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segler said..
If you do a vector analysis of the wind and its forces, you see that downwind the apparent wind hits you from the side, not the front. This is why you release the clew with adjustable outhaul to curve the leading edge to make it catch the sideways apparent wind with little drag..

The most efficient downwind craft sail "upwind" when off the wind. AC boats, foilers, etc. If you look at the sail shape of an AC boat downwind, it's flattened out just like upwind (edit: because it's going upwind in its apparent wind).




Look at the iqfoilers going downwind at 53:15-20. They are pretty much sheeted into the centerline of the board

Yes, you will be fully sheeted in on the downwind runs, but the wind is much weaker compared to when we go upwind. This comes from the summation of vectors. The outhaul is mostly tuned to have enough power in the sail in order to get enough lift from the fin, so we need to let go of the outhaul for a fuller sail profile when going down-wind (fully sheeted in) due to the lower wind strength.

The addition of vectors becomes even more apparent when sailing on ice. Here the low friction against the ice means than one can use very flat sails, both when going upwind and downwind.

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SurferKris said..

The addition of vectors becomes even more apparent when sailing on ice. Here the low friction against the ice means than one can use very flat sails, both when going upwind and downwind.

And extremely quickly, eons before anything else was doing apparent wind sailing ice boats were already doing well over 2times wind speed upwind/downwind when it started to become a thing for waterbourne craft