There have been a number of 21-inch torpedoes in service with the United States. These have been used on ships and submarines of the U.S. Navy. American 21-inch torpedoes are in diameter.
Ship classes that carried 21-inch torpedoes include:
Allen M. Sumner-class destroyers
Atlanta-class cruisers
Bagley-class destroyers
Balao-class submarines
Barbel-class submarines
Barracuda-class submarines
Benham-class destroyers
Benjamin Franklin-class submarines
Benson-class destroyers
Cachalot-class submarines
Caldwell-class destroyers
Cassin-class destroyers
Chester-class cruisers
Clemson-class destroyers
Colorado-class battleships
Connecticut-class battleships
Dealey-class destroyer escorts
Ethan Allen-class submarines
Farragut-class destroyers
Forrest Sherman-class destroyers
Fletcher-class destroyers
Gato-class submarines
Gearing-class destroyers
George Washington-class submarines
Gleaves-class destroyers
Glenard P. Lipscomb-class submarine
Grayback-class submarines
Gridley-class destroyers
Halibut-class submarine
James Madison-class submarines
John C. Butler-class destroyer escorts
Lafayette-class submarines
Lexington-class battlecruisers - canceled
Los Angeles-class submarines
Mahan-class destroyers
Mitscher-class destroyer leaders
Narwhal-class submarine
Nautilus-class submarine
Norfolk-class destroyer leader
O'Brien-class destroyers
Ohio-class submarines
Omaha-class cruisers
Permit-class submarines
Porter-class destroyers
PT boats
R-class submarines
Rudderow-class destroyer escorts
S-class submarines
Sailfish-class submarines
Sampson-class destroyers
Sargo-class submarines
Seawolf-class submarine (1957) Seawolf-class submarines (1997) Sims-class destroyers Skate-class submarines Skipjack-class submarines Somers-class destroyers South Dakota-class battleships - canceled Sturgeon-class submarines T-1-class submarines Tambor-class submarines Tang-class submarines Tench-class submarines Tennessee-class cruisers Tucker-class destroyers Tullibee-class submarine V-boat submarines Virginia-class battleships Virginia-class submarines Wickes-class destroyers