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      A Few Simple Rules   05/22/2017

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olsurfer

Centerboard Vibration/Hum

17 posts in this topic

What are the causes of centerboard vibration/hum and what are the fixes? Can the shape/taper of the trailing edge be the cause?

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Not trying to be a dork, but problem solving demands more info, boat class, even pics...kind of centerboard/daggerboard (plate steel) wood etc. Lots of causes, some can be fixed, some no!

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Cause: Vortex shedding coupled to a resonate structure.

Running the tip into some rocks might help. Or somehow disturbing the flow down there. I have only tried the former. Experiment with little taped on disturbances: vortex generators? 

Changing the mass at the tip might help...changes the resonate frequency.

Make the centerboard trunk less stiff...more compliant...rubbery.

Slow down...why the rush?

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Chisel point the trailing edge ie make it assymetric bevel to one side Nothing drastic only a 1mm or so.

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Like he says it works.  30 degrees will do but it's not scientific, I don't think it matters that its 30 at one point and 60 at another just so long as one side wins.  The effect on steering etc is not noticeable.

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It could just be the pendant vibrating when you hit the right speed(s).

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7 hours ago, UNCIVILIZED said:

It could just be the pendant vibrating when you hit the right speed(s).

Harmonics...and they seldom go away...I still do not understand how one can off help not knowing rig...

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F - Boat, wood / epoxy centerboard with a 30 degree taper towards a planed flat trailing edge. . Hum changes pitch relative to speed. Hum starts around 7 knots and will rattle your fillings at 18 knots.

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Centerboard or daggerboard? If you can, use some sort of inserts inside to pin to edge...but that may not eliminate...am guessing, it vibrates and what happens is it transfers to box and hence to boat and you rear end...depending on feel...you might want to keep as is...humor...I think there are stips made that will insert for daggers, not do certain with centerboards...and most are steel that rotate on pin, which might also be a vibe source that can be adjusted.

Sounds like definite harmonic thingy...you say you hit something and it began?

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4 hours ago, BobBill said:

Harmonics...and they seldom go away...I still do not understand how one can off help not knowing rig...

I don't understand how you think that water molecules flowing over a foil care anything about the details of the boat or rig to which it is attached...

the primary reason foils sing is an overly wide squared off trailing edge that causes vortices to shed off the trailing edge. When the shedding frequency corresponds to a resonant frequency of the foil, the foil vibrates and sings. 

The phenomenon can be eliminated by sharpening the trailing edge .  

 

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Up and at 'em...every man a molecule...I know squat, laker...had some vibes in past and fixed with wedges...I had not the slighest clue the wee suckers could think...just quenched 'em.

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Changing the surface finish will change the onset of hum. My old boards used to hum at 12 knots, then we faired and wetsanded. The hum kicked in at more like 16 knots. It felt slow because I used that 12-knot hum as an indicator of boat speed upwind. Anything below 12 (ie not humming) and I should foot. When the board went quiet I lost that feedback.

 

As others said, "breaking" the trailing edge will help, too. If you want to beanplate it, break the inboard side. 

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5 hours ago, lakepee said:

I don't understand how you think that water molecules flowing over a foil care anything about the details of the boat or rig to which it is attached...

the primary reason foils sing is an overly wide squared off trailing edge that causes vortices to shed off the trailing edge. When the shedding frequency corresponds to a resonant frequency of the foil, the foil vibrates and sings. 

The phenomenon can be eliminated by sharpening the trailing edge .  

 

+1000!

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The fix is easy and has already been stated.

When my F85 hit the water I had a hum from both the C/B and rudder. Not loud but still it was a noise that I didn't need.

If I liked noise I'd have a power boat.

I sanded one side of the trailing edge to make a chisel point shape and I can now sit back and enjoy the quiet.

Neither make any noise at all.

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14 hours ago, Doug Halsey said:

+1000!

Get no argument from this swab...I just solved it doing something else but have done both...whatever works...even looked at "whale bumps!"

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On 5/15/2017 at 7:47 PM, Try Flying said:

Chisel point the trailing edge ie make it assymetric bevel to one side Nothing drastic only a 1mm or so.

This.  Been there; done that. Times a few. Worked every time on dinghies and multis.

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13 minutes ago, Wess said:

This.  Been there; done that. Times a few. Worked every time on dinghies and multis.

Ahhh....+1 I remember from 14 footer days.

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