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Old 30-06-2009, 16:29   #1
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Mast Pumping - Is This Normal?

Hi Every one
I need some advice regarding mast pumping if I may call it that.
When at anchor and tied up to dock if there is a strong wind from aft
I feel the boat shudder, hard to describe. I have 2 back stays and it feels like someone would be pulling the back stays and letting go which would be transmitted to the Mast which I suspect would transfer up and down movement to hull. I have tightened forestay and back stay turnbuckles thinking they were not tight enough. Then I thought maybe this is normal and most cursers get used to it or should I be concerned. Any advice would be welcomed.
Also when my main is reefed and the wind is puffing and I look along the length of mast I can see the mast flexing. I know people use running Back stays to prevent mast pumping when using Cutter style forsail but have never heard of running forstay when reefing main
Forgive my ignorance
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Old 30-06-2009, 16:54   #2
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I know what you are talking about. It's pretty much got to be rig adjustment. Is there any pre-bend in your mast at the dock? May be best to just do a whole rig adjustment from scratch. I have ahd a rig do that in certain conditons even though I thought it was adjusted well...
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Old 02-07-2009, 10:49   #3
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Sergy,

You have two options:

- get a good rigger to check it out and re-tune your rig. Tell him about the mast pumping.

- buy a good book about rig tuning and study. This is better & cheaper but be prepared to spend a couple of days on it. After that, you know exactly what you're doing and it's easy to learn.

ciao!
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Old 02-07-2009, 10:54   #4
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A baby stay might help.
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Old 02-07-2009, 11:12   #5
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s/v Jedi - any advise on a good book for the rigging info?
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Old 02-07-2009, 12:31   #6
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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.

Mast pumping, UNDER SAIL, requires Rig Tuning.
See the discussion, and my links at
Rig tuning
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Old 02-07-2009, 15:08   #7
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Quote:
Originally Posted by tongabound View Post
s/v Jedi - any advise on a good book for the rigging info?
Illustrated Sail & Rig Tuning by Ivar Dedekam. ISBN 1 898660 67 0

This is a thin book with all the information plus explanations. I never saw a better book, no matter how thick.

The things to pay special attention to are:

- a little rake aft is good. However, this means adjusting the base of the mast when the rake isn't there. This might be easy to do or not. How much rake and how to measure that is in the book (bucket with water hanging from main halyard, measure distance between bucket & mast etc.)
Rake is adjusted with fore and back stays.

- at least some bend. And I mean a bend to the aft (yes I did see masts bend forward). Adjusting bend is different for every rig-type. If you have square (not angled aft) spreaders, you use cutter or baby stay or forward lowers in combination with back and fore stay.
With aft swept spreaders, the capshrouds (the ones that go all the way to the mast head) do the same. As they get tensioned, the spreaders push the mast forward.

- capshroud pre-tension. This one is forgotten by so many while it's the easiest to do. The book has a very clear explanation on this. The easiest way to measure this tension is by measuring the stretch of the wire. This is from memory: with slack capshrouds, measure and mark a 2 meter section. Now tension until that section is 4 mm longer. Now it is tensioned to 20% of breaking strength of the wire, regardless of the diameter of the wire. With spreaders that are angled aft, this creates the pre-bend in the mast. The lowers counter this bending but can often only be tightened when capshrouds are very loose so it's quite a job. A good rigger has a feel for this and can do it in just one or two tries.

- when in the marina, it is good to take the tension off the backstay and the jib halyards.

- check that the spreaders are okay: the capshrouds should have the same angle to the spreaders (under and above the spreader). This is almost never obtainable but pointing a little up is better of the spreader allows that. Pointing down is the recepy for loosing the mast. Spreaders are only designed to take compression forces.

- when you have the "open-style" turnbuckles where you can see the threaded fittings, use a caliper to measure the distance between the fittings at 0.1 mm precision and keep that as reference for the next time. Much better than tape markers that shift or disappear.

Like Gord writes, pumping while not sailing isn't really a problem. While sailing it is a big problem and the cause for many dismastings. Spreader failure is high too plus of course chainplate failure.

ciao!
Nick.
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Old 03-07-2009, 00:34   #8
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Thanks every one.
I will be going around Vancouver Island in 2 weeks so I will get busy and tune from scratch
JEDI I have purchased the said book and will give it a go.
I am hoping that I can achieve this without having to touch the forestay but only adj. The 2 back stays
MY PERTICULER ARRANGMENT GETS COMPLICATED TRYING TO GET TO THE Forestay turn buckle.
(I have to remove the roller furling sail then partly dismantle the old Hood furling in order to get to the forestay turnbuckle )
Gord I will also try the fender idear if Tuning does not correct.
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Old 03-07-2009, 21:05   #9
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Sergy,

The forestay is only used for initial rake adjustment and doesn't have to be touched after that.

I would advise the following:

1. find a spot without waves & wakes. Anchor or moor.
2. measure and document settings of all turnbuckles you're gonna touch.
3. loosen all but the forestay... just so you feel the tension is off.

now check your rake like the book explains. If the bucket touches the mast, use a big wrench. Also test on a jib/spi halyard. When the mast is really raked forward, you need to loosen the forestay (tighten the backstay to compensate) or shift the base of the mast forward. These two options are not the same, they change the balance under sail. Also, when changing the rake of a keel stepped mast: you must take out the blocks/wedges between mast and deck first. But I think the book explains all that too.

If the mast is straight up or raked a little aft, you're good and don't have to touch the forestay nor move the base of the mast.... if you are happy with the balance ;-)

Also: get a jar of Lanolin (I think that's what it's called). It's from forespar. Use this on the threads of the turnbuckles to prevent galling. Clean (small stainless steel brush) and lube these threads before putting the wrench on.

cheers,
Nick.
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Old 13-08-2009, 19:05   #10
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Quote:
Originally Posted by s/v Jedi View Post
Also: get a jar of Lanolin (I think that's what it's called). It's from forespar. Use this on the threads of the turnbuckles to prevent galling. Clean (small stainless steel brush) and lube these threads before putting the wrench on.

cheers,
Nick.
You can also order/get Lanolin from some pharmacies.
Pretty cheap.

Extemp.
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Old 13-08-2009, 21:00   #11
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Okay, I just used the stuff today so I know the real name again: LanoCote, by Forespar.

cheers,
Nick.
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