USS Napa, an 1175-ton Casco-class light draft monitor built at Wilmington, Delaware, was converted to a spar torpedo vessel while still under construction in 1864. She was delivered to the U.S. Navy in May 1865, but was never placed in commission. While laid up at the League Island Navy Yard, at Philadelphia, Pennsylvania, Napa's name was changed to Nemesis in June 1869, but reverted to Napa less than two months later. She was scrapped in 1875.