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Old 04-10-2013, 06:53   #16
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Re: Mast shake

Disruption of the vortices, ya ya, that's the ticket.

I will try that....

Thanks everyone!

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Old 04-10-2013, 07:21   #17
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Re: Mast shake

i had that problem after the install of new rigging. it started when the wind was blowing at 90 degrees to the boat .. it was a fore and aft mast pumping. never tried running a fender up the mast cause i though it would look a little too strange. boat has been sold so not a problem anymore.

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Old 04-10-2013, 07:43   #18
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Re: Mast shake

I get some oscillation but only when the wind is from behind the boat. Have you gone out and looked for the culprit? On my boat it's clearly the jib/furler/forestay that is resonating.
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Old 04-10-2013, 08:22   #19
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The sock I haul up around the furled headsail sets up a vicious oscillation in high winds, so much so that I fear for the whole rig. Then when I take home on the two compression lines that cisscross up the sock, the oscillation completely vanishes. Mind you, the sock never flogs. I believe the main effect of the compression lines is breaking up the previously roughly cylindrical shape into smaller "parcels" breaking up the flow pattern around and behind it. Therefore, even a thin line in spiral around the mast may be more efficient than a single fender. Go out and test ir somebody that still haven't hauled out their boat for the winter, and - please come back and tell.
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Old 04-10-2013, 08:32   #20
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Re: Mast shake

GordMay is absolutely right and many sailors have no idea what the mast pumping is or that this occurs until they spend a significant amount of time on their vessel, usually when they become live-aboards. . Then when they discover the Mast Pumping they think it is a new consition, since they never experienced it before.

With certain winds moving around the mast it tends to want to move or "Pump", no matter what type of rig you have or how tight the stays are. Most of the time mine pumps when the wind is coming from the quarter or stern, so while at anchor, there is no problem.

As GordMay suggested, hoist a fender (or any other large object) up a halyard with a tag line on the bottom and secure it so it is against the mast. It will break up the wind and stop the Mast Pumping. Sometimes even large flags will do the trick.
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Old 04-10-2013, 08:56   #21
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Re: Mast shake

I've had it on lots of boats. I think it's a harmonic occillation (remember... that brought down the Tacoma Narrow bridge!) Your rig can be tuned perfectly for sailing and still do this. Often it can be reduced or eliminated by a staysail halyard being tightened up well. If no staysail on your boat, consider using a spinnaker pole lift if you have one. Neither of these is the final solution necessarily though because the halyard rolls through the block as the mast pumps. Maybe a stopper knot would help...
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Old 04-10-2013, 09:03   #22
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Re: Mast shake

As my boat does the same thing in the right conditions, how far are you guys hauling the fender up the mast?
Back in Annapolis waiting for October!
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Old 04-10-2013, 10:31   #23
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Re: Mast shake

Ain't harmonic frequencies a B*&#h!
Mine started pumping when I removed the topping lift for my spinnaker pole, guess I need to put it back
Pretty much anything to disturb the flow of air around the mast section should alter the frequency, maybe even enough to stop it, hopefully without creating another issue.....slapping of lines against the mast!

I think that tightening the rig only changes the natural frequency but would not stop it.
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Old 04-10-2013, 12:51   #24
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Re: Mast shake

People perhaps your extrusions are just too fine???

If there is a spinnaker topping lift you can run it to the bow and slightly tension it on a winch. This normally stops any pumping/wobbling or shaking.

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Old 04-10-2013, 14:42   #25
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Re: Mast shake

Originally Posted by akkebeng View Post
To be honest, turbulence may actually relieve the problem. It's the periodic release of the airflow around a cylindrical object (Karman vortex) alternatingly on each side that drive the pumping. If the period of the Karman Vortices approach the natural period of lateral swing of the mast, you get this cyclic build up of shake until it more or less abruptly vanishes just to begin the next cycle. In other words, certain combinations of mast height, stiffness and diameter will in certain wind speeds display this behaviour. If you have an unfortunate combination of these three parameters, your only choice is to either change one of the parameters or break at least part of the periodic Vortices into turbulence by the introduction of features that break up this periodic release of vortices into mere turbulence as described in a previous post.
I'm not really disagreeing with you. My point was change the dynamics of the mast. If it's pumping witha spinnaker pole stored on it, take it off and see if that fixes it. On mine I could see the TV antenna start to vibrate before the mast started pumping. Removing the antenna changed the existing dynamics of the mast. Maybe removing the spinnaker pole would accomplish the same thing, maybe not. It's a simple and cheap test though.
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Old 05-10-2013, 01:11   #26
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Re: Mast shake

I think that akkebeng is describing the effect that I am getting and I understand his solution - similar to the helical wrapping of tall slender chimney flues to prevent them oscillating. I will have a play with a line around the mast to see if I can disrupt the airflow.

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