Dunderberg, a casemate ironclad ram of 16 guns, was the largest warship built during the Civil War. Launched at war's end and never commissioned, she was returned to her builder in accordance with the Navy's policy to reduce its number of vessels, and subsequently sold to France in whose navy she rendered notable service under the name Rochambeau.
Prior to the Civil War, the United States Navy - unlike its European counterparts in Britain and France - had not shown an interest in ironclad warships. Ship designs were little different than when its first ships were launched in the late-1700's, with the exception of engine technology and rifled guns. By 1860, when the the French had tested La Gloire in the Crimean War and the British had put Black Prince and Warrior to sea, the most powerful American warships were the six ships of the Merrimack class, all of them wooden. Just over a year later, the Civil War would begin, and the Navy would lose its largest facility to the Confederacy in Norfolk, Virginia.
It was a short time after Norfolk's loss that rumors reached Washington that the Rebels had raised one of the damaged ships and had begun the process of converting it into an ironclad; she would later raise havoc in Hampton Roads. It was during this period that the Navy commissioned a board to evaluate and accept designs for ironclads, which would operate primarily within the rivers and coastlines of the South, and from this board three designs were accepted: Keokuk, Galena, and New Ironsides. A fourth one - Monitor, came in late, but was actually built and launched ahead of the others, and would stop Virginia near Norfolk, but not before the Confederate ironclad had demonstrated the power of iron over wood.
With the end of the Monitor/Virginia engagement, a flurry of new designs came in to the board, and one of them was submitted by John Lenthall of New York. His ship was of a casemate design much like Virginia, yet massive: well-over 300 feet long, over 7,000 tons, and carrying nearly twenty guns, with four encased in a pair of turrets. The final draft of the ship removed the turrets, and in July, 1862, Dunderberg was authorized for construction by William H. Webb's shipyard in New York City.
Her hull was constructed of unseasoned oak, three feet thick on most places, and over seven feet thick where the casemate met the hull. Over this framework were placed iron plates 4 ½ inches thick on the casemate, and 3 ½ to 2 ½ inches everywhere else. Within the casemate were four 15-inch and twelve 11-inch Dahlgren smoothbores. Her other, primary weapon jutted from her bow: an iron-plated ram nearly fifty feet long.
Two horizontal steam engines fed by six boilers powered a single propeller, capable - on paper at least - of 4,500 shaft horsepower at 15 knots maximum speed. For economy at sea, she was given a sail rig. Additionally, watertight bulkheads, a double bottom, armored gratings within the funnel, and other features for survivability at sea and in battle were added, which drove up the cost. This, plus the fact that iron was scarce due to other service commitments, slowed down Dunderberg's contruction considerably. She was not launched until July, 1865, after the war had ended.
Service in France
After the war, the Navy began the process of decommissioning and selling ships it no longer had a reason for having, and Dunderberg was one of them. Finished by 1867, she went to sea for trials, and came in below the planned expectations: 11.7 knots at 3,778 shaft horsepower. This was the excuse needed to formally-reject the ironclad, and Webb not only was made to take it back, he had to refund $1,041,660 of the $1,250,000 paid to the company by the government.
Webb didn't sit still very long: he found a foreign buyer in France, which needed a powerful ship to meet the threat posed by Prussia, as well as catch up with Britain in the ironclad arms race. Renamed Rochambeau after the general that the British under Cornwallis had tried to surrender to instead of Washington during the American Revolution's final battle, she performed admirably there, rendering service in an attack on Kolberg during the Franco-Prussian War. Gone were the Dahlgren muzzleloaders, to be replaced by four 10.8-inch and ten 9.4-inch breach-loading rifles, and her speed was claimed to have been 15.07 knots at 4,535 shaft horsepower.
But she had only a few years of life as Rochambeau. The ship's wooden framework - constructed of unseasoned, green timbers to save time and money - experienced the decay which properly-seasoned timber never has, condemning her to be stricken from the French Navy in 1872, just five years after her first sea trials off New York. In 1874, she was scrapped.