(idiomatic, nautical) With sails unfurled; powered by the wind.
A ship under full sail.
In motion.
Source: en.wiktionary.orgI am not a yachting person, by nature, but I have just enough experience on the sea under sail to feel a certain nostalgia for it when I see a big white racing yacht heeled over at cruising speed on the ocean, and I can still tie a mean bowline knot on just about anything in less than 10 seconds. Hunter S. Thompson
A Ship under sail and a big-bellied Woman, Are the handsomest two things that can be seen common. Benjamin Franklin
Since there were still nearly 1,900 ships under sail by 1870 the regulations were long overdue. Source: Internet
Veterans groups from around the country had proposed that she should make the tour under sail, but due to the schedule of visits on her itinerary she was towed by the minesweeper USS. Source: Internet
Since the solo circumnavigation of Joshua Slocum in the 1890s, long-distance cruising under sail has inspired thousands of otherwise normal people to explore distant seas and horizons. Source: Internet
The currents produced by the tidal action in the Black Sea and the Sea of Marmara are such that ships under sail must await at anchorage for the right conditions before entering the Dardanelles. Source: Internet