How to kill keel harmonics?

Discussion in 'Sailboats' started by jim lee, Sep 8, 2011.

  1. jim lee
    Joined: Feb 2007
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    jim lee Senior Member

    We have boat will hum pretty loudly at different speeds. Keel? rudder? Anyone have experience with something like this?

    Thanks!

    -jim lee
     
  2. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    More info is needed. Where does the hum come from? Can you give a more detailed description of your boat: materials it is made of, engine(s), drivetrain, propeller data, appendages, what speed and RPM regime causes the noise. A few pics of your boat would be useful - in particular the underwater part, if possible.
    Cheers!
     
  3. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    rudders on beach cats do it
    rigs on keel boats do it ususally notice at the dock in the wind
    PS carbon rigging does it real bad
    If the rig is doing it you will feel it when you touch it
     
  4. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    Jim,

    The humming comes from the flow on either side of the keel leaving the trailing edge at exactly the same time, thereby forming a vibration harmonic that is at the natural fequency of the keel blade (or rudder blade) itself. This is because the trailing edge is squared off. To cure the problem, grind the trailing edge back to a 45-deg angle so that it is totally chamfered to one side (doesn't make any difference which side is longer, which is shorter). So looking at it in section, the trailing edge will look like a chisel point. You can round the longer sharp edge ever so slightly if you wish, kissing it with a bit of sandpaper, so that anti-fouling paint will stick. What this does is it causes the flow off the trailing edge on both sides to occur at two different places and times, causing them to be OUT OF SYNCH with each other. That way, they cannot set up a harmonic vibration with the blade. This usually works like a charm.

    Eric
     
  5. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Eric,

    While beveling off the TE as you discribe is the perfered solution for singing props, I'm not sure I would recommend it for symetric foils that reverse AoA like keels and rudders without a good inspection due to seperation point rollover issues. Beveling the TE can cause flutter depending on the sign of the AoA and I would generally recommend to avoid it, especally on a rudder. Depending on the foil thickness, boat speed, hull/foil mass stiffness, and the TE cutback the humming may just be the unavoidable manifestation of a properly bound TE vortex or another item altogether such as the horseshoe or tip vortex. FWIW I have often used TE humming/singing as a rough speed tuning/design verification device on high speed foils. A stable tone that changes with speed is an indication of proper TE vortex attachment.

    Anyway,Jim, have a look at this thread.
    http://www.boatdesign.net/forums/sailboats/humming-foils-slow-20193.html
     
  6. peter radclyffe
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    peter radclyffe Senior Member

  7. jim lee
    Joined: Feb 2007
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    Location: Anacortes, WA

    jim lee Senior Member

    Ok, I read the other thread. I absorbed what I could about it for now. Seems like people are seeing this as three different types of problems. Wobble or Flutter, Low frequency humming and high frequency "Singing".

    Our boat..

    26 ft long.
    It has a bulb on a keel fin that's about 20" chord. Draws 6 feet with the keel down.
    No motor in the picture because its out of the water when sailing.

    Rudder draws about 47" and has a chord from about 12 1/2" to about 6"

    I'm getting reports of the humming coming in a various speeds. My racing crew is beating me up about this.

    I've been on these boats and felt the humming. I'd guess that is -around- 60hz. I've not measured it but it seems around that ball park. It doesn't seem to change over the speed range. It will fade in and out. This makes me think it may not be a foil thing.

    I have seen the rig hum like this. (A mix of PBO, amsteel and SS) I'd assumed it was the rig originally, but the crew is insisting its not. They are insisting that there are two sources, sometimes its the rig and sometimes its the foils. I find it odd that both would give the same frequency.

    The weather is beautiful, the boat's currently ready to go. Maybe I should drop all this for now and go do some "field testing" for myself.

    -jim lee
     

  8. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    JEH,

    I have used this trimming technique on keels primarily and have recommended on both keels and rudders. I have not seen any appreciable bad effects on either, whether tacking or turning. On some of my keels where this has happened, the humming is quite bothersome--very loud--to the point that you cannot talk easily over the noise! Trimming the keel works and makes life on board bearable.

    Eric
     
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