The technique of composite ship construction (wooden planking over a wrought iron frame) emerged in the mid-19th century as the final stage in the evolution of fast commercial sailing ships. Construction of wrought iron hulled vessels had begun in the 1820s and was a mature technology by the time of the launch of the SS Great Britain in 1843. However, iron hulls could not be sheathed with copper alloy (due to bimetallic corrosion) and so would become festooned with drag-inducing weed during long voyages in the tropics.