Professor emeritus Lionel Casson discusses this topic in the context of Mediterranean sailing during the classical antiquity in his book Ships and Seamanship in the Ancient World. He estimates that most sailing ships of the time made an average speed of 4-6 knots under favourable wind over open waters, and less than 2-2˝ knots sailing against the wind. (p. 288, 291) The actual speed for any particular voyage during that era could be significantly faster or slower than these average figures, though. This also consides only the typical speeds of individual ships, which as professor Casson notes, cannot be taken as an indication for the speed of a fleet of ships. A fleet only travels as fast as it's slowest element. For fleets he provides the estimate of 2-3 knots under favourable wind, and no better than 1-1˝ knots under light or unfavourable winds. (p. 296)