USS Dunderberg (meaning "Thundering Mountain" In Swedish) was the brainchild of one America's most brilliant ship designers, William H. Webb. But Webb's design, though accepted for service, was fought with problems, including monitor fever, bureaucratic meddling, graft, shortage of materials, and the interference of rivals such as John Ericsson. After multiple redesigns, she was finally nearing launch in 1863, only to be re-designed again by order of Secretary Wells, a complex job that required essentially tearing her apart and rebuilding her piece by piece. She eventually launched in 1865, but was not ready for service in 1867, at which point the navy seems to have lost interest.
Webb fought a hard legal battle to get the Navy to honor its contract.The Navy counter-sued for the money the Navy Department had invested in the ship. Eventually the Federal Courts issued a somewhat favorable verdict for Webb. He would be sold the ship by the Navy, minus the cost they'd invested in it, permitted to keep all onboard equipment, and granted a special dispensation to sell it abroad. At first, Prussia's Bismarck expressed interest, but Emperor Napoleon III of France quickly snapped her up in a bid to slight Prussia, paying far more than Webb was asking. Webb and his family duly sailed to Brest from New York, and her commanding officer reported no troubles on the journey. Upon acceptance in France, she was laid up, overhauled and recommissioned as "Rocheambeau." She spent most of her career attempting to unsuccessfuly bait the Prussian fleet into engaging her, and was retired relatively early- ostensibly, this was because she'd been built with unseasoned timber, but the real reason was that France had lost the Franco-Prussian War and was desperately low on money.
At any rate, here she is. Another first in the scale, as designed for US Navy use (so, not the right ship if you're looking to field Rocheambeau.) She could conceivably have been completed as early as 1863 or 1864 if not for political shenanigans, and makes for another interesting choice for the Union. She is essentially an improvement on many of the concepts used with USS New Ironsides, and would make a powerful ally for a Union player looking for more than the usual Monitor centered fleet.
Strongly recommend BSF with this vessel, due to her masts. However, it's likely that in service, she would have had the masts removed.
We don't know what colors she would have been painted in USN service, so I've chosen to paint her in a style similar to that of New Ironsides: a grey casemate with green vertical surfaces and black masts.