USS Onondaga was an ironclad monitor built for the Union Navy during the American Civil War. Commissioned in 1864, the ship spent her entire active career with the James River Flotilla covering the water approaches to the Confederate States capital of Richmond, Virginia, although her only notable engagement was the Battle of Trent's Reach. After the war, she was purchased by France where she served as a coastal defense ship in the French Navy ().
Onondaga saw little active service with the French, spending most of the next four decades in reserve, although she was mobilized during the Franco-Prussian War of 1870–1871. The monitor became a guard ship in 1898, but she was stricken from the naval register and sold for scrap in 1904; the ship was demolished in 1905.
Description
Onondaga had an overall length of and a beam of . Onondaga was powered by a pair of two-cylinder horizontal back-acting steam engines, built by the Morgan Iron Works, each driving one propeller using steam generated by four vertical water-tube boilers. The engines had a combined rating of that gave the ship a speed of . which gave her a range of . The propellers were protected from underwater obstacles by iron rings.
As designed the exposed area of the hull was protected by wrought-iron plates, but John Ericsson, designer of the , suggested that the side armor be reinforced with additional wood inside the armor belt in April 1863. As most of the armor had already been mounted by this time, the shipyard simply added of wood to the exterior of the armor and sheathed it with a armor plate. The weight of the wood was partially responsible for increasing Onondagas draft by almost a foot more than was designed. The armor of the turrets and the pilot house atop the forward turret consisted of layers of armor plates totaling in thickness. A soft iron band was fitted around the base of the turrets to prevent shells and fragments from jamming them as had happened during the First Battle of Charleston Harbor in April 1863. Onondagas deck was protected by two layers of one-inch plates and the base of the funnel was armored as well. The Navy ordered Onondaga from Quintard on 25 May
Onondaga, the first U.S. Navy ship to bear the name, was launched on 29 July 1863 and was sponsored by Sally Sedgwick, daughter of former U.S. Representative Charles B. Sedgwick. She was commissioned at the New York Navy Yard on 24 March 1864 with Captain Melancthon Smith in command. Construction was delayed by shortages of material, labor and the closing of the shipyard during the New York City draft riots in July 1863. The ship cost $759,673.08, a budget overrun of almost 22 percent.
When the Confederate squadron steamed down river to attack the weakened Union forces, Onondaga dropped downstream to a position where she could maneuver more easily. She and her supporting gunboats awaited them only to see the attack aborted when elements of the Confederate fleet—the casemate ironclads Virginia II and Richmond, the gunboat Drewry, and the torpedo boat Scorpion—all ran aground as the tide ebbed trying to pass Union obstructions at Trent's Reach, after the casemate ironclad Fredericksburg and the gunboat Hampton had passed through the obstruction at 01:30. The latter two ships continued downriver and anchored below the Dutch Gap Canal to await the rest of the squadron.
The squadron commander, Flag Officer John K. Mitchell, recalled them several hours later and ordered them to anchor further upstream under the guns of Battery Dantzler. As the Confederate ships were refloated at 10:45, Onondaga approached the Union obstructions and opened fire at an estimated range of . Her Dahlgren guns hit Virginia II twice with solid shot; the first shot penetrated her armor and badly damaged the wooden backing and its supporting beams. The second shot killed one crewman and killed two others as it penetrated the armor and its backing. Richmond was only hit by a glancing shot that damaged the armored shutter that protected the ship's stern gun port before the Confederate ships were able to withdraw upriver.
The decision to move Onondaga downstream resulted in her commanding officer, Commander William Parker, being court-martialled for "neglect of duty in not offering battle to the Confederate ironclads" during this engagement. Secretary Welles set aside the guilty verdict on a technicality.
Onondaga continued to support Union troops for the rest of the war and steamed north after its end on 9 April. The ship was decommissioned at New York City on 8 June and was laid up at League Island, Pennsylvania. By an Act of Congress approved on 7 March 1867, the monitor was sold to back to her builder who resold her to France
French service
thumb|Onondaga, at [[Brest, France, circa the late 1860s or the 1870s]]
Onondaga retained her name in French service and was towed from New York City to Halifax beginning on 2 September 1867. She was towed across the Atlantic to Brest by the transport departing on 15 June 1868, escorted by the aviso . The ship began an extensive refit after her arrival on 2 July that include the replacement of her original armament with four Modèle 1864 or Modèle 1864-66 rifled breech-loading guns. The French measured her freeboard at although it is uncertain if this was before or after these modifications. Onondaga began her sea trials in May 1869 and was subsequently placed in reserve.