SM U-81 was one of the 329 submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s serving in the Imperial German Navy (Kaiserliche Marine

Kaiserliche Marine

The Imperial German Navy was the German Navy created at the time of the formation of the German Empire. It existed between 1871 and 1919, growing out of the small Prussian Navy and Norddeutsche Bundesmarine, which primarily had the mission of coastal defense. Kaiser Wilhelm II greatly expanded...

) in World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

.
U-81 was engaged in the naval warfare

Naval warfare of World War I

Naval warfare in World War I was mainly characterized by the efforts of the Allied Powers, with their larger fleets and surrounding position, to blockade the Central Powers by sea, and the efforts of the Central Powers to break that blockade or to establish an effective blockade of the United...

 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 143 miles off Fastnet

Fastnet Rock

Fastnet Rock is a small island in the Atlantic Ocean and the most southerly point of Ireland. It lies southwest of Cape Clear Island and from County Cork on the Irish mainland...

, forcing her crew to abandon her. She then shelled the drifting hulk until being chased away by the Acacia-class

Acacia class sloop

The Acacia class was a class of twenty-four sloops that were ordered in January 1915 under the Emergency War Programme for the Royal Navy in World War I as part of the larger "Flower Class", which were also referred to as the "Cabbage Class", or "Herbaceous Borders"...

 sloop

Sloop

A sloop is a sail boat with a fore-and-aft rig and a single mast farther forward than the mast of a cutter....

. 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.