SM U-73

SM U-73 was one of 329 submarines serving in the Imperial German Navy in World War I. She engaged in the commerce war as part of the First Battle of the Atlantic.

U-73 has the distinction of being responsible for sinking the largest ship sunk in World War I, the 48,758 ton hospital ship Britannic, shortly after she laid the mine which Britannic struck.

Operations
After completion at Danzig in November 1915, U-73 was commissioned by Kapitänleutnant (Commander) Gustav Sieß. She joined the Kiel School, where she remained until February 1916, conducting trials and crew training. She then left for the North Sea and was attached to the 1st Half Flotilla, still under Her activities were monitored throughout the war by Room 40, & most of her recorded movements are based on that information. Her first operational cruise began 1 April 1916, when she left Heligoland Bight, bound for the Mediterranean by way of the North Sea. En route, she attacked one steamer in the Atlantic and laid mines off Lisbon and Malta. On arriving Cattaro on about 1 May (the date is uncertain), she joined the Pola-Cattaro Flotilla.

The minelaying cruises of U-73 in the Mediterranean cannot be reconstructed. On 7 October 1916 she is reported to have left Pola, and the French put down to her the mine sunk off Cape Male on 12 October, as well as a minefield in the Gulf of Salonika, and mines in the Gulf of Athens on which two Greek ships were blown up. It seems certain U-73, still commanded by Sieß, laid the mine by which the hospital ship HMHS Britannic was sunk, one hour after it was laid. It is possible the hospital ship Braemaer Castle was also sunk by one of her mines. U-73 suffered from constant machinery trouble in common with her class. At the end of October 1918, now in the hands of Kptlt. Fritz Saupe, she was scuttled at Cattaro.