 Iron ships had first been proposed for military use in the 1820s . In the 1830s and 1840s , France , Britain and the United States had all experimented with iron @-@ <unk> but <unk> gunboats and frigates . However , the iron @-@ <unk> frigate was abandoned by the end of the 1840s , because iron hulls were more vulnerable to solid shot ; iron was more brittle than wood , and iron frames more likely to fall out of shape than wood . 
