A sail was, in human terms, any type of surface intended to generate thrust by being placed in a wind—in essence a vertically-oriented wing. Sails made of textile fabric were used as the main propulsion system for sailing ships.
The ship in question's sail plan, meaning the various combinations of sail proposed for its use, had great influence on its performance. Merlin Athrawes introduced a number of new, more efficient sail plans to the Royal Charisian Navy and merchant marine. (OAR)