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Old 06-01-2008, 10:45   #1
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Rigging tension in a storm

Kind of an unuasual question but it will probably make some of you think......
Had a storm roll through here a couple days ago, and as always while setting in a slip, my backstay was slacken off.. When the furler started whipping around, I went out to tighten the rig up..Along with the check stays and anything else that was lose..
So now with everything taunt, a gust of wind hits and the whole boat goes into vibrate mode,
So in a rigging aspect, which is better for the rig, (while setting in a slip)?
If its tight, the boat shakes to hell, and rigging vibrates, If its lose, you work harden the fittings...
Anyone with some good rigging experance?
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Old 06-01-2008, 11:26   #2
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As with most things I think moderation is the best. If you tighten the rig too much it will distort the shape of the hull. If you leave it tooo loose it will whip around. A moderate amount of tension is good. If your boat is in Nor Cal during that storm well that wind doesn't come around all the time and I think it would have been prudent to go to the boat and adjust the rig a little tighter before the storm hit
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Old 06-01-2008, 11:45   #3
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You really don't want anything too loose so that it flops around or works. If you are getting vibaration/harmonics you can tape the shrouds together. Make shure the halyards are spread and if you have lazy jacks use ties to pull them away from the spar. It is amazing how loud some of those things can be below and it seems like the always start buzzing at 3 in the morning.
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Old 06-01-2008, 18:46   #4
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If you have a performance boat with a bendy mast then I would tune it for a balanced cruising mode that maintains the design shape and orientation of the mast. By repeatedly tacking and slowly removing the excessive slop on the leeward shrouds, you can find that balance without over tightening. Those should be your marks and I would fix it there. The fore and aft stays can be done at the dock.

I would not worry about high wind vibration as that is only the harmonics of closely tuned stays interacting with each other. Much better than slop!

Small suggestion! Living in a Typhoon area, when we are expecting hurricane force winds we always remove the furling headsails to lessen the weight and windage aloft. Makes a big difference.
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Old 06-01-2008, 20:27   #5
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Excellent question, and one I'd like to know the answer to as well.

My boat has a set of aft shrouds that are set fairly loose, or at least slack off a ton when on the lee side in a good blow.

At the dock when the wind pipes up the whole rig buzzes, though I can silence most of it by wrapping a towel around the topping lift.

In a 40+/- knots the mast takes on a bit of rythmic thumping, on deck it doesn't look like its about to jump over the side, but down below it can feel like it. Whole boat quivers. I'm wondering if it has to do with the fractional rig, where the wavelength of the vibration is different between the forestay and back stay.

You are quite right... this is a topic I've pondered, but never really understood.
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Old 06-01-2008, 22:52   #6
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I know exactly what you guy's are talking about. It is the mast that is vibrating. It is a Harmonic set up by the mast under compression. Once you have everything tight, the mast will be under compression and it harmonics at certain wind speeds. Once the wind speed reaches a speed that matches the harmonic, the mast happily shacks the boat. Mine will shake the 26tonne boat in a light breeze and then the next harmonic point is somewhat much higher. The easiest way to stop the shake is to take a line from the bow or stern, which ever is easier, and wrap around the mast half way between the deck and the shrouds. Place a bit of tension on the line, it doesn't need too much, but it will stop the vibration if you are concerned by it.
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Old 06-01-2008, 23:02   #7
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Are you guys talking about the low frequency "shakes" that come through every now and again? I get them in about 25 knots or so. Feels like the whole boat is getting jiggled a bit. Not at all violent; just weird.
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Old 06-01-2008, 23:09   #8
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Quote:
Originally Posted by Zach View Post
this is a topic I've pondered, but never really understood.
If you don't understand harmonics then don't worry as engineers and architects are still fazed by it!

Recently a large bridge was built in Sydney, Australia (just near where Boracay lies to the pick) which vibrated when very light rain hit its 'shrouds' causing scared designers to panic
The resolution was very simple bits of, initially, rope attached to the shrouds and anchored to the road changing the moment of harmonic resonance. Now replaced by SWR they can be seen from the road, and indeed, seen in the bottom left of this photo
Credit Singing Bridges | Viator Travel Blog which is an interesting read.

Also see Google for stories on LondonsMillenniumBridge that started waving up and down when people walked on it!

The solution can be extremely simple in some cases, and may be as said in a post above just a lanyard or tape connecting the 2 shrouds to change their harmonic moment.
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Old 07-01-2008, 01:01   #9
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We use bungees to stop the vibrates on the mast and shrouds. They take the vibe instead of the shroud, topping lift or mast then. Just like the pic above.
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Old 07-01-2008, 01:11   #10
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Well everyone has probably seen this before. Maybe someone should have run out and put some bits of string on this one.




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Old 07-01-2008, 01:37   #11
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. Maybe someone should have run out and put some bits of string on this one.

John
LOL. Maybe they could have I remember we saw that film when I was doing Engineering at University. I think the wind was only about 30 knots.. OK I checked... I'm lying again.. it was actually: "wind of 42 miles (67 km) per hour"

Its amazing that such a small amount of pressure can do it!
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Old 07-01-2008, 05:16   #12
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A note of caution: Don't screw the leeward shrouds down while sailing lest you drive the spar through the bottom of the boat. Use a tension gage on the shrouds or a ram on the spar to set tension. If you are not sure what tension the caps require, talk to a rigger or spar maker. Make sure the screws are clean and lubricated and don't gall them by over tightening.

http://www.navtec.net/docs/RiggingService.pdf
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