(MB: t. 41; 1. 72'0"; b. 14'0"; dr. 3'8"; cpl. 14; a. 1 1-pdr., 1 mg., 4 dc.)
The third Vigilant (SP-406) was a motorboat designed by Charles D. Mower; built in 1909 by Henry L. Blatz, of Philadelphia, Pa.; and originally named Marguerite II. Acquired by the Navy from E. B. Smith, of Philadelphia, Vigilant was commissioned on 19 May 1917 at the Philadelphia Navy Yard, Ens. John B Yarnall, USNRF, in command.
The boat proceeded from Philadelphia to Essington, Pa., on 28 May for installation of armament, loaded ammunition at Fort Mifflin on 5 June and, two days later, reported to the 4th Naval District. Throughout the remainder of World War I, Vigilant patrolled the Delaware River and the coastal waters of the Middle Atlantic States, ranging from Atlantic City, N.J., to the mouth of he Chesapeake Bay. She remained alert to detect any U-boats which entered her patrol area as she 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 merchantmen, as she did on 9 June 1918 when a U-boat had sunk SS Del Rio.
After the armistice ended hostilities, Vigilant proceeded to Philadelphia early in December for inactivation. She was decommissioned there on Christmas Eve 1918 and was returned to her owner.