Sailboats represent an ancient (but newly relevant) sustainable form of transportation. They work off the relative velocity between the air and the water. Sailboats can sail upwind (by tacking), they can sail downwind faster than the wind (also by tacking), and they can sail crosswind much faster than the wind. I present the simplest possible momentum transport model of a sailboat, and explain all of these capabilities. In this model, the sailboat is defined by three dimensionless numbers: The sail-to-keel area ratio, a lift ratio for the sail, and a lift ratio for the keel. The model makes a number of amusing "predictions" that explain the properties of commercial and competitive sailboats. There are many connections to sustainable energy and astrophysics.