Photo credit: Benjamin Tillier on Elix F1X
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? Apparently not, then it’s only a “cosmetic” issue.
Fin whistle usually means that there's a slight imperfection in the foil:
Foil whistling happens when the point where the flows of water join at the back of the fin oscillates between two positions. This happens typically when the trailing edge is square and the flow reattaches on either side of it.
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.
Most people recommend sanding it to get rid of the sound. In that case you should use a very thin sand paper (800 or 1000 grit). You don’t want to damage your €2000 foil!
It’s also recommended to only sanding one side of the foil to create a (small) asymmetry of the trailing edge. It also works if you sand both sides, but you may have a very sharp edge, which could become dangerous!
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:
This is well explained on the drawing below, from "Principles of Yacht Design (3rd edition), by Lars Larsson and Rolf E. Eliasson"
You want a trailing edge with 0 in vibration amplitude.
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. It sounds like grandma's secret recipe, but it's worth trying!
To finish off, here a good example of whistling foil: