Dunderberg, which is a Swedish word meaning "thunder(ing) mountain",[1] was an ocean-going casemate ironclad of 14 guns built for the Union Navy.

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.

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 7,000 long tons (7,100 t) to the US Navy Department.

[Note 1] The main deck, from the rear of the casemate to the stern, was to consist of 4.5-inch armor plates that tapered to 2.5 inches in thickness.

[4] The engines, designed to produce 4,500 indicated horsepower (3,400 kW),[5] and boilers were both subcontracted by Webb to the Etna Iron Works of New York City.

The most important of these was the eventual elimination of her turrets[12] which began in October 1863 when Webb wrote to Gideon Welles, Secretary of the Navy, saying that he concurred with the General Superintendent of Ironclads, Rear Admiral Francis Gregory's suggestion that the "turrets be dispensed with and the casemate lengthened to accommodate an additional number of guns"[13] Welles did not approve the change until September 1864 when he authorized an armament of four 15-inch and twelve 11-inch guns in the casemate.

[14] Dunderberg was built with a plough-shaped 44-foot (13.4 m), ram bow of which the forward 12 feet (3.7 m) were sheathed in cast iron.

The only significant problem encountered during the trials was that the gun deck was made of soft pine and did not withstand the force of recoil well.

The New York City draft riots in July 1863 and several machinist's strikes further slowed progress on Dunderberg.

Webb failed to account for these problems when negotiating the ship's contract and he repeatedly tried to charge the Navy for alterations as well as use cheaper materials to reduce his costs.

The Navy agreed to pay for some of the changes made and it also reduced the reserve amount held back in case the ship did not meet her specifications in 1865.

The following year, the government rejected offers by Peru and Chile to purchase the ship, both then at war with Spain, lest the sale prejudice its lawsuit against Great Britain for selling warships to the Confederacy.

Webb did manage to get a private bill passed by Congress in March 1867 that allowed him title to the ship once he repaid all monies advanced to him.

[18] The exact date of purchase is unknown, but Welles noted in his diary on 7 May that Webb had told him of the sale.

A little over a week later, he attempted to sail to France, but had to return to port the following day due to engine problems.

After briefly running machinery trials two days later, Rochambeau began an overhaul in the naval dockyard at Cherbourg to fix problems that had become apparent during her delivery voyage and to modify her in accordance with French practices.

These changes included the addition of a pilothouse on top of the conning tower, the replacement of the main rudder and the length of her stern was reduced somewhat.

In service the ship proved to burn a lot of coal, full bunkers and 30 tonnes (30 long tons) stored on deck provided her a range of only 2,200 kilometres (1,200 nmi) at a speed of 8 knots (15 km/h; 9.2 mph).

[23] Rochambeau completed her refit on 18 May 1868 and was briefly under the command of Captain (Capitaine de vaisseau) Jules-François-Émile Krantz (a future Minister of Marine) before being decommissioned on 1 August and placed in reserve.

She was refitted from August to December and had her forward hull armor plates replaced by a one-piece cast iron ram.

Rochambeau's shallow draft meant that she and the armored corvette Thétis were ordered to search the mouth of the Jade Estuary on 26 September for Prussian ships, but they found nothing.