What is "sixth-rate"

Sixth-rate was the designation used by the Royal Navy for small warships mounting between 20 and 24 carriage-mounted guns on a single deck, sometimes with smaller guns on the upper works and sometimes without. It thus encompassed ships with up to 30 guns in all. In the first half of the 18th century the main battery guns were 6-pounders, but by mid-century these were supplanted by 9-pounders.