321 (SP-2235), also written Herreshoff #321, was a United States Navy patrol vessel in commission from 1918 to 1921.

At the suggestion of Assistant Secretary of the Navy (and future President of the United States) Franklin D. Roosevelt (1882–1945), businessman Payne Whitney (1876–1927) of New York City, seeking a way to contribute to the U.S. war effort during World War I, contracted with the Herreshoff Manufacturing Company at Bristol, Rhode Island, for one of a small group of 60-ton steel-hulled steam-powered boats built to a naval patrol boat design for private owners with the intention that they would be made available to the U.S. Navy in time of war.

After spending a short time at New London, Connecticut, fitting out with listening gear, Herreshoff No.

308 (SP-2232) bound for the Panama Canal Zone, stopping at Charleston, South Carolina; Key West, Florida; and Guantanamo Bay, Cuba, en route.

321 to Keyport, the towing hawser connecting the ships parted off Southern California on 7 October 1921.