USS Napa was a Casco class twin-screw light draft monitor built during the American Civil War for operation in the shallow inland waters of the Confederacy.

These warships sacrificed armor plate for a shallow draft and were fitted with a ballast compartment designed to lower them in the water during battle.

Though the original designs for the Casco-class monitors were drawn by John Ericsson, the final revision was created by Chief Engineer Alban B. Stimers following Rear Admiral Samuel F. Du Pont's failed bombardment of Fort Sumter in 1863.

It was discovered that Stimers had failed to compensate for the armor his revisions added to the original plan and this resulted in excessive stress on the wooden hull frames and a freeboard of only 3 inches (76 mm).

He was forced to raise the hulls of the monitors under construction by nearly two feet and the first few completed vessels had their turrets removed and a single pivot-mount 11 inch Dahlgren cannon mounted.