Mast pumping in slip

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BillyK

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Jan 24, 2010
502
Catalina 310 Ocean City, NJ
Last night I was trying to sleep and the wind must have been from just the right direction and speed that it caused my mast to pump or oscillate.. So I twisted some heavy line around a halyard and wrapped the mast below the spreaders.. The crazy part of this is that it totally worked! Figured I'd share with the forum
 

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RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Retune your rigging! Your mast was probably set up without a slight 'forward bow' or 'prebend'.

Everything in the universe has a natural oscillation frequency. The wind was inducing the mast/rigging to oscillate.
The natural frequency can be changed either by changing the mass (hanging a rope, etc), by changing the rig tension .... or better by adding a slight forward bow (prebend) to the mast. This will change the natural frequency ... to a far different frequency.

A single spreader mastheaded rig 'should' have 3/4" forward prebend, a multi-spreader mast will have 1/2" bow for each spreader set. Your mainsail is cut by the sailmaker expecting this prebend ... or the sail will have more draft than designed.

How set prebend: http://www.seldenmast.com/page.cfm?id=6740 then download according to your choice language This guide will apply to all 'common' masts and rig tuning.
There is a section in this manual that explains how to properly tension the rig without a tension gage ... just a 'meter stick' - you set up the rig by its measured 'stretch' or elastic strain simply by measuring that strain.

;-)
 
Dec 2, 2003
1,637
Hunter 376 Warsash, England --
Look at any tall slim stainless steel chimney on any industrial site and you will see spiral ribs projecting out from the surface.
These are "vortice breakers" and serve to suppress the parasitic vortices that occur behind all round vertical tubes.
Your halyards did this job just as well.
Nothing to do with pre bend or rigging tension - unless there is none!

Hope you had a good nights sleep.
 
Mar 8, 2011
296
Ranger 33 Norfolk
Before I re-tensioned my rig I would get wicked mast pumping like I thought the boat would come apart. . .I initially tied the bottom of a fender to the mast and hauled it up to just above the spreaders and it eliminated all pumping. It was there to break up those vorticity things. A radar dome might do the same if you needed another reason to buy one :D

Once I re-cored the side decks at the aft lower chain plates and re-tensioned, the mast still pumps a little, but stops as soon as it starts (one or two oscillations and it stops) . . .and it is only when the wind is on the beam in the slip, and like 8-12 mph. Just FYI

But, if it is as easy to wrap a line around the mast, I will try that with my spinnaker halyard. . .at least it will have somewhere to go when at dock lol. . .
 

capta

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Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
Just out of curiosity, how loose are your lee shrouds when you are beating? Does your mast pump when you are sailing, too?
Wow, pretty scary to me that your mast is pumping in the slip.
 

BillyK

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Jan 24, 2010
502
Catalina 310 Ocean City, NJ
Just out of curiosity, how loose are your lee shrouds when you are beating? Does your mast pump when you are sailing, too?
Wow, pretty scary to me that your mast is pumping in the slip.
I've gone from loose to tight, there simply hasn't been a tune i've been able to set that eliminated the pump. Even with a significant bend in the past she'll still pump, and i don't see any way around it. Other than to break it up with the halyard and another heavy line wrapped around it.
 

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
I've gone from loose to tight, there simply hasn't been a tune i've been able to set that eliminated the pump. Even with a significant bend in the past she'll still pump, and i don't see any way around it. Other than to break it up with the halyard and another heavy line wrapped around it.
Did you add the proper amount of 'pre bend' to the mast? on your single spreader rig you SHOULD have about 3/4" of forward 'bow' to the mast. This will radically change the 'natural frequency of oscilation' (a 'harmonic frequency') of the mast and reduce the 'sensitivity' to or to stop 'mast pumping'.

http://www.riggingandsails.com/pdf/selden-tuning.pdf if you need directions on setting up proper 'prebend'.
FWIW, your mainsail was cut expecting this 'prebend' and if your mast is without it, the mainsail will be 'too full' in draft.
 
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