The third USS Union was a heavy (1,114-ton) steamer with a powerful 12-inch rifled gun purchased by the United States Navy during the American Civil War.
Union served the U.S. Navy successfully during the blockade of ports and waterways of the Confederate States of America, capturing numerous blockade runners. Towards war’s end, she was also assigned the role of dispatch boat and, because of her large size, of storeship, at the same time continuing to capture blockade runners.
She was a screw steamer built at Mystic, Connecticut, was chartered by the U.S. Navy on 24 April 1861 at Philadelphia, and was commissioned there on 16 May 1861 with Commander John R. Goldsborough in command.
Source: Wikipedia