USS Modoc, a single turret, an 1,175-ton Casco-class light draft monitor built under contract by J. S. Underhill at Greenpoint, Brooklyn, was completed as a spar torpedo vessel in June 1865.

Though the original designs for the Casco-class monitors were drawn by John Ericsson, the final revision was created by Chief Engineer Alban C. 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 armour 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 were converted to torpedo boats with the weapons listed to the right.

This article incorporates text from the public domain Dictionary of American Naval Fighting Ships.