Using a POD any time after 1800, keep wooden sailing ships the standard of transportation and warfare for as long as possible (OTL steam-warships appeared in the 1820s and became widespread by the 1850s, and it was from 1860-65 that warships 'turned the corner'--any pre-1860 ship could stand up to any other pre-1860 ship, but any post-1865 ship could take any pre-1860 ship).