My guess would be there is virtually no effect.
Look at the separation between the wind turbines in the wind field.
At what one of those units cost, I'm pretty sure that they place them so
the upwind turbine does not negatively effect any turbines downwind.
So, the distance between them is probably greater than some calculated or
actually physically tested distance at which the upwind turbine affects the
efficiency and output of the downwind turbines.
You could do your own test...... take a hand held wind meter and a GPS (for elevation/
altitude) and take some readings upwind a mile and downwind a mile as see if there is any measureable difference.
If the turbines are on the top of a hill (which they often are) and your sailing site is lower, I fail to see how there could be any effect at all.
One of the windiest spots in the USA (Cabezon Pass (The Ponds near Palm Springs)
had us windsurfing on small narrow "Perk ponds" right in the middle of a wind farm.
Didn't see any effect there, and that was when "wind farming" in the USA was very much in it's infancy.