I am optimistic about the return of commercial sail. Hybrid sail/electric vessels will proliferate by harvesting energy from the wind.
Two near-inexhaustible energy sources—sunlight and the angular momentum of the rotating earth—combine, via the atmosphere, to produce the energy flux we know as wind. We have two well-proven methods of capturing this energy: windmills and sailing ships. Windmills are real-estate limited, since most available land surface is already spoken for, and distribution-limited since wind-swept areas tend to be far from where large concentrations of people live. Sailing ships turn wind energy directly into long-distance transport, but the practice was abandoned in an era of cheap fuel. The prospects deserve a second look. It is possible to not only conserve, but even accumulate, fuel reserves by sailing around the world.
Modern sailing vessel design, so far, has been constrained by two imperatives: racing (for sport or against commercial competition) and ability to sail upwind. Under favorable conditions sails produce far more horsepower than is needed to drive a ship. At marginal sacrifice in speed, by running the auxiliary propulsion system in reverse, this energy can be stored for later use. Hybrid vessels, able to store large amounts of energy—in conventional batteries, in flywheels, or by disassociation of seawater—would be free to roam the world.
The trade winds constitute an enormous engine waiting to be put to use. When oil becomes expensive enough, we will.