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Old 25-01-2010, 12:09   #1
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Hull / Rig Flex ?

As we sit here at Morningstar Marina in JAX trying to locate a new turbo, the wind is blowing like crazy. Last night we had a huge thunderstorm blow over with gusts over 40 knots and today it is blowing a fairly steady 20-25 with some gusts over 30 and more.

When it really blows like this and we are in a slip and sitting below not doing much you really notice the hull flex and/or rig pumping. I am not sure exactly what this is but you feel a sort of mild flexing in a rhythmic sort of way as if the rig is transmitting forces to the hull and the hull is flexing or twisting a bit in response. Its not major or scary but you can clearly feel it and it definitely coincides with very strong wind gusts.

Anyone else experience this? Is if fairly typical or normal?



Terry
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Old 25-01-2010, 12:27   #2
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It's called Vortex Shedding or Von Karman Vortices. There is a good explanation here: Vortex shedding - Wikipedia, the free encyclopedia The vortex shedding causes the rig to pump which in turn shakes the whole boat. Not much can be done aboard a sailboat to stop it though on my boat it only happens when the wind is exactly from abeam. The rig doesn't pump at other wind angles. Some sailors have reported success in reducing the problem by wrapping halyards spirally around the mast... a little difficult to do with spreaders in the way :-)
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Old 25-01-2010, 12:53   #3
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This is normal. The degree of pumping/flex and the conditions which produce it will vary from boat to boat. Also, it is important to have your standing rigging properly tuned. I actually saw a boat with permanently warped deck/hull because the owner kept tightening the shrouds and stays in an effort to reduce flexing.
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Old 25-01-2010, 12:56   #4
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Old 25-01-2010, 14:08   #5
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My boat is a Stevens 47, same boat as a Hylas 47. The mast is in column and I have had the rig closely inspected twice in the past 6 months. I do feel confident that the rig is very solid.... It is significantly oversized.

Thanks for the info!


Terry
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Old 25-01-2010, 15:52   #6
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We get a fair amount of "noise" from our rig and it is all but impossible some days to stop it from pumping. To try to stop it we will crank on some backstay and runners and checks. To keep the rigging from ringing together, like a cap against a d2, we whip a small block between them. Since we are in fresh water I'm not worried about corrosion.

Rigs can be noisy but the worst are the roller furling masts that are not gasketed.
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Old 25-01-2010, 15:57   #7
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Hoisting a fender in a halyard about half way up the mast and tightening it good with a line downwards to a cleat on the mast will help. Try it and you'll notice that in most cases the pumping stops. Just tighten the fender good, you don't want it banging against the mast all night
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Old 25-01-2010, 18:38   #8
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Quote:
Originally Posted by Hampus View Post
Hoisting a fender in a halyard about half way up the mast and tightening it good with a line downwards to a cleat on the mast will help. Try it and you'll notice that in most cases the pumping stops. Just tighten the fender good, you don't want it banging against the mast all night

This is an interesting idea. The pumping I am referencing is fairly mild and really doesn't bother anyone its just interesting and something to think about when the wind gets really gusty. Still, it would be easy enough to hoist a fender as mentioned and see what happens. I may give that a go.


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Old 25-01-2010, 20:05   #9
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Quote:
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Hoisting a fender in a halyard about half way up the mast and tightening it good with a line downwards to a cleat on the mast will help. Try it and you'll notice that in most cases the pumping stops. Just tighten the fender good, you don't want it banging against the mast all night
I was going to mention the thick black bungee straps but the fender sounds like a good idea too. I usually wrap the bungees around the shrouds. And I loosen up the hyd. backstay, the pumping all goes away. The object is to get the harmonics out of the rigging. Your rig is like a string instrument and the wind is doing the strumming.
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