Captain could be an address without being the officer's rank

How would a master and commander normally be addressed aboard his ship?

Often as captain because he commanded the vessel, even though his formal rank remained master and commander; a post captain's commission carried the stronger traditional status of master under God aboard ship.

Captain could describe the function of commanding a ship as well as a formal commissioned rank. A master and commander was commissioned to command a vessel without becoming a post captain, so the crew could still address him as captain while his rank remained lower. The exact customs varied between navies and circumstances. A master and commander also differed from the post captain's traditional status as master under God aboard his ship. This distinction prevents dialogue and forms of address from being mistaken for an officer's substantive rank.