You can increase the resonance frequency by stiffening the fin.
e.g. by adding an extra layer of glass or even carbon.
I vote for softening the leading edge…call me crazy if you like.
I beefed up all four fins with six layers of glass. Took it out for a spin today and no problems at all. I guess that shows that thicker fins are less likely to hum.
I’m a big fan of thick fins but to say they’re less likely to hum based on the work you did might be jumping to a conclusion. It would stand to reason that you refoiled and sharpened after the glass was added. So there’re two possible conclusions, or a combination of the two, the sum of the parts. Either way, you ended up with a foil that didn’t hum and glad it was resolved.
… he changed the stiffness of the fin by adding glass, which changes the resonance frequency which is all you need to stop the hum.
So three factors were changed and could have contributed.
Yeah, I guess you’ve summed up the humming fins problem pretty well; i.e. it’s very difficult to figure out what’s causing it. I tried both sharpening and blunting the fins with no luck, so in the end I just added the glass. As I mentioned at the start the fins had been in about three different boards over about fifteen years so they were pretty thrashed to begin with. Lots of rocks where I live. They look pretty sweet now though.
On a related note, I make my own fins now (though not the four in question) and I must say for me the most difficult to foil are the centre fins. It’s quite easy to end up with a bit of a wobble. Side fins are easier since they have a flat surface to use as a reference point.
Never had a removable fin hum (glass, so-called glass flex, etc). But I have had several glass-on fins hum. Wish I knew why.
Hans, in my experience, the fins hummed only when hitting a higher speed. Your resonance comment make a lot of sense to me.
My understanding is that fin hum is the same thing as wing flutter in aviation.
https://en.wikipedia.org/wiki/Aeroelasticity#Flutter
Flutter happens typically at high speeds and is very dangerous for aeroplanes as they can cause the wing to fail (break off). One of the last tests flights done with new aeroplanes is the “flutter test” and it’s one of the most dangerous tests.
Stiffness is a very important parameter to change flutter characteristics.
Flutter is a complex feedback system between a structural responce to the fluid dynamic forces it doesn’t have to occur at a structural harmonic frequency:
With flutter the forces generated by the fluid have a tendency to twist the foil; as the foil twists it will reduce the effective angle of attach and suddenly unload the foil, as soon as it returns to it’s original geometry the fluid dynamic forces will regenerate and the foil will again twist until it unloads.
Hum could also be caused by trailing edge vortices; keels and rudders of race yachts typically have a square or angle trailing edge 3-5mm thick.
Hum could equally be caused by the fluid flow detatching at the leading edge if there are any imperections or if the edge radius is too small.
Another possible reason would be detatchment and reattachment of the flow mid chord (a bit like the buffeting when you only have one car window down).
Adding thickness would increase the stiffness and hence structural responce but also change the foil, assuming the job was nicly sanded it could be acase of all of the above?
And I was afraid I would lose the reader 
You’re definitely right, there are a lot of effects into play.
Actually the message I want bring, is that fin hum is a vibration carried from the flow to the board through the fin. Changing the fin stiffness changes the frequencies that are propagated or filtered. Assuming your fin is properly finished, stiffness is your best bet at fighting fin hum. But like you correctly highlight, the source of the vibration remains largely unaffected.
BTW did you notice finfoil creates an angled trailing edge with a configurable thickness?
“BTW did you notice finfoil creates an angled trailing edge with a configurable thickness?”
I’m not sure I understand what you mean by that.
Meagain said the following:
“Hum could also be caused by trailing edge vortices; keels and rudders of race yachts typically have a square or angle trailing edge 3-5mm thick.”
This image illustrates the angled (almost square) trailing edge:
http://www.swaylocks.com/sites/default/files/test_111_minthick_asym.jpg
finFoils minimum thickness function is also useful when printing fins standing on trailing edge: Set it to 1.3mm, and the supports can be printed 0.8mm wide. Enough difference in strength to allow snapping off the supports without breaking the trailing edge.
My one data point doesn’t confirm a theory, but last time I had fin humming, the only thing i did to fix it was to square off the trailing edge just a little. At the time I wasn’t interested in the “why”, just happy it worked.
All the best