Boats can travel faster than the wind because (ignoring surface resistance), they make their own 'wind' thanks to their speed. That wind combines with the true

Boats can travel faster than the wind because (ignoring surface resistance), they make their own 'wind' thanks to their speed. That wind combines with the true wind in generating the lift on sails even when the boat is travelling at wind speed or higher. ~~This can't happen if the boat is going straight downhill, because the apparent wind would be zero (as the boat is moving with the wind).~~ [ETA: that's not actually true in the general case. You can't have a sailboat move downwind faster than the wind, but if you have a gear mechanism and a turbine you can do it.] But when tacking downhill, the movement of air around sails still generates lift.