Ironclad Draughts is a set of one-page rules for playing out battles between ironclads.
The Jackson Gamers have a page with everything you need to know to start gaming Civil War Ironclads
I have a large collection of 1/1200 Civil War Ironclads but have never played many games with them becuase I haven’t really found a set of rules I liked. I may give theis free set called Smoke on the Water a try.
Pete Pellegrino has written a set of free miniature wargames rules called Damn The Torpedos. In the game, you customize your ironclad, and then take it out to fight in the Cvil War battle of Mobile Bay.
Heart of Oak, Skin of Iron is a set of free wargame rules for naval combat in the years 1852 – 1870.