Two experimental Mackerel (SS-204) class diesel submarines were approved in the FY1940 building program; one built by Electric Boat, the other by Portsmouth Navy Yard. They commissioned March and August 1941. They displaced about half that of contemporary fleet boats and were 70-feet shorter. Surface speed was significantly less than that of fleet boats at 16 knots though submerged speed was slightly better, at 9 knots.
These submarines were assigned training duties early in the Second World War, operating out of New London CT. They also assisted the Underwater Sound Laboratory in developing anti-submarine sensors and tactics. On 14 April 1942 Mackerel (SS-204) narrowly avoided two torpedoes fired by a German submarine off the coast of Virginia, and managed to fire several torpedoes back, which missed.
Both were decommissioned in November 1945 and scrapped.
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