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Old 08-05-2012, 03:16   #1
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Swing Keel Vibration / Shudder - Help?

I have recently bought a swing keel Beneteau 26 from 1986. On our first passage from Salcombe to Dartmouth yesterday there would be shudder / vibration from the keel as we went over 6.5 - 7 Kts. It is a fairly disconcerting and it makes the whole hull vibrate.

Does anyone have any experience with this and or any recommendations?
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Old 08-05-2012, 03:59   #2
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Sounds as if the keel has been faired unevenly. It only takes millimetre differences on one side to the other to make it vibrate at speed. Not familiar with the boat, but as you mention swing keel you could check the bushings on the pivot point to see if they are worn thus allowing the keel more play and ability to vibrate.
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Old 08-05-2012, 07:00   #3
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Re: Swing Keel Vibration / Shudder - Help?

Greetings and welcome aboard the CF, northcave.

Could it be the cable vibrating?

If the keel winch cable is stretched tight, when the keel is down, the cable may vibrate like a guitar string. Lower the keel all the way down, and then give the cable some extra slack.
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Old 08-05-2012, 07:02   #4
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Re: Swing Keel Vibration / Shudder - Help?

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Originally Posted by GordMay View Post
Greetings and welcome aboard the CF, northcave.

Could it be the cable vibrating?

If the keel winch cable is stretched tight, when the keel is down, the cable may vibrate like a guitar string. Lower the keel all the way down, and then give the cable some extra slack.
Interesting thought. I'll give that a go!
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Old 08-05-2012, 15:37   #5
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Re: Swing Keel Vibration / Shudder - Help?

Welcome to CruisersForum, northcave
I didn't know that some Bénéteau yachts had a cable-operated swing keel. IME, all have a big vertical screw to raise the keel.

When the keel vibrated, were you sailinging upwind or downwind? Sometimes, a keel (fixed or swinging) vibrates when sailing (fast) dead downwind because there is no lateral pressure from leeway. The same phenomenon can happen on the rudder, too. An effective cure is to change course by a few degrees.

Alain
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Old 08-05-2012, 16:49   #6
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Re: Swing Keel Vibration / Shudder - Help?

A humming or high pitch vibration is common on lighter boats when they go above hull speed.

It may be prudent to check the play on the keel and rudder, but it doesn't sound like anything out of the ordinary.
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Old 08-05-2012, 17:31   #7
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Re: Swing Keel Vibration / Shudder - Help?

If the above doesn't work, try grinding the forward edge of the board at a 45-degree bevel ONE side so it is slightly asymmetric. Don't overdo, you don't need to fully bevel it.... just flatten it "a bit".
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Old 09-05-2012, 13:36   #8
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Re: Swing Keel Vibration / Shudder - Help?

I would rather chamfer the *trailing* edge of the keel. This is the recommended cure for "singing" propellers (the same phenomenon, but higher-pitched). See for example: http://www.hydrocompinc.com/knowledg...Propellers.pdf

If you alter the leading edge, you risk triggering a local separation of the boundary layer on the keel, resulting in a substantial increase in drag and a reduction in lift for the same angle of attack. The final result would be increased leeway and reduced speed.

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