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  #21  
Old 10-01-2009
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Quote:
Originally Posted by RichH View Post
Crapaud has it.

The hum comes from the flex due to the viscous drag forces causing the thin plate centerboard to flex. If these forces excite the natural frequency of vibration (or a harmonic) of the keel/centerboard, then the board will vibrate quite excitedly.
I have a hard time imagining keels 'flexing'. Turbulant flow around or off the trailing edge of a foil yes. But having thousands of pounds of lead or cast iron 'flex', even under 15+ knots, I just don't see that happening.
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  #22  
Old 10-01-2009
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zz - the original poster implied/stated dinghies (usually with thin flat plate boards) such as 420s and lasers.

Nontheless, EVERY solid is by definition elastic to some degree and will vibrate when induced at its natural frequency or a harmonic of the natural frequency of the object. The 'flex' (amplitude of vibration) doesnt have to be all that great as the 'heard sound' may be further amplified by the adjacent shapes/geometries .... like the 'sound box' around which a guitar, etc. is built. It can also happen because the turbulance @ the edge/tip is in harmonic sync with the natural frequency of the object thats vibrating ... many many causes for a solid object to 'vibrate'/resonate, I only listed the principal and most common causes.
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  #23  
Old 10-01-2009
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Quote:
Originally Posted by zz4gta View Post
I have a hard time imagining keels 'flexing'. Turbulant flow around or off the trailing edge of a foil yes. But having thousands of pounds of lead or cast iron 'flex', even under 15+ knots, I just don't see that happening.
When I was in the Marine Corps, our Boeing rep told me a story about the company the developed the torque gauges for the H-46 helicopter. The torque is determined by measuring the twist in a 1.5 inch solid shaft over a span of about 8 inches. When the Boeing rep toured their facility there was a 10 inch thick piece of marble with sensors on it. They could measure the deflection in that marble when he set his coffee cup on it. Everything flexes. When the frequency of the flex matches the items natural resonance, the flex compounds itself, and causes a hum.
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  #24  
Old 10-01-2009
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My limited knowledge of physics says that pretty much everything has a resonant frequency. When something passes over the surface of an object at a specific speed it will cause that object to resonate. The hum you're hearing is the result of that resonance.


Of course I'm probably WAAY off but at least it sounds good
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Old 10-01-2009
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That hum can be reproduced just at the docks. It usually means some aspect of the rigging is tighter than the rest and as the result - the mast vibrates while being more tightly secured in some areas than the others. You'll never hear the mast vibrate it will be channeled through the tightest or loosest part of the rigging just depends on direction of pressure. Usually it will be the loosest but not always.

It also indicates that you are not dialed in and there is too much pressure exerted on the mast and all its components in one particular area. Has nothing to do with the keel unless you have a centerboard arrangement. The singing keel is different than singing rigging.

In instance of example - my roller furler is sometimes a bit with slop - but where moored - the wind blows into the furler putting pressure on it. The running backs will start to hum unless I tighten them and thus tighten the mast and forestay...


Humming / singing is an indication that you are not dialed in with your sailing plan and thus not getting the most efficiency out of it... IMHO...
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  #26  
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Quote:
Originally Posted by RichH View Post
zz - the original poster implied/stated dinghies (usually with thin flat plate boards) such as 420s and lasers.

Nontheless, EVERY solid is by definition elastic to some degree and will vibrate when induced at its natural frequency or a harmonic of the natural frequency of the object. The 'flex' (amplitude of vibration) doesnt have to be all that great as the 'heard sound' may be further amplified by the adjacent shapes/geometries .... like the 'sound box' around which a guitar, etc. is built. It can also happen because the turbulance @ the edge/tip is in harmonic sync with the natural frequency of the object thats vibrating ... many many causes for a solid object to 'vibrate'/resonate, I only listed the principal and most common causes.
I agree, everything does flex to some extent. However, I think that the finish on the foils itself and the trailing edges (non-laminar flow) will cause the hum before a "flex" or vibration hits it's natural harmonic frequency. Or am I looking at this completely wrong? Not trying to be a smart ass, just trying to learn.
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Quote:
Originally Posted by zz4gta View Post
I agree, everything does flex to some extent. However, I think that the finish on the foils itself and the trailing edges (non-laminar flow) will cause the hum before a "flex" or vibration hits it's natural harmonic frequency. Or am I looking at this completely wrong? Not trying to be a smart ass, just trying to learn.
The non-laminar flow is what is causing the flex. As the eddys exert a small amount of pressure on one side then the other, you get a small amount of flex either direction. As the frequency of the flex reaches the harmonic frequency, you get a hum. The water doesn't hum, just like the wind doesn't hum or whistle. The finish of the foil and the shape of the trailing edge (and the rest of the foil) will effect the laminar flow, and the resultant eddys.
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Hum vs. Speed?

Quote:
Originally Posted by zz4gta View Post
I agree, everything does flex to some extent. However, I think that the finish on the foils itself and the trailing edges (non-laminar flow) will cause the hum before a "flex" or vibration hits it's natural harmonic frequency.
If the turbulence occurs at the trailing edge, it should not cause vibration or hum - unless the turbulence wash from the keel is effecting the rudder. I would think at the higher speeds, the fluid flow is separating (Transitioning from laminar to turbulent) slightly behind the widest part of the foil. This would then effect the trailing & thinnest section of the foil - and noise (hum) could be generated.

I think you are correct in saying the surface finish is a factor, but it helps promote the vibration not the reason for it. Because of the shear stress the hull, keel and/or rudder generates while moving through the water - any disruptions on the surface finish will cause the flow to transition - resulting in the eddies and pressure variations that cause the vibration.

The word "flex" might be misleading - think of it more like the continuous shifting from tension to compression.

I do believe the vibration can and does promote fatigue in components (how much - is debatable), but does it effect speed? When I think of vibrations as mechanical energy lost into sound waves - the thought is that it could slow a boat down. Another line of thinking is it reducing the effects of friction - like the old football game with the figures moving on the vibrating game board. Any feeling one way or the other? Not that I an suggesting adding vibration generators to sail faster. . .if that be the case.

JP

Last edited by Crapaud; 10-01-2009 at 11:08 PM.
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Old 10-01-2009
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Quote:
Originally Posted by Crapaud View Post
If the turbulence occurs at the trailing edge, it should not cause vibration or hum - unless the turbulence wash from the keel is effecting the rudder. I would think at the higher speeds, the fluid flow is separating (Transitioning from laminar to turbulent) slightly behind the widest part of the foil. This would then effect the trailing & thinnest section of the foil - and noise (hum) could be generated.

I think you are correct in saying the surface finish is a factor, but it helps promote the vibration not the reason for it. Because of the shear stress the hull, keel and/or rudder generates while moving through the water - any disruptions on the surface finish will cause the flow to transition - resulting in the eddies and pressure variations that cause the vibration.

The word "flex" might be misleading - think of it more like the continuous shifting from tension to compression.

I do believe the vibration can and does promote fatigue in components (how much - is debatable), but does it effect speed? When I think of vibrations as mechanical energy lost into sound waves - the thought is that it could slow a boat down. Another line of thinking is it reducing the effects of friction - like the old football game with the figures moving on the vibrating game board. Any feeling one way or the other? Not that I an suggesting adding vibration generators to sail faster. . .if that be the case.

JP
Humming is always bad - it indicates part of your rigging is not in tune and too much slack... just that simple. You can't hum a guitar string if tuned correctly nor should your rigging hum unless you want to be the next sailboat musical sensation..
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  #30  
Old 10-01-2009
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Quote:
Originally Posted by artbyjody View Post
Humming is always bad - it indicates part of your rigging is not in tune and too much slack... just that simple. You can't hum a guitar string if tuned correctly nor should your rigging hum unless you want to be the next sailboat musical sensation..
I think we're talking mainly about keel or rudder hum at high speed. This is completely different than the rig humming.
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