Complex sentence: Two years thereafter, at the age of twenty-three, he signed on as third mate of the "Duchess", which was being readied for a perilous journey into the South Pacific Ocean to raid ships and towns along the Spanish Main.
Split into simpler sentences: Two years thereafter, at the age of twenty-three, he signed on as third mate of the "Duchess". The "Duchess" was being prepared for a perilous journey into the South Pacific Ocean to raid ships and towns along the Spanish Main.