USS Vigilant (SP-406)

The third USS Vigilant (SP-406), later USS SP-406, was a United States Navy patrol vessel in commission from 1917 to 1918.

Vigilant was built as the private motorboat Marguerite II in 1909 by Henry L. Blatz at Philadelphia, Pennsylvania. She had been renamed Vigilant by the time the U.S. Navy acquired her from her owner, E. B. Smith of Philadelphia, in 1917 for use as a section patrol vessel during World War I. She was commissioned as USS Vigilant (SP-406) on 19 May 1917 at the Philadelphia Navy Yard with Ensign John B. Yarnall, USNRF, in command.

On 28 May 1917, Vigilant proceeded from Philadelphia to Essington, Pennsylvania, for installation of her armament. She loaded ammunition at Fort Mifflin on 5 June 1917, then reported to the 4th Naval District for duty on 7 June 1917. For the remainder of World War I, Vigilant patrolled the Delaware Bay and the Atlantic coast of the United States from Atlantic City, New Jersey, to the mouth of the Chesapeake Bay. She also carried messages, men, and mail to ships and stations within the 4th Naval District. Occasionally, she ventured out to sea to search for survivors of torpedoed merchant ships, as she did on 9 June 1918 after a German submarine had sunk SS Del Rio.

In 1918, Vigilant was renamed USS SP-406, apparently to avoid confusion with the tug USS Vigilant (YT-25), which was in commission at the same time.

After World War I ended on 11 November 1918 and defensive patrols were suspended on 24 November, SP-406 proceeded to Philadelphia early in December 1918 for inactivation. She was decommissioned there on 24 December 1918 and was returned to her owner.