Wiki Sail by GRF

Originally posted by pondmonkey

   Glad to see you seem to be getting the point in the second paragraph your last post, GRF - you need moor up to the buoy for speed over the ground to make any odds.
I would hope, Bootscooter, that if the sailboard were going at 10 knots it would be planing, else we are talking about a real mega-length board here.  

But the question originally asked was, in your terms, "why [with the true wind on the beam] will the sailboard go at 12 knots in the up-tide direction relative to the water but slower in the opposite direction?".  

The speed difference means that the drag of the water on the sailboard will be higher in the one direction than the other.  It follows that the thrust generated by the rig of the sailboardboard must be higher in the one direction than the other.  

So the question translates as "why does the rig generate more thrust in one direction than the other?"

The answer the wise respondents have been giving is that it is because the angle of the apparent wind is freer going up-tide than down-tide. 

It is interesting to note that the strength of the apparent wind will be stronger when the sailboard is going down-tide, since the 'board's speed through the water and the tide are additive, but despite this James was generating more thrust going up-tide thanks to the force vector being more forwardly directed at the point where he was about to plane.