S.M.U. was the common designation for the submarines of the Austro-Hungarian Navy. The first submarine, S.M.U. 3, was launched on August 20, 1908. In the years prior, knowledge of submarine technology had been perfected with trial boats. By 1912, the Austro-Hungarian submarine fleet already comprised six boats, and during the war years, it grew to 27 operational boats. An additional 32 were planned but could not be completed by the end of hostilities. Nevertheless, the sinking successes of Austro-Hungarian submarine commanders from 1914 to 1918 were significant, with a total tonnage of 269,383 GRT (sunk and damaged). They inflicted heavy losses on the Allies in the Adriatic during World War I, sinking and damaging, among others, 32 ships from the British Navy and 34 from the Greek Navy.