SM U-51 was a Type U 51 submarine, one of 329 submarines in the Imperial German Navy in World War I. She engaged in commerce warfare during the First Battle of the Atlantic. She was ordered from Germaniawerft, at Kiel, on 23 August 1914 and laid down there on 19 December. She was launched on 25 November 1915 and commissioned on 24 February 1916. Kapitänleutnant Walter Rumpel was her captain for her entire career.