How guns replaced crew size in the ship rating system

Earlier systems emphasized crew size because boarding parties mattered heavily. By the later seventeenth century, professionalization and heavier artillery made a ship's gun power more useful than its complement as a shorthand for fighting strength. The Pepys-era system ran from first through sixth rates, and gun-deck count could distinguish ships carrying similar numbers of cannon. A two-deck fourth rate and a one-deck fifth-rate frigate therefore occupied different tactical and structural categories even when their nominal totals overlapped. Ratings were administrative descriptions that changed with naval practice.