mast repair

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Scott
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mast repair

Post by Scott » Fri Jul 06, 2012 8:21 pm

Unfortunately Toby and I snapped his mast this week, sailing on Lake Mendota in Madison, WI. It's a Willets rig for B3 1135. It broke fairly cleanly at the GNAV attachment, and I think it will go back together willingly. However, having never attempted a mast repair before, I figured we had best gather as much advice as we can before we start.

I was lucky enough to watch Hendo do his magic on Joe's mast at San Diego NOOD in 2011, so most of my assumptions are coming from that. I'm thinking 12-18" or so internal sleeve, and then grind enough original material off the outside to get a couple layers of carbon on the outside. Does that sound about right?

How much wall thickness do we want for the internal sleeve? what should we spec for carbon on the outside layers? Is vacuum bagging warranted (we do have it available), or should we just get a tight wrap with peel ply?

Thanks!
Scott

Dan H
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Re: mast repair

Post by Dan H » Sun Jul 08, 2012 8:04 am

Scott,
the sleeve should be app 12" long if the break is clean and perpendiular to the mast - longer if the break is complex or diagonal.
Most important thing is a tight fit to the inside of the mast tube.
You can get an oversize tube and dart it so that it bends inside the mast.

I would also apply 45deg bevels to each end of the sleeve to reduce stress concentrations

Wouldn't worry too much about the thickness - anything around 1.5mm or more is good - its main purpose is to get the two halves aligned and also provide a substrate onto which you laminate the subsequent structural layers which do the real work.

When positioning the sleeve, after prepping and buttering inside and sleeve of one half with glue (gurit 345 if you have time, fast epoxy with fibres if not) locate one half, then drill and locate a temp spigiot, like a rivet or screw to stop the sleeve wriggling about when you stick the other half on.

Use a (straight/clean) ladder as a jig to align the two halves once joined before taping it all down.

Once you have let that go off, and ground all the damaged material away, then ground at least 8:1 taper either side, you are ready to measure and prep for each of you subsequent structural layers.

How many layers depends on mast and position, and any local reinforcing for shroud attachments. usually I have gobne for 1 layer 400gsm biax, then anything between 4 or 8 layers of carbon uni, depending on tube and positiion. If you can measure local wall thickness, you can assume 0.35mm thickness for a single layer of 300gsm cloth at a decent (i.e. vac consolidated) volume fraction, so if wall thickness is 2.1mm, there are 6 layers of 300gsm material there. You are prob best overestimating and grinding back then applying the cosmetic layer afterwards.

I've never vac bagged - use shrink tape instead - this is quite tricky, and the clacs of your laminate and quality of your taper will become evident when you try and wind the tape on at a constant pitch. Tape gets yellow taped in place before aggressive heat gunning.

Enjoy!

Dan

tlunt
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Re: mast repair

Post by tlunt » Sun Jul 08, 2012 5:40 pm

Thanks, Dan. This is helpful. Although it's a bit of a bummer to have broken the thing, fixing it sounds like good fun. As long as we don't botch it.

Toby

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