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 Post subject: Kevlar on a bottom job?
PostPosted: Fri Jan 02, 2015 3:14 pm 
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Joined: Mon Sep 15, 2014 12:34 pm
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Location: Hartland, WI
Has anyone else used Kevlar on a bottom job over fiberglass?
I repaired a Hobie 18 hull for my brother which was worn and damaged at the bottom. After reshaping it with fiberglass and polyester resin, I put a 4" wide strip top layer of Kevlar tape along the bottom. It had to be stretched and held tight to the hull with masking tape and then resin added. It wasn't possible to feather the edges of the Kevlar after it set up, even with a metal file. Luckily it was down tight. I had to fill the edges with resin as filler. After gelcoat was sprayed and wet sanded, it came out pretty nice. This one was for practice, as it will be mated to a newer hull.
So now my brother wants me to do his other 18. I would need to strip the gelcoat along the bottom and lay the Kevlar. What could I add to the resin to make it like a puddy filler for the edges? I will be going over the whole hull with gelcoat on this one to freshen it up. The Kevlar won't be running the full length, but rather the main wear area. The idea is he'd never have to worry about wearing through the glass on his sandy beach.

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PostPosted: Sat Jan 03, 2015 3:51 pm 
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Joined: Mon Jul 14, 2003 7:11 pm
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Location: Detroit, MI
Kevlar has a very high tensile strength but is not very abrasion resistant. It fuzzes up when it's exposed and you can't sand off the fuzz - it just gets worse. Not good long term on a bottom job.


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PostPosted: Mon Jan 05, 2015 7:12 pm 
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I believe the Canadians put Kevlar on their yellow H18 when they sailed/dragged across the Northwest Passage in 1988.


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PostPosted: Mon Jan 05, 2015 7:56 pm 
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Location: Detroit, MI
HRob wrote:
I believe the Canadians put Kevlar on their yellow H18 when they sailed/dragged across the Northwest Passage in 1988.

That was for impact/penetration resistance (which Kevlar is very good for), not abrasion resistance.

From page 13 of the paperback edition of Polar Passage:
"I named her Perception, and before she left the factory the bottoms of her bright yellow hulls had been reinforced with thin strips of bullet-proof Kevlar plastic, which I hoped would make her impervious to holing by ice when sailing. To protect her and ease her way when she was being hauled over the rough ice, we'd made a set of thin plastic runners, which strapped onto the banana-shaped hulls like a hockey player's shin pads."

Scary how fast I found that. My basement's more organized than I thought.

Anyway, the most cost effective, abrasion-resistant material that won't fuzz up when it's exposed by abrasion is . . . s-glass.


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PostPosted: Tue Jan 06, 2015 8:28 am 
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Location: Jersey Shore
I agree, just use glass and polyester resin. Kevlar, as you found, will not be able to be sanded/faired. It is also hygroscopic, which means that once the resin/putty/filler is worn away and the raw Kevlar is exposed, it is going to soak up water like a sponge. Not a good application for this material.

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PostPosted: Tue Jan 06, 2015 4:39 pm 
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What about carbon fiber tape? ( assuming the black doesn't bother you )
Anything tougher than plain fiberglass assuming your not going to fair it or paint it in any way?

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PostPosted: Tue Jan 06, 2015 5:26 pm 
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Fxloop wrote:
What about carbon fiber tape? ( assuming the black doesn't bother you )
Anything tougher than plain fiberglass assuming your not going to fair it or paint it in any way?


Carbon fiber tape would be a waste of money (and good CF tape). Like Kevlar, carbon has exceptional tensile strength, but poor abrasion resistance. Unlike Kevlar though, it won't fuzz up - the fibers will break off at the surface.

Check this out: http://www.ewp.rpi.edu/hartford/users/p ... astics.pdf

The plastic with the best wear resistance? Ultra-high molecular weight polyurethane (UHMWPE). You can get adhesive-backed UHMWPE film from McMaster-Carr - http://www.mcmaster.com/#uhmw-polyethyl ... ts/=vcrr0h. The adhesive would probably give out long before the film did (edges would start to peel).


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PostPosted: Mon Jan 12, 2015 4:31 am 
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Joined: Wed Mar 31, 2004 7:20 am
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Location: New Brighton, PA
Check out 'West System' epoxy for bottom repairs. Much stronger than normal resin, you can add color pigment and silica thickeners to come out with a very durable repair that is stronger, takes a lot to wear down and keeps it's color.

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PostPosted: Mon Jan 19, 2015 7:58 am 
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Joined: Fri Dec 28, 2007 11:23 am
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Location: Lake Norman NC
My quick and easy bottom jobs for the last 25 years have been done with a putty knife and Marine tex. Unless you are in the top of A fleet racing this is the way to go. Prepare surface exactly to directions Mix exactly to directions and pick a good warm day. Worried about white on the hobie bottom well the only time it is noticeable is when you are turned over. Tape around the wear area of the bottom and have at it no flipping the boat no working for days on many layers of cloth and resin no sad time at the beach seeing all your hard work just come off.
Former Hobie Admiral Gary who always carries around the large kit of marine tex in the Hobie hell box


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