On the night of 25 September 1925, "S-51" was operating on the surface near Block Island, with her running lights on. The merchant steamer "City of Rome" spotted a single white masthead light but were unable to determine its course, speed, or intentions. The ship altered her course away from the unknown light to give whatever it might be greater leeway. Meanwhile, "S-51" spotted the ship's masthead and green sidelights, and held her course as she was required to do by the Rules of the Road then in effect. Shortly after altering course, "City of Rome" spotted the submarine's red sidelight and realized that they were on collision courses. She turned and backed her engines, but it was too late. Twenty-two minutes after first spotting the submarine's masthead light, the steamer rammed her at the position .