you are right about that, it was the steam drum....

i have been to the Cairo many times. it is a great place to go and a great resource of information.

it is believed by many including me that the reason Selfridge put railroad iron on the forward quarters of Cairo is exactly because of what happened to the Mound City. It probably wouldn't have stopped a large caliber bolt like the one that disabled the Mound City though. In addition to the field modification of railroad iron on the casemate, a railroad iron barrier was built inside the Cairo arond the engines and boilers. apparently that was lost during the salvage or too severely damaged to display.

The book "Hardluck Ironclad" by Ed Bearass has detailed info on most of this.