(MB: l. 51'; b. 10'3"; dr. 2'5"; s. 22 mph.; cpl. 10; a. 1 1-pdr.)
Seatag was built in 1917 by the Great Lakes Boatbuilding Corporation, Milwaukee, Wis.; acquired by the Navy from Donald Ryerson of Chicago by a free lease on 9 July 1917; and placed in commission on 1 August 1917, Ens. Sylvester Aparling, USNRF, in command.
During World War I, Seatag (SP-505) was based on the Great Lakes. She served primarily on the Detroit and St. Clair River Patrols until 4 October 1918. On 18 November, Seatag was placed out of commission and was returned to her owner on 7 March 1919.