USA, USS Dictator (1863)
Designed as an ocean going monitor with 2 × 15 in (381 mm) Dahlgren smoothbores and a speed of 10 knots (18.5 km/h; 11.5 mph). In service untill 1877.
USA, USS Puritan (1864)
Designed as an ocean going monitor with 2 × 20-inch (508 mm) smoothbore Dahlgren guns and a designed speed of 15 knots (28 km/h; 17 mph). Never completed due to issues with casting the guns. Deteriorated in the stocks and "administratively rebuilt".
CraigH