Ironclads

Ironclads were ships that were quite literally cladded with iron. Cladding a ship with iron would make it heavier, but also stronger structurally. Ironclads were not needed until the Paixhans Gun came into the warfare scene in the 1820s. While the gun would easily destroy wooden ships, iron cladded warships were a bit more resistant.

The mettle of the Union was tested when the USS Monitor went up against the CSS Virginia. These were both ironclads, but in the end, the Monitor prevailed in a naval battle.