At last, I have found a good video explaining how to suppress the vibration of fins and foils (aka the Von Karman effect), and giving the name of this technique : the Donaldson trailing edge, described in 1956 by ... Donaldson
Up to now, I had only seen the diagrams for symmetrical foils but this video goes into the details and show that you must sand the inner face of asymmetrical foil (flat one), not the curved one.
Thanks to Mario Legenstein from Levitaz for the video:
You can google "Donaldson trailing edge" to find many detailed references. For instance these images show the alternating vortexs creating the vibrations on a blunt edge on the left, and how the Donaldson edge locks the vortex in place on the right:
But this is usually only when taking the drop down head high or bigger waves. It eases off to nothing after levelling off & riding the swell. So it's not annoying to me & is a good indicator to know when I'm in the high speed band.
