Complex sentence: Two years thereafter, at the age of twenty-three, he signed on as third mate of the "Duchess", which was then being readied for a long and difficult journey to the Pacific coast of 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 vessel was then being readied for a long and difficult journey to the Pacific coast of the Spanish Main.