Gravity does not actually provide a force in the sense that a sailboat would require. (It acts on all parts of the vessel equally.) There is some work on the i

Gravity does not actually provide a force in the sense that a sailboat would require. (It acts on all parts of the vessel equally.) There is some work on the idea of using both a photon sail and a magnetic sail on a single vessel. In many interesting locations, these would push in different directions, allowing you to sail.