The Sphinx-class sailing sixth rates were a series of ten post ships built to a 1773 design by John Williams. Although smaller than true frigates, post ships were often referred to incorrectly as frigates by sea officers, but not by the Admiralty or Navy Board. The first vessel in the class was launched in 1775, six more in 1776, two in 1777 and the last in 1781. The vessels of the class served in the Royal Navy during the American Revolutionary War.