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02-10-2015, 22:44
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#1
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Registered User
Join Date: Feb 2013
Location: Live-aboard Cruiser
Boat: Lagoon 450
Posts: 628
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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
__________________
Steve
SV Emerald Sea
L450
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03-10-2015, 00:44
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#2
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Senior Cruiser
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,888
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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.
http://www.cruisersforum.com/forums/...ing-32478.html
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03-10-2015, 04:15
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#3
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Senior Cruiser
Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 49,139
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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
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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03-10-2015, 12:26
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#4
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Registered User
Join Date: May 2009
Location: Lorient, Brittany, France
Boat: Gib'Sea 302, 30' - Hydra
Posts: 1,245
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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.
Alain
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04-10-2015, 07:58
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#5
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Senior Cruiser
Join Date: Dec 2010
Location: Cruising Indian Ocean / Red Sea - home is Zimbabwe
Boat: V45
Posts: 1,352
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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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04-10-2015, 08:07
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#6
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Moderator Emeritus
Join Date: Aug 2005
Location: Carlsbad, CA
Boat: 1976 Sabre 28-2
Posts: 7,505
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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.
__________________
Peter O.
'Ae'a, Pearson 35
'Ms American Pie', Sabre 28 Mark II
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04-10-2015, 08:11
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#7
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Senior Cruiser
Join Date: Dec 2010
Location: Cruising Indian Ocean / Red Sea - home is Zimbabwe
Boat: V45
Posts: 1,352
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Re: Harmonic Vibrations on Aluminium Rig
Try wrapping another line around it first.
Quote:
Originally Posted by roverhi
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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04-10-2015, 09:22
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#8
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Registered User
Join Date: Dec 2008
Location: SC
Boat: None,build the one shown of glass, had many from 6' to 48'.
Posts: 10,208
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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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04-10-2015, 09:47
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#9
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Registered User
Join Date: Apr 2015
Location: Melbourne, Fl., Patrick Airforce Marina
Boat: 1965 Pearson Alberg 35 #190
Posts: 322
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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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04-10-2015, 09:55
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#10
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Senior Cruiser
Join Date: Dec 2010
Location: Cruising Indian Ocean / Red Sea - home is Zimbabwe
Boat: V45
Posts: 1,352
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Re: Harmonic Vibrations on Aluminium Rig
Quote:
Originally Posted by Cadence
A humming shroud makes for good sleeping, it's like white sound.
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Until its from the boat next to yours. It can be as irritating as a slapping halyard.
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04-10-2015, 10:37
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#11
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Registered User
Join Date: Aug 2012
Posts: 811
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Re: Harmonic Vibrations on Aluminium Rig
Quote:
Originally Posted by Emerald Sea
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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Glen Campbell sang about just that.
"I hear you singing in the wire; I can hear you through the whine."
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04-10-2015, 11:06
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#12
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Registered User
Join Date: Dec 2008
Location: SC
Boat: None,build the one shown of glass, had many from 6' to 48'.
Posts: 10,208
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Re: Harmonic Vibrations on Aluminium Rig
Quote:
Originally Posted by Bulawayo
Until its from the boat next to yours. It can be as irritating as a slapping halyard.
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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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04-10-2015, 11:12
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#13
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Registered User
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,483
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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.
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard
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04-10-2015, 11:13
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#14
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Registered User
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,483
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Re: Harmonic Vibrations on Aluminium Rig
Quote:
Originally Posted by jreiter190
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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hah! I'm rollin' on the floor.....
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard
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04-10-2015, 11:33
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#15
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Registered User
Join Date: Dec 2008
Location: SC
Boat: None,build the one shown of glass, had many from 6' to 48'.
Posts: 10,208
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Re: Harmonic Vibrations on Aluminium Rig
Quote:
Originally Posted by Cheechako
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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Or where are those damn bungees, at 0 dark thirty, to hold the halyard off the mast.
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