Kevlar hull repair

Discussion in 'Boatbuilding' started by zember311, Aug 12, 2004.

  1. zember311
    Joined: May 2004
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    Location: FL

    zember311 Junior Member

    I have a friend who has a boat hull that was made of resin and kevlar.

    There are some stress cracks in the hull, nothing major, but in need of repair.

    One boat shop he took it to said they can't work on a kevlar hull.

    I don't know why, i never worked with the stuff.

    I recall hearing at one point that kevlar floats in the resin and needs to be vaccum formed.

    but, if the area around the kevlar was scuffed, and clean,

    could i get away with using fiberglass cloth and epoxy to patch the areas?

    1 reason, he does not want to spend the price on kevlar,

    2 I have a huge stock of epoxy/fiberglass cloth left.

    would htis work out ok?

    thanks in advance for yout responce,

  2. dereksireci
    Joined: Jun 2004
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    dereksireci Senior Member


    I never repaired kevlar either but I will share what I know. Kevlar is best suited for use in laminates in areas which may encounter inpact, like the keel. You need special tools to work with it. If you take a grinder to it, it doesn't grind away but rather creates a giant kevlar brillo pad. Show it to somebody who has experience working with it. I don't think the issue is with the materials used to repair it, rather the method to prep the existing part.
  3. zember311
    Joined: May 2004
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    zember311 Junior Member

    I found some information on this topic

    Q: How do I repair my Kevlar canoe? Need more Help?
    Name: Default Admin (admin)

    A: To successfully repair a hole in a kevlar canoe you should sand blast the break. Grinding will abrade the cloth enough that it will very fuzzy and difficult to repair. Sand blasting makes the surface ready for the patches. Sand blast a good inch or two on each side of the break both inside the canoe and outside of the damaged area. Use the West System brand epoxy to lay your repair patches in. On any kevlar repair patches use about a 6oz. glass cloth on the surface. This will give you a surface that can be sanded lightly. Brush the epoxy mixture onto the sand blasted area and then cover it with the cloth making sure that the cloth is well saturated with the resin mix. Cover with clear plastic (thin (2 mil) vapour barrier) stretch tight and secure with masking tape. Work out the trapped air bubbles. The epoxy won't stick to this type of plastic. Wait a day until the epoxy has set. Sand to get it smooth then refinish with paint only, don't attempt to use gel coat. Do the same for the inside but use kevlar cloth instead of glass cloth. For major repairs (canoe broken in half and several splits 3 feet long or more) it is best to leave it for the professionals as the canoe will need several batten strips to keep it in alignment.

    Gonna give it a try
  4. mistral
    Joined: Jul 2004
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    mistral Senior Member

    I don't think it's a good idea to use fiberglass (E-glass or S-glass) to fix your hull; I suppose it's a quite high performance boat; anyway, fiberglass and kevlar have dramatically different mechanical properties, so your hull will keep on cracking with your glass patches 'cause of the different stiffness between kevlar and glass, the patches simply won't be enough stretched to develepoe their strenght.
    Other tip: never underestimate the delamination on carbon and kevlar hull, they have a unpleaant tendence to sudden failure

    fair wind
  5. Joe Kenkel

    Joe Kenkel Guest

    Does anyone know how to tell if your boat hull is kevlar or fiberglass? I have a 1988 Concord CC-7. It seems I have heard somewhere that the hull has kevlar in it. I need some minor repair and want to make sure I am getting it fixed properly.
    Thanks in advance.
  6. lucas adriaanse
    Joined: Sep 2004
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    Location: netherlands

    lucas adriaanse

    Hey guys, don't be too scared about working with Kevlar. What zember311 says about sanding Kevlar is true. His sand blasting tip might be a good solution, I'll try it next time. How we do it is (still) by sanding with an electrical powerfile, letting it turn/run from the outside to the middle of the hole/patch. This way you do not get the fluffy stuff at the exact spot you want to be putting your new laminate to. If any small threads stick out however we cut them away with very, very (new) sharp razorblades.

    Mostly Kevlar in yachts is used in between layers of e-glass. In fact you should NOT laminate Kevlar straight onto a foamcore, since it will shear. So, a combination of both aramides (Kevlar/Twaron/carbon) and e-glass is not a problem. The use of e-glass in between 2 layers of Kevlar is even favourable, since the glass stretches more than Kevlar it makes for less chance of delamination of the layers. (as long as it stretches, it does not break ...)

    Kevlar and Twaron 'float', or better: you won't get the core of the threads wetted. Not even by vacuum infusion. That's why it is such a fluffy mess when you start sanding it down. Of course vacuumtechniques help, but even carefull hand lay up with a lot of squeegeing or de-aerating with a roller will do the job.

    We have been using Twaron (the other 'Kevlar-brand') on all our yachts' hulls since 1995, to make them more impact resistant. Several layers in bow and slamming area, but also all-over. In places of compression (such as decks) it is better to use carbon; that's why spinnakerpoles are carbon, not Kevlar.

    Conclusion, based on the little info about the stress cracks received: e-glass NO PROBLEM

    To find out if there is Kevlar in there ? Drill a small hole. If your find light-yellow dust/grit on your drill, you have struck ............ Kevlar.

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  7. rbickle
    Joined: May 2008
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    rbickle New Member

    I have an inexpensive Mohawk Kevlar canoe that has developed cracks from running over logs in coastal rivers. I patched in once with fiberglas but it cracked again this past weekend. I do not have access to sandblasting gear or a powerfile. Am I doomed?

  8. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    As was stated, most hulls are a combination of fibers if they say Kevlar or Carbon construction. Neither product is that good by itself and epoxy should be used for best results. Many Kevlar or Carbon Hulls have only a few strips of the fabrics in them, just enough to advertise it, but it's of little real value.

    Not knowing the actual construction of his boat and if there's even any Kevlar in the area that needs to be repaired, I would just sand a small area and see what you find, either way you can fix it easily.

    As for the canoe, did you use epoxy or polyester resin to repair it? A good kevlar canoe should be made with epoxy, so if you repaired it with polyester it won't hold up.
    1 person likes this.

  9. callum.boase
    Joined: Jul 2009
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    callum.boase New Member

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