For me it is 8-9 knots measured on the shoreline with a calibrated anemometer with no immediate obstructions to back up the wind (tall trees right on a shoreline will reduce the windspeed measured on the shore line). 8-9 knot gusts makes a light ripple on the water with no white caps. Now their maybe 1 second of wind in the 10 knot range at the front of the gust, but that passes me by before I get up. Steady 8-9 knots allows me to foil both ways, if it is a little less will only be able to foil one way, downwind going with the minor waves. AFS Wind95 foil with F1080 cm2 wing, Goya Bolt 135 board, Aerotech Freespeed 7.2 sail, weigh 84 kg.