My pleasure. I believe I've read somewhere that the metal boats used an early version of steel hulls. The hulls would have been lighter for a given displacement if this is true. If this is true they were pushing the technology for the period. The early steels for maritime use were actually closer to wrought iron than what we think of as steel. They were more brittle and it is difficult to project what they would have done under load and in high vibration conditions. I have a suspicion that the infamous Will o the Wisp blockade runner's problems were in part caused by the effects of high-speed runs through rough water. The flex of the hull may have been more than the builders anticipated. Also, the length to beam ratio is important in these vessels. Wooden hulls had some "give" to them and a limited amount of flex was expected. The metal hulls also deflected under load or lack of support in a portion of the hull was out of the water in rough seas. The purpose built runners tended to have greater length to beam ratios for purpose of speed. Some had the semi-conical foredecks and were intended to drive through waves rather than over them. Metal structures of the period had definate life limits (duty cycles) for deflections before you got cracking and then later structural failures. The war ended before the early steel hull vessels got enough operating hours for this phenomena to become apparent. For the technologies available 1860-65, the "composite" hulls proved the most durable. Typically you had a primary framing of iron/steel and wooden planking. One big advantage of this construction was that water-tight bulkheads became more effective.