|Shipyard||Blohm & Voss, Hamburg (Werk 271)|
|Ordered||29 Aug 1915|
|Launched||1 Apr 1916|
|Commissioned||12 Sep 1916|
|Successes||98 ships sunk with a total of 134,063 tons.|
5 ships damaged with a total of 11,826 tons.
1 ship taken as prize with a total of 148 tons.
1 ship damaged with a total of 778 tons. (View ships hit by UC 21)
|Fate||30 Sep 1917 - Departed Zeebrugge for the Bay of Biscay on 13.9. Lost by unknown cause on or after Sept. 16, 1917. 27 dead (all hands lost).|