All About Ventilation

Ventilation! The foe of any foiler. Fast, mysterious, and sudden; it catches even the best of us, but with a little knowledge, skill, and maybe some time with the sander we can learn to harness and even use it to our advantage!

The Action: 

Ventilation is a wing sucking air over the low pressure side, resulting in a loss of lift. most of the time in foiling, this happens in a turn or pumping when the wingtip breaks the water's surface. On most foils only one side of the wing will ventilate, creating an imbalance in spanwise lift and throwing you off your board. 

Mast ventilation is the hardest to identify and catch, kinda feels like you ran into a plastic bag. If you are sure its not the wingtip, it could be the mast.

If you're crashing kicking out, in any turn, or going upwind winging ventilation is probably the cause.

The Fix:

Ventilation problems are mainly caused by 3 things: 

-Wingtip breach angle

     In my experience, most ventilation can be eliminating by making the angle between the wingtip and surface of the water more perpendicular.  Winglets and flat tips are very good at preventing ventilation, while downcurved or hooked winglets suffer. On foil, this angle can be changed by the addition of a winglet, or by chopping off some downcurved portion.

-Airfoil

     The airfoil is another extremely important factor in ventilation performance, especially on masts. airfoils with a sharp leading edge or more even velocity distribution are very forgiving and can even prevent ventilation on a compromised design

-Surface roughness

     Smooth is fast, smooth is forgiving. this is another big one for the mast. Dings, scratches, and even graphics or stickers on the leading edge can create separation that will trap and allow air to spread. Sanding with at least 240 grit paper can help. 

-Tip loading

     High tip loading because of a bad lift distribution can make ventilation a big problem. if a designer doesn't properly adjust the lift distribution when changing sweep, twist, or planform the wing will have ventilation along with efficiency issues.