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What is going on is whats known as an 'induced harmonic vibration response' in the rigging or mast system. This is what makes a violin or cello, etc. string vibrate. The Rigging or a mast (or combo of both) is being 'excited' by wind conditions flowing over or around the rigging/mast ... and is producing a sound that is 'harmonic' to the 'energy, etc. conditions' of the wind.
Such vibrations are dependent on two things: rig tension and the mass of the wire/rigging. If you change the tension or the mass (by simply 'hanging' some weight in the rigging) you will either change the 'note being played' or will arrive at a vibratory condition where there is no harmonic response; Rx: change the tension or hang a weight onto the 'wire thats vibrating'.
For a mast that is 'pumping' or radically vibrating, add the proper 'prebend' to the mast by correct rig tension - typically 3/4" forward bow to a single spreader rig or 3/4" forward bow per each spreader set on a multi spreader rig --- the mast manufacturers all have specific 'pre-bend' specifications on each mast section offered. Using a mast without 'prebend' is an open invitation to 'mast pumping' / radical induced harmonic vibrations (and your mainsail was cut to match that expected 'pre-bend'; without that pre-bend the mainsail will always be slightly 'powered-up'). Adding 'pre-bend' changes the 'apparent stiffness' of the mast and changes the 'natural frequency' at which the mast vibrates.
To find the part of the rig that is the 'culprit', when the 'note is being played' simple go to each component and with your hand & 'feel' the part that is 'buzzing' and affect the correction (tension or weight change). If you cant 'feel' the errant vibration, borrow a vibratory tuning gage from a stringed instrument musician and when the gage shows a B#, etc. ... thats the wire that needs 're-tuning' --- just re-tension up or down until the harmonic ceases (but may issue another 'note' when the conditions are correct for that new tension).
In cases of severe 'harmonic response' (mast pumping) where the mast is actively and noticeably 'rocking back and forth' (can be 'violent' rocking/bowing), especially in high wind conditions ... this must be quickly corrected as such high energy harmonic vibrations can soon cause mast / rigging FAILURE (the classic example of destruction due to 'harmonics' is the failure of the Tacoma Narrows bridge in the late 1930s). The quickest emergency remedy is to pull the wire or mast with a rope attached or tied to it and pulled at a perpendicular angle and led to a winch, etc.
However if you have an in-mast furler (acting like a 'flute' with air flowing across the slot and producing the 'note') then the only way to solve this is cover the 'slot' by any means possible - raising a 'twisted or 3 stand 'rope' in very close proximity to the 'throat of the slot', etc. ... anything to disturb the air flow 'across' the open slot, or closing the slot.
Last edited by RichH; 01-16-2012 at 03:31 PM.