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Old 03-10-2013, 20:48   #1
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Mast shake

Hi, I live in a marina aboard my 37 foot sloop and in certain wind conditions get significant fore and aft wobbling / shaking in the mast - the movement is transmitted to the hull and can easily be felt when lying down on my innersprung mattress! The 'wobbling' is cyclic in that it starts off quietly and builds to a peak before dropping off. The mast and boom are aluminium and the rig is a simple masthead rig with cap shrouds over single (not splayed) spreaders and with a pair of lower shrouds each side attached below the spreaders. I'm not an expert rigger but the tension in the shrouds seems ok - they are certainly not slack. The mast is keel mounted and is set up with a slight fore and aft back curve. I assume that the movement has something to do with the cross sectional profile of the mast and a natural resonance being activated in certain conditions!? Has anyone come across this 'problem' and have any suggestions as to how I might fix it?
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Old 03-10-2013, 21:10   #2
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Re: Mast shake

I had a similar issue, but with a fractional rig. I eventually figured out that it was starting with a disk shaped TV antenna I had mounted on the front of the mast. Got rid of the antenna and the problem. I did notice it never occurred when I was anchored out and only happened when at the dock and the wind was more or less on the beam. It sounds similar to your problem, but my antenna was mounted on the unsupported section of the mast above the stays. I don't know if a masthead rig would be as susceptible as a fractional rig to this problem.
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Old 03-10-2013, 21:56   #3
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Re: Mast shake

Tighten up your shrouds until they are "piano wire tight"...not crazy tight, but firm, and see if that helps. They may not feel loose, but if there is room for the mast to set up some oscillation in a wind then you'll get the syndrome you describe. Aside from that, if you could better describe "certain wind conditions" that might help with a diagnosis.
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Old 03-10-2013, 23:17   #4
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Re: Mast shake

Have the same problem on my ketch but only started when I tied the 2 masts together with a trapezeodal ??? stay. I like the idea of the masts tied together but I might have to take it off if I can't sort out the oscillation problem using the lower shrouds
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Old 03-10-2013, 23:26   #5
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Re: Mast shake

It's hard to eliminate it entirely. Halyard tension, stay tension, wind direction, wind force, and doo-dads are always changing. It doesn't really matter; it's just weird.
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Old 04-10-2013, 02:10   #6
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Re: Mast shake

Your could just use the spinnaker Poles topping lift to take up some slack at the centre of the mast when needed. But it really sounds like you need to speak to a rigger.
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Old 04-10-2013, 03:19   #7
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Re: Mast shake

Hi, Thanks for the interest and comments to date. Like Capt. Bill the shaking only occurs when alongside the dock, but in my case when the wind is more fore and aft than abeam. Coincidentally last night I tried using the spinnaker pole topping lift, as suggested by Ozskipper, to apply some tension at mid mast height, but wouldn't you know it, the wind died before I could tell if it was effective!
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Old 04-10-2013, 03:27   #8
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Re: Mast shake

I notice a slight harmonic vibration in my keel stepped mast under certain wind conditions at the dock as well.

I've had the rig retensioned for different purposes twice in a year by professional riggers. The vibration is still there. It doesn't bother me and I never feel it when under way or at anchor.
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Old 04-10-2013, 04:31   #9
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Check your topping lift tension. I get the vibration and noise when I tighten my topping lift and mainsheet. If I slack these, it goes away. I had the vibration problem in the past when I over tightened the shrouds.
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Old 04-10-2013, 05:04   #10
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Re: Mast shake

Objectionable mast pumping, AT THE DOCK, can often be ameliorated by twisting together, then wrapping the twisted halyards around the mast, and hauling them very tight.
This disrupts the formation of the Von Karman vortices, that cause the harmonic oscillations.

Another easy way to do this is to run a small fender up a main halyard, with a line attached on other end, so it can held tight.

You could also move to a dock, oriented at 90 degrees to your present berth, changing the mast foil orientation to the prevailing winds (probably not very practical).

Changing the tension of the shrouds (rig tuning) will also change the resonant frequency, but may have other unintended deleterious consequences.
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Old 04-10-2013, 05:32   #11
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Re: Mast shake

Quote:
Originally Posted by GordMay View Post
Objectionable mast pumping, AT THE DOCK, can often be ameliorated by twisting together, then wrapping the twisted halyards around the mast, and hauling them very tight.
This disrupts the formation of the Von Karman vortices, that cause the harmonic oscillations.

Another easy way to do this is to run a small fender up a main halyard, with a line attached on other end, so it can held tight.

You could also move to a dock, oriented at 90 degrees to your present berth, changing the mast foil orientation to the prevailing winds (probably not very practical).

Changing the tension of the shrouds (rig tuning) will also change the resonant frequency, but may have other unintended deleterious consequences.

I used to run a fender up the mast of my last boat. Cured the problem.
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Old 04-10-2013, 05:33   #12
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Vibration you say. With what frequency?
My experience of vibrations is from the topping lift itself, beginning to hum at around 5 m/s gradually changing into a nerve-racking howl above say 12 m/s. A useful feature if you want to be automatically waked up by a windshift while at anchor, but a real nuisance otherwise. One simple remedy for me was to hang a fender in the topping lift, 1 m above the boom. That silenced it more or less completely. If the mast or a roller furled headsail begins to pump the frequency is much lower like one to a few cycles per second.
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Old 04-10-2013, 06:10   #13
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Re: Mast shake

I too have a medium frequency hum from the topping lift when I have it too tight. I release a little pressure from the sheet and it goes away. This noise occurs even at anchor in winds above 10-12 knots. The shaking problem I had was much lower frequency, maybe only 5-10 Hz. Though the OP may not have a tv antenna it may be a problem with soething attached to the mast creating turbulance that makes the problem start. He mentioned a spinnaker pole, if it's attached to the mast, perhaps that is where it starts. Perhaps he should leave it on the deck the next time the wind comes from an offending direction.
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Old 04-10-2013, 07:30   #14
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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.
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Old 04-10-2013, 07:35   #15
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....sorry! Too fast on the send button.
The last sentence in my previous post contains two formulations of the same thing squashed together.
Pbflllllt!
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