There are two other issues with iron hulls. They were thinner than wood hulls and thus increased the usable volume within the hull. They were usually about 20% lighter than wood hulls for a given displacement (actually the cross over line was about 500 tons gross displacement). The down-side was that the early iron hulls and the newer "steel" versions were really forms of wrought iron. With prolonged exposure to cold water, the crystaline structure of the metal became more brittle and subject to damage from impact. Viable steel hulls appeared when alloying became common, allowing the metal of the hull to be tailored to a wider range of temperatures with increased resistance to shock. Much of this evolution was empirical. There were some real turkeys until they found what actually worked. The blockade-runner Will-of-the-Wisp is an example. Most historians assume that her well-known maintenance problems were due to hasty and inferior construction. It is more likely that her seams worked under loads in heavy seas and that this was compounded by vibration from her engines. There were a number of these early steel ships that had similar problems. The earlier iron hulled vessels may have actually performed better and would continue to do so until alloyed steel became common.