Do you have more information on that? I assumed the keel was for balancing the torque that the forces on the sail generate so one doesn't capsize. I guess this

Do you have more information on that? I assumed the keel was for balancing the torque that the forces on the sail generate so one doesn't capsize. I guess this is part of the reason. As for why it isn't the full reason, think if the boat was a hovercraft or somehow otherwise had zero friction. When the wind hit the sails it would just move the vehicle in the direction of the wind (possibly turning it sideways in the process). In land sailing or ice yachting you don't have two medium with different densities and still achieve forward movement using a sail. In land sailing or ice sailing you can steer against a hard surface. I'm not a specialist in fluid dynamics but this should count for something, IMO it should even get better results.