Have to agree with AdamLein. A more simplistic approach to proving the existence of wind abeam would be this thought experiment: Say that you are going downwind, which direction is the apparent wind? Astern? Now suppose you are going upwind, the apparent wind is now coming from somewhere forward of the beam... Now consider how you change between the two. It is logically necessary that at some point the apparent wind is on the beam, but I agree with AdamLein that even then the flow would be laminar enough to allow the leeward sail to be useful, if somewhat less so than on other points of sail.
Interesting topic to research DDWFTTW just google it. I will give you a clue, it is possible to go dead downwind faster than the wind, multihulls and foilers do it all the time.
With this in mind, mitiempo is correct in some capacity, fast multis spend most of their time with the apparent wind forward of the beam, this makes sense when you consider that they can go dead downwind faster than the wind by broad reaching and jibing.
However, when jibing, the apparent wind will be astern at some point, and this is a sufficient condition to satisfy my previously stated "thought experiment." I have avoided a rigorous analysis of this subject just because it would reach a narrower audience, nobody likes a pedant.
Last edited by tager; 01-15-2010 at 02:49 AM.