I just made a nice quad with glass on fins that hums a bit when it goes in a straight line. I’ve been searching the archives and there seem to be two strongly held schools of thought on this matter. One group swears black and blue that you need to sharpen the trailing edge of the fins to stop the humming. Another group says the exact opposite; i.e. you need to blunt the trailing edge. They can’t both be right can they?
In case anyone is interested the fins I used were pretty old and battered and had been in a couple of other boards over the years. My gut feeling is that the foiling is a bit off.
Spuds, I too have had this problem many times and after years of testing different ideas and searching for insight on hydrodynamic sites I recently remembered something Bill Thrailkill said many years ago and it still holds true today.
He said that fins hum because they don’t know the words.
(Not a help at all, but it’s a surfing ‘dad joke’ I couldn’t resist.)
I have always thought it was because foil is a bit “off”. Blunting trailing edge or sharpening, either could even out the foil depending on what is there to start with. So like Cert’s, “breath or candy?”, both are right
It would be a good first step to hand sand the fins overall with 180 and then finish with 220. Sand by hand and go lightly on the edges. See what that does before you start any grand experiments.
Gene is 100% correct. A common cause of fin hum is often a bead of resin that was left when the fiinish coat was brushed on. Like a paint drip/run on a vertical surface. But generally speaking it’s due to a blunt trailing edge.
common foiling in naval industry is to choose the profile of the keel or rudder 5% longer and cut that 5% off, so the trailing edge is square. you cant make make trailing edge completely sharp, when I go down to 0.5mm on my carbon fins, it beginns to be seriously dangerous.
Even a subtle bead is a big deal. Dragging a single edge razor repeatedly on both sides of the trailing edge is my preferred method to sharpen and totally flatten the bead. Then a little polish by hand if it’s a G&P.
The great majority of humming fin corrections, that I’ve done, involved tuning the trailing edge. Fin flex is another culprit, that slows a board, but is such a low frequency that you can’t hear it. Sometimes, it can be felt through the back foot, as the board slows down, but even that is rare.
Thanks for the input everybody. The rear fins are pretty thin so I reckon I’ll start by beefing them up a bit. it’s interesting to see the two firmly held positions I was talking about in evidence here.
Agree with the naval guy…Square off the trailing edge just a wee bit with a flat sanding block I’ve read the scientufic reason why it sets up a harmonic and it made a lot of sense but only remember the practical bit which was square it off to stop it.
It can have many reasons why fins hum.
Long story short is that you just got unlucky you’re hitting the resonance frequency.
Any modification you do has the potential to fix it.
I had a killer 7-4 years ago that I rode a lot at a grinding reef break over here. I had checked all my edges (3 glass ons), and couldn’t find the spot. I finally found a micro pit on the flat inside of a front fin, a small little air pit in the fin lam., sanded it out, and solved the hum problem.
So what is the problem with a fin hum?
When I first had a board with a fin hum, a 9’0’’ Nigel Beckham longboard, I enjoyed the hum. It only happened when I was going really fast (by my beginner standards back then).
I think Hans is spot-on with his explanation of hitting the resonance frequency.
So what actually happens when a fin starts to vibrate at it’s resonance frequency? Does it change drag or lift in any significant way?
It could be a fun project to make fins that produce particularly loud and / or harmonic hums.
Hey welcome back Rick, I still have an awesome board you made many years ago, I picked it up from Ambrose while on Kauai… thanks for the pointer, have had fin hum (on a Brewer 8’4") that I could not kill off, never thought to check the leading edge. Got so annoyed I ground all the fins off and installed boxes. Ps fin placement on that humming board was way off too.
The hum you hear, is the signal that something ain’t right. It is irritating, at best. The subsonic hum, (low frequency vibration) that you don’t hear, is the real culprit, that can/will slow your board in a dramatic way. So, yes, it really does create significant drag on your board. I learned this by direct experience, with the only board I ever used a flex fin on. So, how did I know about a hum I couldn’t hear, you ask? As the board slowed dramatically, I felt the low frequency vibration through my back foot, is the answer.
I blunted the trailing edge of the fins a bit on the board in question. Yesterday I took it out in reasonably solid down the line surf. Still humming like a humming bird on crack when I hit top speed. I noticed a significant drop in speed with the humming. The humming stops when I either slow down or turn.
As I mentioned earlier the fins have been glassed into a couple of other boards over the years and were fairly chewed when I glassed them into this board. I reckon I’ll beef them up a bit with five or six layers of glass, then re-foil them in-situ. Will report back.
