One of my favorite sounds, only happens when I am going flat out on my board. Haven't tried to fix it on my board as I actually like it and I'm not racing anyone.

I thought it was the trailing edge of the fin being to sharp and that was resonating causing the high pitch whistling. On catamaran foils and aircraft control surfaces I ensure the trailing edge has a flat on it to stop any whistling/vibration.

Guess I will go and google it, should be an interesting read,
cheers,
Darryn

Hi JoffaDan. Sharpening fins? I'd rather the harmonic whistling sound, kinda cool.
Honestly don't want to be a smart ar5e but after receiving a nasty chop myself, and reading the following article, I'd be leaving fin sharpening to the pros on the big juice.
www.seabreeze.com.au/News/Stand%20Up%20Paddle/The-fin-chop-from-hell-Kai-Lennys-surfing-accident_7918038.aspx
I take the sharp edges off my kid's board fins with a bit of wet and dry, would hate them to end up looking like me!

Can be coming more from fin positioning rather than side fin shape:

Whistle happens when fin positioning is across the main flow direction of the board, as a drag of high frequency when the planning reaction of board is more important than the fin foil reaction across the main flow.


I explain , except time it happens on short board going fast on extreme situation but in case of short board surfers use more fins areas than planning area of the tail shortboard , whistle did happen most of the time on big board like longboard or SUP.

Did happens to me longtime ago on 7'6" single fin but was obviously from a very very sharp fin. (got a large scare in my buttock from it)

Recently, i did wonder about it again when it happens on my 7'11" on rear fins of my quad set up made of TC aqualine . Those fins won't be bad construction . So This make thinking than positioning is the reason of whistling as well as trailing edge can be too.


Any feed back in this way

Whistles happens then 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.

The solution can be to have a sharp trailing edge (case 5 below), but in my opinion, it is too dangerous.

Better is to sand to round one side (case 7) so the one edge is favored to "anchor" the vortexes, trying to leave no cutting edge.





From Principles of Yacht Design (3rd edition), by Lars Larsson and Rolf E. Eliasson

Fin hum is a bummer; the vibrations cause drag, not to mention it's annoying as hell and a defect in a production fin. Take it back. I've tried refoiling hummers multiple times... not so easy.

the wise and knowledgeable colas strikes again - always come out smarter after reading your posts.