Cube, I would say that sail will come back because it costs nothing to fuel. It costs a lot in maintenance, but so does a boat that uses gas. Remember that a boat is a hole in the water you throw money in to.
I just got back from using my solar boat. It's the easiest thing imaginable. A battery, a flexible solar panel and an electric trolling motor. No charge controller, just a blocking diode. It sits and charges for a few days, and I use it to cruise around the harbor.
It's the cheapest and easiest boat I've ever had.
Why couldn't this be scaled up? We could use flexible solar panels as the sails and use electric motors to move the boat when the wind isn't blowing or when maneuvering in port.
The hull would have to be different from the ones we're using now. I don't think we'll be using much of the stuff we made in the 20th century in the 21st. It will provide some nice raw materials maybe.
I checked out about 10 schooners with my solar boat. The Victory Chimes has been carrying lots of people and cargo for over 100 years around the coast of Maine and is still doing it:
http://www.sailmainecoast.com/victorychimes.htm








