"Chainclads" in a nonsense.
It derives from the USS Kearsarge. The design of the Kearsarge was very bad in that she had a vertical boiler above the waterline (RN screw ships of the time put a horizontal boiler below the waterline). Obviously one shot amidships that penetrated the boiler would instantly put the whole ship out of action as scalding hot steam filled the entire internal volume.
To give some degree of protection the captain of the Kearsarge draped his anchor chains over the second that the boiler was in to provide some additional protection. This didn't stop Alabama's shells.
Farragut and his fleets used that same measure of adding at least some basic armor at New Orleans and Mobile Bay. Sandbags were also used. Obviously this isn't a replacement for armor plate, but it does add a measure of protection. If nothing else it helps morale.
While a number of smaller gunboats were knocked out of action by penetrating shots that holed boilers (and thus live steam killed or wounded much of the crew below decks), I know of no instance involving a a larger ship running 1,000 or more tons. Do you know of one?