Have you ever experienced hearing a harmonic whistling sound when you get a bit of speed on your foil? I have.
Personally, it doesn’t bother me that much, but the question is if this has a consequence on the performance of the foil?
My research has shown that a whistling foil is mostly a “cosmetic” issue. It doesn’t mean your foil is damaged to the point of harming performance, but it does usually mean there is a slight imperfection in the foil.
Foil whistling happens when the water flowing above and below the wing meet at the back edge of the wing, and oscillate between one another. This typically happens when the trailing edge of the wing is squared, and not smooth.
It makes the foil vibrate when the board is going fast enough. It’s an acoustic phenomenon, like the reed vibrating on the mouthpiece of a saxophone.
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Most people recommend sanding it to get rid of the sound. In that case, you should use very fine sandpaper (800 or 1000 grit). You don’t want to damage your expensive foil!
When sanding, you can either focus on just sanding down one side of the wing or sanding both sides to a fine point, just be careful with how sharp the foil will be in that instance.
The diagram below from “Principles of Yacht Design” (3rd edition), by Lars Larsson and Rolf E. Eliasson shows the “vibration amplitude” that occurs with the shape of each kind of trailing edge. The lower the number, the less vibration you’ll have which means no more whistling.
Some people have also managed to get rid of the whistling noise by applying a 1 cm wide sticker all the way down on one side of the trailing edge.
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Neilpryde has had some issues with their alloy foil, which is known to make some noise. Here is a small video showing how to use the sand paper on their foil:
To finish off, here a good example of whistling foil: