SM U-81[Note 1] was one of the 329 submarines serving in the Imperial German Navy (Kaiserliche Marine) in World War I. U-81 was engaged in naval warfare and took part in the First Battle of the Atlantic.

[1] On 8 February 1917 she torpedoed Mantola 143 nautical miles (265 km; 165 mi) off Fastnet, forcing her crew and passengers to abandon her.

She then shelled the drifting hulk until being chased away by the Acacia-class sloop HMS Laburnum.

The submarine was powered by two 2,400 metric horsepower (1,800 kW; 2,400 shp) engines for use while surfaced, and two 1,200 metric horsepower (880 kW; 1,200 shp) engines for use while submerged.

[2] When submerged, she could operate for 56 nautical miles (104 km; 64 mi) at 5 knots (9.3 km/h; 5.8 mph); when surfaced, she could travel 11,220 nautical miles (20,780 km; 12,910 mi) at 8 knots (15 km/h; 9.2 mph).