Complex sentence: Dorantes sold his slave, Estevanico, to the Viceroy and was preparing to leave when the ship where he was on board, was pronounced unfit to sail, forcing him to return to the port of Veracruz.
Split into simpler sentences: Dorantes sold his slave, Estevanico, to the Viceroy. However, when he was preparing to leave, the ship (where he was on board), was pronounced unfit to sail, forcing him to return to the port of Veracruz.