Dunderberg, which is a Swedish word meaning "thunder(ing) mountain", was an ocean-going casemate ironclad of 14 guns built for the Union Navy. She resembled an enlarged, two-masted version of the Confederate casemate ironclad . She was originally designed to have both gun turrets and a casemate but the turrets were deleted while the ship was still being built. Construction began in 1862, but progress was slow and she was not launched until after the end of the American Civil War in 1865.
The ship was not accepted by the Union Navy so her builder began seeking buyers elsewhere; Otto von Bismarck expressed some interest, and the thought of Prussia armed with such a vessel prompted France to purchase her and commission her in 1867 with the name Rochambeau. She was initially placed in reserve, but was mobilized in 1870 to participate in the Franco-Prussian War. The ship saw no action and was decommissioned after the end of the war. Rochambeau was stricken from the Navy Directory in 1872 and scrapped in 1874.
Design, description and construction
On 11 April 1862, William H. Webb, arguably the premier wooden shipbuilder in the country, sent a model of a large wooden-hulled, casemate ironclad with a displacement of about to the US Navy Department. Webb signed a contract on 3 July with the Navy's Bureau of Yards and Docks for a ship that had an overall length of , a beam of at least and a draft of no more than . His ship was required to make in still water and she was to be armed with four Dahlgren guns in two gun turrets, each protected by of armor, and eight 11-inch Dahlgren guns in a casemate. The ship was to be completed in 15 months at a cost of $1,250,000.
Now named Dunderberg by Webb, the contract was amended on 27 August to specify her armor scheme. Above the main deck, her armor was to be thick. From the main deck to a depth of below the waterline, the armor was to be thick, tapering to at its lower edge. The ship's main and casemate deck armor was thick except at the rear of the ship.