Complex sentence: From the beginning of the nineteenth century steam propulsion and the use of iron, rather than wood, in shipbuilding gradually came into use, making fire ships useless, although ships or boats packed with explosives could still be effective.
Split into simpler sentences: From the beginning of the nineteenth century steam propulsion and the use of iron, rather than wood, in shipbuilding gradually came into use, making fire ships less of a threat. Ships or boats packed with explosives could still be effective.