The aim of the Wind Challenger Project is to substantially reduce fuel consumption by large merchant vessels. Under development by a group including members from the University of Tokyo, the idea is to utilize giant retractable sails, 20m wide by 50m high, to make maximal use of wind energy. The group has done simulations for shipping routes such as Yokohama-Seattle. The results indicate that hybrid ships with sails and engines could reduce annual fuel consumption by about 30% on average.
Using today's technology, it's possible to make big sails, and to control them automatically. Also, navigation technology includes networked maritime information and weather forecasting, so ships like this can travel safely. Using wind energy, as in old-fashioned sailing ships, is actually feasible.