Werner commanded her for most of her wartime career, during which she undertook 14 patrols with II Flotilla, sinking 61 merchant ships for a total of 126,010 GRT.

Her most famous act was the sinking of the British passenger liner RMS Carpathia with three torpedoes, on 17 July 1918, off the east coast of Ireland.

On 26 November 1918 U-55 was surrendered to the Royal Navy at Harwich, along with the rest of the surviving U-Boat fleet.

It was subsequently handed over to Japan, and entered service with the Imperial Japanese Navy in 1920 as O-3, serving as such until 1921.

[6] It was dismantled at the Sasebo Navy Yard between March and June 1921, briefly recommissioning in 1923 as Auxiliary Vessel No.