[1] For surface running, the Electric Boat submarines were powered by two 450-brake-horsepower (336 kW) diesel engines, each driving one propeller shaft.

[2] L-9's keel was laid down on 2 November 1914 by Fore River Shipbuilding Company in Quincy, Massachusetts.

Following the United States's entry into World War I, submarines were needed to protect Allied shipping lanes to Europe.

After an extensive overhaul, preparing her for the task ahead, L-9 departed Portsmouth, Virginia, on 17 January 1918 and arrived Bantry Bay, Ireland, on 21 February.

After the Armistice with Germany on 11 November, L-9 operated out of the Isle of Portland, England, until she sailed 3 January 1919 for the United States.