The ironclad became technically feasible and tactically necessary because of developments in shipbuilding in the first half of the 19th century . According to naval historian J. Richard Hill : " The ( ironclad ) had three chief characteristics : a metal @-@ skinned hull , steam propulsion and a main armament of guns capable of firing explosive shells . It is only when all three characteristics are present that a fighting ship can properly be called an ironclad . " Each of these developments was introduced separately in the decade before the first ironclads .