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 Post subject: Bowed mast
PostPosted: Mon Jul 02, 2012 7:01 pm 
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Location: Clinton Lake Kansas
OK all you mechanical engineers, metallurgists, and old salts put there, here's the opportunity...

I have a mast that has a nice gentle bow, created due to the boat being blown on to it's side, and laying there for a long time...maybe several months. I've created a jig where I can over bend/bow the mast in the opposite direction and it seems to be working, however very slowly.

I've moved the mast/jig setup to the center of the yard where it gets direct sunlight all day (and for the last 10 days, 100 degree ambient temperature)

My question is, should I release the tension when the sun goes down and reapply first thing in the morning?
Or leave tension on all the time?
Will having tension off in the cooler overnight temps (~75) help speed the process?
Will one method, over the other, speed the process?
Maybe this will just take...several months

Still have a 1.5 inches to go :) :?

Impatient in Kansas

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 Post subject: Re: Bowed mast
PostPosted: Mon Jul 02, 2012 7:07 pm 
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Location: BC, Canada
Is the bend within you comp tip? Or partly at the aluminum to comp tip joint?

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 Post subject: Re: Bowed mast
PostPosted: Tue Jul 03, 2012 9:57 pm 
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jackB wrote:
Is the bend within you comp tip? Or partly at the aluminum to comp tip joint?
no

center of the bow is 2/3 way up the comptip mast...closer to the top

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 Post subject: Re: Bowed mast
PostPosted: Wed Jul 04, 2012 8:16 pm 
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What you are looking at is visco-elasticty of plastics. It is typical to plastics, but not to metals. It is time stress-strain behavior known as creep/relaxation. If you apply constant load for a long time, you see creep. If deformation is constant, then stress in material goes less with time, that is relaxation. Best solution for you is use constant force.

What you are doing is basically correct. If you apply a deformation opposite to the original loading pattern, it will eventually go back to a the original shape, or gets close enough. The trick is to apply force as it was deformed, but from the opposite side. For example, if the tip was resting on the ground, apply main force the same position and direction, but opposite. Otherwise you will end up with a S-shape.

You are also correct with applying heat to accelerate stress relaxation. One issue is direct sunshine, which will also degrade your fiberglass. One trick you can do is to wrap your comp tip with aluminum foil and then spray your bend section with black paint. This way you will protect the fiberglass from UV but keep the heat from the sun.

There is no need to release the load at night. Stress will continue to relax at night, but much slower. You may also adjust the tension every few days to keep it at the same level, but if you have constant load then you fine. More stress, faster the process, but you don't want to push it to close the material fracture.

You are also correct, it is very slow. If the original bend happened in several months, the fix will also take about the same time. Heat and close control over the load will make it faster.

The not so good news is -- your mast will never be the same. There will be residual stresses that will make your comp-tip a bit compromised for both strength and stiffness and shape stability. It is hard to say if it will be critical or even noticeable, but it will be there.

What may also happen -- after your mast looks straight, after a few sailings, a bit of the bend may come back. This is because material still contains some residual stresses and creep/relaxation continues.

Good luck

How about some pictures?

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 Post subject: Re: Bowed mast
PostPosted: Thu Jul 05, 2012 6:47 am 
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J_Eaton wrote:
jackB wrote:
Is the bend within you comp tip? Or partly at the aluminum to comp tip joint?
no

center of the bow is 2/3 way up the comptip mast...closer to the top


Just to be clear, you're saying the bow is in the comptip and not in the lower aluminum section, right?

In that case, I would say just keep the mast bent in the opposite direction of the bow for a while. It should eventually bend back to straight. We see this a lot in composite windsurfing masts. If they are left rigged for weeks or months, a curve will set into the mast due to the constant loading. It can be corrected by flipping the mast around and re-rigging to bend it back the other way.

I would probably just rig the boat up in the yard, rotate the mast so the bow faces forward, then hoist the boom up using the main halyard, and sheet in the mainsheet to bend the mast back the other way. Leave it like that for a week and then release tension and see where it is.

sm


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 Post subject: Re: Bowed mast
PostPosted: Thu Jul 05, 2012 8:52 pm 
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srm wrote:
Just to be clear, you're saying the bow is in the comptip and not in the lower aluminum section, right?
sm
NO, sorry...in my second post I was only stating that yes, it is a comp-tip mast.
(Apparently) the boat was blown on to it's side, tip of the mast on the ground, as I said for a long time. The comptip section is not bowed, and where it joins the aluminum section is not damaged. The bow is in the aluminum portion of the mast, and looking again today, just past half way up the aluminum portion. Since the boat was on its' side, the bow is in the minor axis of the section.

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 Post subject: Re: Bowed mast
PostPosted: Fri Jul 06, 2012 3:39 am 
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In that case, I don't think time or heat are likely to have any effect. Since it's the aluminum section, you would need to bend the mast in the opposite direction beyond it's yield point. There have been many discussions on the forum about bending aluminum masts, most involve using jacks, come alongs, trees, and trucks.

sm


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 Post subject: Re: Bowed mast
PostPosted: Fri Jul 06, 2012 6:06 am 
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srm wrote:
you would need to bend the mast in the opposite direction beyond it's yield point.
from my first post
Quote:
I've created a jig where I can over bend/bow the mast in the opposite direction and it seems to be working,
It's currently overbent, in the opposite direction, approximately 8 inches

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