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Old 02-10-2015, 23:44   #1
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Harmonic Vibrations on Aluminium Rig

Has anyone experienced where their Al mast and possibly rigging begins to make a 'humming' noise, while stationary, when the wind blows through it? I believe this is harmonic vibration (wind building up on the windward side of the mast/rigging and creating eddies in the vacuum leeward side i.e. vortex shedding). On my rig, it seems to start at about 10kts, it starts 'singing' and I can feel the high speed vibrations on the shrouds and mast. These are not excessive movements and its invisible to the eye. Certainly disturbing while trying to sleep! Its non-existent while sailing.

Has anyone come across this before and what have you done about it? Perhaps its a rig tuning issue and I need an TA (technical authority) to review it?

There is a related CF thread on this subject, good info, but it wasn't conclusive and was for a different rig.

Thanks
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Old 03-10-2015, 01:44   #2
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Re: Harmonic Vibrations on Aluminium Rig

Not uncommon. Is this the thread? There is some good advise there on things to try.

Mast Pumping / Vibrating
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Old 03-10-2015, 05:15   #3
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Re: Harmonic Vibrations on Aluminium Rig

Objectionable mast pumping, AT THE DOCK, can often be ameliorated by twisting together, then wrapping the twisted halyards around the mast, and hauling them very tight.
This disrupts the formation of the Von Karman vortices*, that cause the harmonic oscillations.
Another easy way to do this is to run a small fender up a main halyard, with a line attached on other end, so it can held tight.
You could also move to a dock, oriented at 90 degrees to your present berth, changing the mast foil orientation to the prevailing winds (probably not very practical).
Changing the tension of the shrouds (rig tuning) will also change the resonant frequency, but may have other unintended deleterious consequences.

* ➥ https://www.google.com/search?q=Von+...utf-8&oe=utf-8
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Old 03-10-2015, 13:26   #4
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Re: Harmonic Vibrations on Aluminium Rig

Sometimes, the vibrations originate in the boom topping lift. Then, it is possible to suppress them, just by reducing this tension.

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Old 04-10-2015, 08:58   #5
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Re: Harmonic Vibrations on Aluminium Rig

Have you checked the rigging tension recently? Has this always been the situation or is it a recent phenomenom? If its recent something has obviously changed so its time to start checking. I can recommend buying a Loos rigging tension meter - it is simple to use once the principals are understood.
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Old 04-10-2015, 09:07   #6
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Re: Harmonic Vibrations on Aluminium Rig

Switched from a wire to an equally thin synthetic rope topping lift. The new lift sure do love to sing in winds over 10k. The wire lift destroyed the tabling on the leech of the main so won't switch back. Will go to a larger diameter topping lift which will hopefully eliminate the noise or at least change the pitch.
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Old 04-10-2015, 09:11   #7
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Re: Harmonic Vibrations on Aluminium Rig

Try wrapping another line around it first.

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Originally Posted by roverhi View Post
Switched from a wire to an equally thin synthetic rope topping lift. The new lift sure do love to sing in winds over 10k. The wire lift destroyed the tabling on the leech of the main so won't switch back. Will go to a larger diameter topping lift which will hopefully eliminate the noise or at least change the pitch.
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Old 04-10-2015, 10:22   #8
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Re: Harmonic Vibrations on Aluminium Rig

A humming shroud makes for good sleeping, it's like white sound.
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Old 04-10-2015, 10:47   #9
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Re: Harmonic Vibrations on Aluminium Rig

Hold up a guitar flat so that a good wind blows across the strings, move it around a little until the harmonics begin . You won't believe the concert you hear. Oh, it helps if you take some mescaline.
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Old 04-10-2015, 10:55   #10
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Re: Harmonic Vibrations on Aluminium Rig

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A humming shroud makes for good sleeping, it's like white sound.
Until its from the boat next to yours. It can be as irritating as a slapping halyard.
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Old 04-10-2015, 11:37   #11
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Re: Harmonic Vibrations on Aluminium Rig

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Originally Posted by Emerald Sea View Post
Has anyone experienced where their Al mast and possibly rigging begins to make a 'humming' noise, while stationary, when the wind blows through it? I believe this is harmonic vibration (wind building up on the windward side of the mast/rigging and creating eddies in the vacuum leeward side i.e. vortex shedding). On my rig, it seems to start at about 10kts, it starts 'singing' and I can feel the high speed vibrations on the shrouds and mast. These are not excessive movements and its invisible to the eye. Certainly disturbing while trying to sleep! Its non-existent while sailing.

Has anyone come across this before and what have you done about it? Perhaps its a rig tuning issue and I need an TA (technical authority) to review it?

There is a related CF thread on this subject, good info, but it wasn't conclusive and was for a different rig.

Thanks
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Old 04-10-2015, 12:06   #12
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Re: Harmonic Vibrations on Aluminium Rig

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Originally Posted by Bulawayo View Post
Until its from the boat next to yours. It can be as irritating as a slapping halyard.
Your right with the humming coming from someone else's boat. But nothing is as irritating as a slapping halyard on their boat or yours.
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Old 04-10-2015, 12:12   #13
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Re: Harmonic Vibrations on Aluminium Rig

I've not had it happen while sailing, but anchored it does often. Usually curable with hanging where halyards etc are tied off.
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Old 04-10-2015, 12:13   #14
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Re: Harmonic Vibrations on Aluminium Rig

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Originally Posted by jreiter190 View Post
Hold up a guitar flat so that a good wind blows across the strings, move it around a little until the harmonics begin . You won't believe the concert you hear. Oh, it helps if you take some mescaline.
hah! I'm rollin' on the floor.....
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Old 04-10-2015, 12:33   #15
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Re: Harmonic Vibrations on Aluminium Rig

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I've not had it happen while sailing, but anchored it does often. Usually curable with hanging where halyards etc are tied off.
Or where are those damn bungees, at 0 dark thirty, to hold the halyard off the mast.
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