> I'm not a sailor, but doesn't this rely on the force of water on the keel, to avoid being pushed laterally? The analogy with sailing is actually kind of weak

> I'm not a sailor, but doesn't this rely on the force of water on the keel, to avoid being pushed laterally? The analogy with sailing is actually kind of weak. Since you get elastic collisions with photons on the sail, you can (approach) applying force at 90 degrees from the direction of the light. Unlike with actual sailing, you can never have an "upwind" force component.