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  #11  
Old 09-30-2009
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Quote:
Originally Posted by stpetersburgsailor View Post
sanding with grit! gee I'm hopin' he was being comedic...
I've sailed BRAND new high performance skiffs & if u took a piece of sandpaper within 20 yards ...
-JD
No I wasn't. How exactly do you think they get a mirror finish on racing boats? They wet sand the bottom to 800 or 1000 grit. Most only take it to 400-600.
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  #12  
Old 09-30-2009
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Quote:
Originally Posted by Crapaud View Post
In my experience, the hum is a function of speed and exciting the natural frequency of the rudder, keel or combination of the two. My "Fiz-ix" knowledge dealing with fluid flow is rusty at best, but to add to John's comments it is a vibration brought on by flow inconsistencies.

One of my thoughts has been - could this vibration be a result of flow separation (or eddies) coming off the keel that the rudder then contacts and starts the process? Almost like a flag flapping in the wind generated by a fan?

The nat frequency is determined by the object's geometry and stiffness. So, the frequency would be changed by changing the length of the rudder or the materials it is made from. Stopping the eddies completely would probably require NASA to design and build the foils to have no flow separations at the higher cruising speeds.
NASA does design a lot of foils, and the NACA (the predecessor to NASA) foil sections that most boats use were designed by the same smart guys. Just like a boat, every foil is a compromise. Lift and drag are big considerations, but so is flow separation at the expected top speed. No foil is going to be perfect for all conditions, so they pick the one that works best in the expected range of conditions. My boat uses an 0012 foil for the keel, and my friends Ranger uses a 63009 foil.

All of our keels would be much more efficient if we could only get the lead to change shape from one tack to the other.
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  #13  
Old 09-30-2009
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Gemini

this beauty designed by CCS has twin leeboards for just that John
great post, thanks I enjoyed reading it
'sqawk' to the bird' is he on board all the time w/u?
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Old 09-30-2009
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My experience with the vibration has been a flow noise common with keels, daggerboards or centerboards that have flat instead of foil crossection. It's usually noticeable at higher speeds so I've seen it in racing dinghys and a fix there is to reshape the board or if that's not possible tightening up the fit in the trunk. Vibrations hurt boat speed. The March Practical Boat Owner ( a British sailing magazine) had an substantial article on the problem and how to streamline bilge plates to cure it. They attribute vibrations to rounded leading and trailing edges for the board or keel which causes unnatural flow which causes turbulence and hence vibration.
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Old 09-30-2009
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Originally Posted by stpetersburgsailor View Post
'sqawk' to the bird' is he on board all the time w/u?
No he only makes it to the lake on rare occasions. Hard to believe, but he gets seasick if I take him out on the boat. At the clubhouse he's everyone's new best friend whether they want to or not. Very few parrots are too friendly, but he's one of them. He will latch onto someone's hand or shoulder and insist on being petted, then bite at me if I try to take him away from them. They aren't called "Psycho Chickens" for nothing.
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  #16  
Old 09-30-2009
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I am not going to get into changing the shape of my rudders, or keels. So I will just keep at 18 knots, and less. .......i2f
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Old 09-30-2009
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Quote:
Originally Posted by US27inKS View Post
My boat uses an 0012 foil for the keel, and my friends Ranger uses a 63009 foil.

All of our keels would be much more efficient if we could only get the lead to change shape from one tack to the other.
Interesting. . .

I am guessing the foil # has a meaning - do you know what the numerical code for the foil is?

Many moons ago, I was part of a research design team that looked at resonating frequencies to control eddies in order to increase fuel atomization. Before I left, another project had been submitted by NASA to reduce noise generated by airplane prop blades. The idea was to reduce the effects of the eddies coming off the leading blade so when contacted by the trailing blade the noise would be reduced. . .theory. One idea was to introduce trailing "streamers" that would break-down big swirls into little swirls to reduce impact on the trailing blade, but induce more drag.

I have only experienced the hum on dingys, racing cats and high performance monos - never a full keel. Is this a case of the full keel not being capable of reaching speeds high enough or the fact that the distance between the rudder an keel is nil or the flow is interrupted by a prop? Would a folding prop give different results? I have never been on a boat with a folding prop.
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Old 09-30-2009
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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.

Scows with very thin bilge board hummmmmm easily and regularly when at certain high speeds, if you pull the board ever so slightly up (slightly changing the natural frequency of the board) the hummmmmmm will occur at a different speed. Ditto when a mast starts to pump at relatively high windspeeds. Ditto why your diesel engine 'develops the shakes' at a certain specific rpm.

All solids will have a specific natural frequency of oscilation for their 'geometry' .... excite that item at its natural frequency and listen to the hummmmmm.

The same is what makes a violin string vibrate when rubbed with a rosined bow.
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Old 10-01-2009
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Dear Rich,
I believe you are perfectly correct in what you are saying.

However, I am not talking about a vibrating harmonic frequency;
Therefore, your hum and mine are different. Rushcutter (a KP46), hums on a beam reach and slightly forward of beam in 20-25kn of breeze and we are hooting along at a few kn greater than hull speed. No prop noise (folding) and the girl just sings. Cant explain or describe it.
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  #20  
Old 10-01-2009
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Quote:
Originally Posted by Crapaud View Post
Interesting. . .

I am guessing the foil # has a meaning - do you know what the numerical code for the foil is?
The foil numbers do have meaning, although I'm sure they mean more than I understand. Foils are broken down into 4, 5 and 6 digit foil numbers. The 4 digit foil numbers are very basic foils, I'm guessing developed first. Foils developed later have a "series" number that likely has something to do with a percentage or ratio, but I don't know what. The last part of the foil number is a ratio of chord length vs width. My keel has a 12% ratio, my friends ranger has a 9% ratio. Every foil profile has a slightly different lift to drag ratio, and will stall at a different angle of attack. My friends keel will have a slightly lower drag due to the finer entry, but will stall at a lower angle of attack than mine. The Ranger uses a rudder profile that stalls at a much higher angle of attack than the keel.

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