systems generate very low amounts of energy for propulsion (only about 70 W per rower) and the upper limit for rowing in a fixed position is around 10 knots. Ancient war galleys of the kind used in Classical Greece are by modern historians considered to be the most energy-efficient and fastest of galley designs throughout history. A full-scale replica of a 5th-century BC trireme, the Olympias was built 1985–87 and was put through a series of trials to test its performance. It proved that a cruising speed of 7–8 knots could be maintained for an entire day. Sprinting speeds of up to 10 knots were possible, but only for a few minutes and would tire the crew quickly. Ancient galleys were built very light and the original triremes are assumed to never have been surpassed in speed. Medieval galleys are believed to have been considerably slower, especially since they were not built with ramming tactics in mind. A cruising speed of no more than 2–3 knots has been estimated. A sprint speed of up to 7 knots was possible for 20–30 minutes, but risked exhausting the rowers completely. Rowing in headwinds or even moderately rough weather was difficult as well as exhausting. In high seas, ancient galleys would set sail to run before the wind. They were highly susceptible to high waves, and could become unmanageable if the rowing frame (apostis) came awash. Ancient and medieval galleys are assumed to have sailed only with the wind more or less astern with a top speed of 8–9 knots in fair conditions. Galley slaves Contrary to the popular image of rowers chained to the oars, conveyed by movies such as Ben Hur, there is no evidence that ancient navies ever made use of condemned criminals or slaves as oarsmen, with the possible exception of Ptolemaic Egypt. Literary evidence indicates that Greek and Roman navies relied on paid labor or ordinary soldiers to man their galleys. Slaves were put at the oars only in times of extreme crisis. In some cases, these people were given freedom thereafter, while in others they began their service aboard as free men. Roman merchant vessels (usually sailing vessels) were manned by slaves, sometimes even with slaves as ship's master, but this was seldom the case in merchant galleys. It was only in the early 16th century that the modern idea of the galley slave became commonplace. Galley