According to the Economist, Energy Technologies Institute (ETI) wants to study auxiliary wind power to cargo vessels. The ship would be equipped with huge vertical rotors to be spun by the wind. Some kind of gear train would drive a propeller off the rotors. Huge, like fifteen feet in diameter and 150 feet tall. The idea isn't all that new. The devices are called Flettner rotors, and they were invented by a German aerodynamics guru in the 1920's. Flettner was able to obtain enough funding back then to outfit a medium sized ship with his rotors and "sail" it. For carrying all this top heavy gear, ETI expects, not a real no-fossil-fuel ship, but merely a 5-10% savings in fuel.
Seems like a helova lotta money sunk into equipment for a pretty chintzy fuel savings. Does not sound worth while to me. I'm sure the greenies are all over this, they love expensive and worthless things.
By 1840 the square rigged sailing ship had been perfected to a level that is competitive with modern steam ships in terms of speed, size, and range. Clipper ships could do 20 knots in 1840, where as 1940's convoys of steamers could only do 6 knots. The only reason steamers replaced sailing vessels was cost. Sailing ships required a big crew to handle all the sails. Enough men to furl sail in a sudden blow, enough men to wear ship. Whereas a steamer only needs a man at the wheel and a few men in the boiler room to keep steam up. The world's merchant fleet was mostly sail powered up until WWI when German U-boats sank most of the sailors. Sailors were easier to catch than steamers.
Should the price of fuel go back up (way way up) the square rigger would become practical again, yielding a true no-fossil-fuel ship. It would probably take something like $200 a barrel oil to do that.