The submarine depicted as U-172 in the film is the USS Argonaut, SS-166, Ex V-4. At the time the largest submarine in the world! The real German WW II U-Boat U-172 was not launched till July 31, 1941.
The Navy placed at John Ford's disposal a flotilla of destroyers, cruisers and submarines, as well as the services of naval personnel.
The three-masted schooner used as the decoy mystery ship was the same one used in the movies The Sea Wolf (1930) and The Painted Woman (1932).