Been switching around fins on a quad lately, and today, in clean, head high surf, I heard a high pitched hum. I’ve only ever dealt with humming fins on glass-ons, so I was pretty amazed to hear it as all fins are fcs plugs… two FG5 FCS fins leading (the blue ones with the infinity sign on them), and two fiberglass speed dialer fins trailing (but I’m using the single foiled front fins as rears in the setup).
So… I’m only hearing the hum on my frontside bottom turns, and I’ve never heard the hum on any other setup on this board. Now I’m starting to think that there’s some kind of strange dynamic between the front and rear fins, where water’s not flowing properly from the fronts to the rears. But I’m only hearing it on the one side.
I turn the board over, and see a small, less than peas sized bead of wax on the inside of the tip of the toe side leading fin. It must have scraped some wax off another board I had it stacked on top of. I pull the little bugger off and never hear the hum again.
My question is… what was making the hum? The front fin due to irregular flow around the tip? Or the back fin, due to the fact that water was cavetating off the front fin and rattling the rear fin?
I would guess that maybe that little wax was enough to make that particular fin unstable, so that when the flow is coming at it at a certain angle and speed (when you are doing your frontside bottom turn) the fin starts to flutter, or vibrate. I believe that this can also be caused by a blunt trailing edge. I dont know that a little piece of wax could cause this, but maybe. If airplanes have the engines mounted on the wrong portion of the wing the wing can easily become unstable, it will just twist back and forth until they rip off. Maybe that little piece of wax caused something similar. One of my friends had some humming glass-ons and the glass at the base of the fins actually cracked.
It could have been caused by cavitation from the wax bead, Cavitation is caused by an abrupt pressure drop, which could have occurred across the trailing side of the wax bead. Sorry but it’s been too long since my fluid flow theory class to remember what the name of that pressure threshold is.
I plucked the tips of both fins, front and rear, like a string on a bass guitar. The sound the front fin made sounded more like the hum than the sound the back fin made.
The whistles are leading edge anomalies. The leading toe fin wax was probably the culprit - I’ve had this on glass on fins with a leading edge bit of resin. Can get pretty dern loud too. I think you can remove the rear fins, put the wax back on, and get the whistle just as loudly.
I’ve got a whistle from a fin I dropped on the pavement before installing. It’s got a couple nicks on the leading edge. Whistles, nice and hight pitched, but only when I’m going fast and turning hard. It’s my speed whistle, and I love it!