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Old 18-04-2012, 23:23   #1
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Pumping mast!

Quote:
Originally Posted by mitiempo View Post
Rigging ages at the dock or in use. The stress of use will add to this but the corrosion in the swages exists regardless.
In response to another thread but something to ponder.

Had a odd situation today at the boat. While up forward below decks I could hear the occasional chafing of a snap shackle on a padeye. It came in a sequence of about 1 minute on/off.
I went up on deck and looked around and found that my 60' aluminum extrusion (mast) was pumping off & on, in a 7 kt breeze. The boat's on the hard and I've never seen it do this before. The wind must have been coming from just the right direction at the right speed. It makes me want to install running back stays for bare poles.
Anyway, I tightened up the spinnaker boom topping lift to a forward padeye and it stopped the pumping.
SOooo, rigging can get wear just sitting around!
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Old 18-04-2012, 23:25   #2
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Re: Pumping mast!

A little help for the uninitiated,,,,what is a pumping mast?? Thanks :-)


Quote:
Originally Posted by delmarrey View Post
In response to another thread but something to ponder.

Had a odd situation today at the boat. While up forward below decks I could hear the occasional chafing of a snap shackle on a padeye. It came in a sequence of about 1 minute on/off.
I went up on deck and looked around and found that my 60' aluminum extrusion (mast) was pumping off & on, in a 7 kt breeze. The boat's on the hard and I've never seen it do this before. The wind must have been coming from just the right direction at the right speed. It makes me want to install running back stays for bare poles.
Anyway, I tightened up the spinnaker boom topping lift to a forward padeye and it stopped the pumping.
SOooo, rigging can get wear just sitting around!
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Old 18-04-2012, 23:37   #3
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Re: Pumping mast!

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Originally Posted by Nemo55 View Post
A little help for the uninitiated,,,,what is a pumping mast?? Thanks :-)

While at the dock it is usually a harmonic oscillation caused by fluctuating vortexes of air passing the mast.



Under sail is a different matter. Running backstays can be rigged to deal with this under sail. At the dock I've heard of people hoisting fenders to disrupt the air flow around the mast.
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Old 18-04-2012, 23:39   #4
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Re: Pumping mast!

You mean like a reed oscillation on a harmonica?? which would make noise like a heavy bow????
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Old 19-04-2012, 00:01   #5
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Re: Pumping mast!

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Originally Posted by Target9000 View Post
At the dock I've heard of people hoisting fenders to disrupt the air flow around the mast.
That reminds me. I heard that if you wrap a halyard spiral around the mast that helps too.
As for the pumping the mast was oscillating fore and aft about 2-3" just below the top spreaders but no noise, just vibration.
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Old 19-04-2012, 00:47   #6
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Re: Pumping mast!

Ok,,Got it!!
Terms i can understand.

Quote:
Originally Posted by delmarrey View Post
That reminds me. I heard that if you wrap a halyard spiral around the mast that helps too.
As for the pumping the mast was oscillating fore and aft about 2-3" just below the top spreaders but no noise, just vibration.
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Old 19-04-2012, 01:39   #7
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Re: Pumping mast!

I saw a 70 metre tall by about 5 metre diameter propane/propylene splitter column on a refinery pumping in about a 15 knot breeze. It was still under construction, so the feed and reflux lines hadn't been installed yet. Pretty impressive though.
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Old 19-04-2012, 01:58   #8
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Re: Pumping mast!

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Originally Posted by 44'cruisingcat View Post
It was still under construction, so the feed and reflux lines hadn't been installed yet.
G'day, mate. Sounds like chemical engineering speak there! Yup, if the mast starts pumping, time to put on the running back stay, around when the apparent wind speed starts to creep over 20 kts to windward for us. Cheers.
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Old 19-04-2012, 03:10   #9
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Re: Pumping mast!

My furled headsail was doing that in 10 to 15 knots, like you I felt this oscillation vibrating the whole boat. Just pulled on it a bit and it stopped and it never started again.
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Old 19-04-2012, 03:11   #10
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Re: Pumping mast!

Sounds like a harmonic to me also. You may get a similar minimization using the boom and vang to adjust the shape of the mast somewhat and its ability to pump -even back stay if its a 3/4 rig. Though i kinda like your topping lift solution better.

Runners and Check stays and their entire family belong in the 80s in my opinion. They way over complicate a cruising rig.
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Old 19-04-2012, 06:56   #11
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Re: Pumping mast!

Mast pumping is common, but abnormal.

- 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 19-04-2012, 07:00   #12
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Re: Pumping mast!

Chances are that the boat has flexed slightly from uneven blocking on shore. I would be concerned if this was happening while in the water. Don't attempt to adjust anything while hauled out as the boat is not taking its true shape.
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Old 19-04-2012, 07:07   #13
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I found just putting a bungie on my lowers stopped it
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Old 19-04-2012, 07:08   #14
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Re: Pumping mast!

Mine has been doing this at the dock from time to time ever since the wind changed in the spring and started coming out of the south. Shakes the entire boat.

I've tried loosening/tightening the backstay tensioner with no real change. I'll try the fender trick.
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Old 19-04-2012, 07:19   #15
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Re: Pumping mast!

Muss be dese new-fangled alooium masts that think they are pipe organ rejects.
My 48 feet of Douglas fir sits really still.
--but dont let a halyard get loose--all hell breaks out.
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