In the Gathering Storm expansion, the Ironclad requires 1 Coal to train and 1 Coal per turn to maintain.
Being the first armored ship in the game, and thus a precursor to modern vessels, the Ironclad is a good upgrade from the earlier Caravel, although the difference (10 CS) is not as big as in other cases. But more importantly, the Ironclad serves as a necessary stepping stone towards real modern melee ships, which become available quite a bit later. You'll need Coal to be able to build Ironclads, which can be a problem, because you'll never be able to modernize your fleet if you don't get it!
The gist is that the Ironclad (and the later Destroyer) represents the only naval melee unit in your fleet. Without it, your ability to take coastal cities will be severely jeopardized, and non-existent without a ground army.
Civilopedia entry Edit
The best-known clash between ironclads – those steam-propelled, lumbering behemoths covered by iron or steel plating – at Hampton Roads in May 1862 AD ended in a draw. The CSS Virginia had earlier wreaked havoc on the Union’s traditional warships there, and while there was a lot of fighting still to come, the age of wood and sail in warfare was doomed. But the American ironclads weren’t very seaworthy, and tended to sink in open water. The British Royal Navy built the first classes of “seagoing ironclads” (actually armored, iron-hulled frigates) in the early 1860s, and soon enough they added steam-engine driven screws to the designs. Seeing how the Virginia had handled wooden warships far larger, every nation scrambled to build ironclads. The first fleet action fought on the open water between ironclads was near the island of Lissa in the Adriatic between the Austrian and Italian navies; of the 19 ironclads involved, two went to the bottom. But, like that battle, the age of the ironclad didn’t last long.