i just got a new 10ft longboard with a 10"glassed on fin.
the fin humms.does anyone know why.
and dont say…because it doesnt know the words.
my friend thinks it may be crooked.it doesnt look it.
Howzit popey, the most common cause of fin humming is a resin bead on the traiing edge of the fin. Check for this and if there is a bead just sand it off. Aloha,Kokua
No, It looks smooth.
i had an old board that buzzed alittle.and i think that fin was broke off,and put back on.But this is new,and from a good company.this one buzzes alot.
I’m with Kokua. Generally a fin hums when something is wrong with the foiling. Check the front edge and trailing edge. Are both foiled correctly up and down the fin? I have seen many glassed on fins crooked but usually it takes quite a difference to get a fin to hum…
I’m also with kokua. Humming typically results from the shedding of eddies from trailing edge of a blunt body (in this case, the trailing edge of the fin). A bead on the trailing edge is a good generator of such eddies, but eddy shedding is also possible from a rounded trailing edge, or one with a squared-off trailing edge, so some experimentation with trailing edge shape or thickness may be necessary.
The eddies that are shed do so alternating from first one side of the fin, then the other (forming what is called a “Karman vortex sheet”). As each eddy is shed, it changes the pressure exerted on the trailing edge of each side of the fin in an oscillating fashion. Humming occurs when the frequency of shedding of the eddies (governed by the velocity of the flow, and a characteristic dimension of the body–such as the thickness) matches the natural frequency of vibration of the fin (i.e. the system is in resonance). However these eddies are only shed in this fashion for a specific range of Reynolds Numbers–hence the sudden onset of humming with increasing speed, and the possible abrupt cessation of humming at an even higher speed, can be the result of entering and exiting this range of Reynolds numbers, or because the eddy shedding frequency no longer matches (or nearly matches) the resonant frequency of the fin.
mtb
Quote:
I’m also with kokua. Humming typically results from the shedding of eddies from trailing edge of a blunt body (in this case, the trailing edge of the fin). A bead on the trailing edge is a good generator of such eddies, but eddy shedding is also possible from a rounded trailing edge, or one with a squared-off trailing edge, so some experimentation with trailing edge shape or thickness may be necessary.
The eddies that are shed do so alternating from first one side of the fin, then the other (forming what is called a “Karman vortex sheet”). As each eddy is shed, it changes the pressure exerted on the trailing edge of each side of the fin in an oscillating fashion. Humming occurs when the frequency of shedding of the eddies (governed by the velocity of the flow, and a characteristic dimension of the body–such as the thickness) matches the natural frequency of vibration of the fin (i.e. the system is in resonance). However these eddies are only shed in this fashion for a specific range of Reynolds Numbers–hence the sudden onset of humming with increasing speed, and the possible abrupt cessation of humming at an even higher speed, can be the result of entering and exiting this range of Reynolds numbers, or because the eddy shedding frequency no longer matches (or nearly matches) the resonant frequency of the fin.
mtb
Woooo! I’m gonna read that again! That’s awesome! “Eddy shedding frequency”!! “Karman Vortex Sheet”!!! “Humming occurs when the frequency of the shedding of the eddies”!!! WOOO! “The sudden onset of humming at high speeeeed,” man!!! Awesome. Dude look at the eddies shedding off this Reynolds Number!!! Physics R rad!!
(It’s cool when cool people are smart. I’m serious.)
Popey,
I suspect that you have several things going on with your fin. The leading and trailing edges may well need attention as suggested. However I think the major contributer to your hum is that the fin at 10" deep, is not sufficiently rigid ,and is vibrating in response to unequal pressure loading. More glass and thickness with a more carefull foiling will solve the problem.
I noticed a humming on my board a few weeks ago surfing at sunset cliffs. Bigger waves and more speed. There is nothing noticeably wrong with the fin. The board is relatively new cooper fish hornet 9’10".
My best board humms when it gets head high. Late backside drops and the board is singing ! I love it…
Our crew back in the day had a board that hummed. Called it “the Hummingboard”; hummed so much, it massaged your feet. Then one day the hum turned into a flutter and the fin got tore off. We didn’t know why it hummed so it never got reproduced.
It may be possible to “tune” a board to a note, like C-sharp…
As has been described, the problem could be the fin’s trailing edge. It’s also possible that the fin could have
a subtle twist. I’ve found that the longer, thinner, more flexible a fin (especially for use at higher speeds), the more precise the taper, foil and finish. I’ve had high aspect ratio fins that looked fine, but still hummed above
a certain speed… the answer was adding a few thin laminations to each side, resulting in a slightly thicker, firmer shape.
Yes, a change in thickness will change the parameters for the von Karman Vortex street. Another solution is to introduce a “trip”. In this case a trip would be a clear resin pinline that runs the length of the fin at least 3/4" in from the leading edge.
Also, as mentioned sanding the trailing edge of the fin so there is no drip bead or square rear edge will sometimes work. Sanding the rear edge with approx 45 degree bevel from both the inside and outside of the fin will eliminate the resonant shedding (however, this will not eliminate any anomalies upstream of the trailing edge).
Kind of off topic but has anyone ever tried this, a bit like the beadline, it might also solve the humming.
http://perso.wanadoo.fr/scherrer/matthieu/aero/nimbus4e.html
Quote:
Yes, a change in thickness will change the parameters for the von Karman Vortex street. Another solution is to introduce a “trip”. In this case a trip would be a clear resin pinline that runs the length of the fin at least 3/4" in from the leading edge.
Also, as mentioned sanding the trailing edge of the fin so there is no drip bead or square rear edge will sometimes work. Sanding the rear edge with approx 45 degree bevel from both the inside and outside of the fin will eliminate the resonant shedding (however, this will not eliminate any anomalies upstream of the trailing edge).
Yes, I stand corrected, it is “street” (and in fact I typed that first, then decided that “sheet” made more sense and didn’t bother to check). Adding layers of glass (thickness) not only changes the parameters for the Karman Vortex Street (lowering the shedding frequency), but also the natural frequency of the fin itself (increasing the natural frequency) thus moving the two off resonance (although they could still resonate at a different speed if within the appropriate range of Reynolds Number).
FWIW–if anybody is interested–the range of Reynolds Number in which this type of shedding occurs around a circular cylinder is from about 60 to 5000. It is eddy shedding within this range of values that sometimes causes telephone or power lines to “sing” in the wind (“aeolian tones”). These vibrations can cause work hardening of the metal and subsequent failure of the wire. To avoid this, small weights are commonly attached to the wire (they look like a small dumbell, about 1’ long, that hang just below–a few inches–and parallel to the wire) to alter (lower) the resonant frequency of the wire. Although plastics/fiberglass do not work harden, per se, I suppose that the analogous vibrations of the fin that PlusOneShaper mentioned breaking off could have contributed to the failure.
At one time the UCSD sailing club had a sailboat (capable of planing) with a rudder that hummed just about the time the boat would get onto plane. As a gust would hit, and the boat would start to accelerate, the hum would suddenly begin at a low frequency and then build in pitch as the boat continued to speed up. As the pitch got higher, the sound (and vibrations) would suddenly quit and everything would be quiet again–giving the impression that one had just broken the sound barrier.
mtb
I’ve never tried that but I will; I’ll let you know how it works…that’s an innovative solution…
Very cool description of “breaking the sound barrier” and going into “smooth sailing” mode.
Must have been a fun boat almost with a personality of sorts.
Really like the pinking shears being taken to the tape. Wow, you do realize that one could
take a 1" wide piece of Scotch 233 and split it down the middle and lay the two pieces
1/8" apart. Then brush a resin zig-zag “pinline” but don’t pull the tape until fully kicked.
This would leave a crisp-edged trip (turbulator). It could be placed on the fins or along
the bottom of a board. Reduction in drag should be quite noticible.
MTB: what would the length of laminar flow be along the bottom (curved plate) of a board in
cruise mode. Say, wave speed 12mph (fps?) with rider translating at 45 deg (x 1.41) in
salt water? (Unfortunately, I no longer have my fluids texts).
The water entry point for a typical board could be empirically located and a turbulator line
could be placed at the estimated start of the turbulent flow regime. This could be repeated
down the board if necessary…
…potentially could turn a “bad” board with “not-so-good” bottom countours/rocker/flex into
a slippery wonder!
I’ve been on a boat with a humming rudder too. Must be fairly common.
This afternoon I tried the tape turbulator on a board that has bad fin hum (not one I made, I’m pleased to add!). It did NOT work. Maybe with resin it would, but just the tape didn’t do a darn thing…fin hum just as strong as ever…
While I haven’t experienced fin hum for a few years, on the older longboards we used to correct the problem by carefully resanding the foil with a hand block and say, 60 grit paper. I considered it to be the foil being thicker towards the tip than lower down towards the fin base.
Keith, bummer the tape didn’t work. My bet is that the “trip” would have to be several layers of tape thick or it needs to be positioned further aft. I did it with resin because it would be robust yet I also wanted to get a sharp edge. I thought if it didn’t work then I would have to sand it off and re-position it. I got lucky and it worked the first try. I made a clear 1/8" resin pinline using 5 layers of tape and pulled the tape well after the resin kicked.
Really curious what a zig-zag would do, if anyone is in a situation to give it a try…
Aloha all, howzit!!! I’ve been making fins of all kinds ever since I started making boards way back. Hundreds, thousands even. Typically, fin edges, front or back, are the cause of the demon shriek, like everyone has already mentioned. A few years ago, I had a race 7-4 that I used at one of our more rad southside reefs. Had a fin hum that I couldn’t figure out, checked every edge repeatedly, couldn’t find it. One day, I found a small puka on the inside of a front fin, just big enough to catch the water flow and make noise. I sanded it out and noise no more. Something else to consider. Mahalo…RH