Answer:
They could take more damage from enemy power.
Reason this is an answer: Remember that during the Civil War, all sides (as well as the world) continued to use cannon balls, which is generally a sphere made of pig iron, and was loaded into a cannon. These kinds of rounds packed more punch rather than the penetration quality needed to go through iron. Ironclads generally shrugged off the damage that would usually penetrate the wooden-ships, causing a hole in which water would sip in, and slowly drown.
They relied on power.
Reason this is an answer: While I'm not sure what "power" it is referring too, they do rely on many different powers. They first rely on the power of the economical situation of a country, in which the country is able to not only build the ship, but also produce the iron & create enough iron to armor the ship up. Power also can refer to armament as well as crews and protection, in which all of these are heavily relied upon when in battles. Without the experience and battle-hardening of all of these, the ships would not have as much of an impact as it should have.
It can be, but is not necessary, at least during the Civil War:
They could have more weapons and firepower.
Later on as the ironclads evolved they were able to hold more weapons and have a higher % of firepower, however, the early days (or the days of the Merrimack and Monitor) they did not have more fire power than the ordinary wooden ships both navies had. Remember, they relied more on their armor than in firepower. However, this changed over the years.
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