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Hi guys... upon purchasing my Avance24, the previous owners had rigged it quite badly, and I had to tighten everything up. Reading about tightening and tuning a Fractional Rig, I was careful to make sure the mast was vertical and not bias to one side, both looking up the mast and measured with the main halyard to each side of the deck. All good there. Even without specs on shroud loads, I guestimated and the yacht sails and points very well now.

However... when at anchor or in the marina, with a breeze passing through the rigging the mast oscillates in the middle... not bad but the vibration goes through the whole yacht. I was on a friend's yacht (normal rig) and that did it too... is it common, or is the shroud tension incorrect between the lower and upper shrouds? I have tried to reduce the upper shrouds a little, and it didn't help...

Any advice would be great, as I am quite naive in rig tuning.
 

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you sure it's the mast, and not the halyards? If the halyards are external for any length, with tension on them its pretty common for them to "slap" or "hum."

Internal halyards can do it too, but usually because of wave movement.
 

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Your mast is probably set up with insufficient 'pre-bow' / prebend. The prebend is important for two issues:
1. it artificially stiffens the mast so that that induced harmonic vibrations are not induced by certain wind strengths and wind angles.
2. it matches the shape of the luff of the mainsail that the sailmaker intended in the design. Without prebend the mainsail will be set with excessive draft.

Here's how to tune ... choose your rig and follow the directions: http://www.riggingandsails.com/pdf/selden-tuning.pdf .... one of the most comprehensive rig tuning guides on the internet and includes a method of tuning that assures proper rig tune without the use of a tension gage.
 
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