From the Naval Wargames Association comes Ironclad, a set of free wargames rules for ACW ironclads.
Steam Iron and Tin is a set of 15mm ACW naval rules that was distributed to people who attended the 1985 Origins Convention in Baltimore (I was at that convention!).
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.