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harmonic ressonace

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dog
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harmonic ressonace

Postby dog » Fri Apr 25, 2003 1:50 am

just desgined new 4" wave blades for my mutant and noticed a harmonic ressonance when reaching high speed does this mean the fins are ineffeciant?

Ari Raptis
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Postby Ari Raptis » Fri Apr 25, 2003 3:25 am

Yes.

They are vibrating because they are not true.This will cause such uneven pressure,they are sure to cavitate.

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DibutilFtalat
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Postby DibutilFtalat » Fri Apr 25, 2003 3:53 am

Yes: they are inefficient in the sence of lift to drag relation - you are wasitng energy to generate the sound.

No: vibration might provoke cavitation but at those speeds is unlikely the reason of it.

You can change things in several ways.

1. Change (decrease) Q-quality of the resonator. For that you can coat the fins with rubberizing compoud or several layers of thick polyurethane laquer and/or rubberize the sockets. This will dramatically decrease the amplitude of vibrations.

2. Change the resonance frequency - you might be just unlucky and once you change it the initiating pulsations will no loger produce resonance. For that you modify weight and/or rigidity of the attachment.

3. Modify the profile of the fins - a tiny displacement of the profile maxixum along the chord might be sufficient.

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Postby Nico » Fri Apr 25, 2003 4:10 am

Hi Dog,
all of the above might help, but to start off just sand down slightly the trailing edge of the fins, it's probably just one, and you will have solved the problem.
Nico

dog
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harmonic ressonace

Postby dog » Fri Apr 25, 2003 4:55 am

thanks for the info :thumb:

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tweekster
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Postby tweekster » Fri Apr 25, 2003 4:56 am

i agree with nico. happens alot with surfboard fins. it is caused by uneven thickness in the trailing edge. at the point where the edge transitions from thin to thick the material is vibrating. that is the humming you hear. sand the trailing edges down a little at a time until the humming stops. like nico said, its more than likely only one fin.


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