SM U-6 (Austria-Hungary)

SM U-6 or U-VI was a U-5-class submarine or U-boat built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K. Kriegsmarine) before and during the First World War. The submarine was built as part of a plan to evaluate foreign submarine designs, and was the second of three boats of the class built by Whitehead & Co. of Fiume after a design by Irishman John Philip Holland.

U-6 was laid down in February 1908 and launched in June 1909. The double-hulled submarine was just over 105 feet (32 m) long and displaced between 240 and 273 tonnes (265 and 301 short tons), depending on whether surfaced or submerged. U-6's design had inadequate ventilation and exhaust from her twin gasoline engines often intoxicated the crew. The boat was commissioned into the Austro-Hungarian Navy in July 1910, and served as a training boat—sometimes making as many as ten cruises a month—through the beginning of the First World War in 1914.

The submarine had only one wartime success, which was sinking a French destroyer in March 1916. Later that year, in May, U-6 became entangled in anti-submarine netting deployed as part of the Otranto Barrage. Coming under fire from Royal Navy's drifters running the nets, U-6 was abandoned and sunk. All of her crewmen were rescued and were held in captivity through the end of the war.