099 | Sunk | |
| 22 July 1917 | Glow | United Kingdom | 1,141 | Sunk | |
| 23 July 1917 | Vanland | Sweden | 1,285 | Sunk | |
| 24 August 1917 | Springhill | United Kingdom | 1,507 | Sunk | |
| 18 October 1917 | Amsteldam | United Kingdom | 1,233 | Sunk | |
| 19 October 1917 | Gemma | United Kingdom | 1,385 | Sunk | |
| 23 November 1917 | Ocean | United Kingdom | 1,442 | Sunk | |
| 29 December 1917 | Inverness | United Kingdom | 3,734 | Damaged | |
| 29 December 1917 | Patria | 838 | Sunk | ||
| 30 December 1917 | Hercules | United Kingdom | 1,295 | Sunk | |
| 25 March 1918 | Hercules | United Kingdom | 1,095 | Sunk | |
| 8 May 1918 | Constantia | United Kingdom | 772 | Sunk | |
| 10 May 1918 | Anboto Mendi |  Spain | 2,114 | Sunk | |
| 11 May 1918 | Gothia | Sweden | 1,826 | Sunk | |
| 12 May 1918 | Haslingden | United Kingdom | 1,934 | Sunk | |
| 4 July 1918 | Mentor | Norway | 539 | Sunk | |
| 26 September 1918 | Paul |  Belgium | 659 | Sunk |