Complex sentence: The Admiralty design was approved at the end of December 1858 but, having no experience with iron hulls, the Board of Admiralty called for designs from the country's most prominent iron shipbuilders.
Split into simpler sentences: The Admiralty design was approved at the end of December 1858. Having no experience with iron hulls, the Board of Admiralty called for designs from the country's most prominent iron shipbuilders.