SM U-81 was one of the 329 submarines serving in the Imperial German Navy (Kaiserliche Marine) in World War I. U-81 was engaged in the naval warfare and took part in the First Battle of the Atlantic. U-81 had one 10.5 cm gun with 140-240 rounds. On 8 February 1917 she torpedoed the SS Mantola 143 miles off Fastnet, forcing her crew to abandon her. She then shelled the drifting hulk until being chased away by the Acacia-class sloop HMS Laburnum. Laburnam took the floating hulk under tow, but the line parted in the rough seas, and Mantola was left to sink, which she did on 9 February.