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Old 28-01-2007, 19:49   #1
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Kevlar over fiberglass

Just curious: Is it practical? Would it provide a strong sheath over a fiberglass hull? How many layers of kevlar matt would be required? Is there a special resin?

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Old 28-01-2007, 20:32   #2
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I don't know any specific answers to your questions, sir, but I will say that Insatiable has a hull that is a combination of fibreglass and kevlar. As far as I have been able to tell, there is a single layer of fairly heavy kevlar cloth throughout the whole hull. There is a 2nd layer of kevlar below the waterline. There are a couple of lightish layers of fibreglass outside the kevlar layer. Inside the kevlar layer there are a couple of layers of fibreglass above the waterline and a lot of fibreglass below the waterline.

I have had to lay some glass over the kevlar in places on Insatiable, and have managed to do so with no problems using vinylester resin. I think that vinylester is the recommended resin when working with kevlar. I guess this begs the question of what type of resin was used on the fibreglass hull that you are planning to sheath... if it was vinylester, you have no worries, if a different resin, seek professional advice.

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Old 28-01-2007, 20:43   #3
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Kevlar has some unusual properties that making working with it difficult. It's lighter in density than resins, so it tends to "float" on top of the resin. Therefore vacuum bagging is always recommended to push it into place while the resin cures. It fuzzes big time (doesn't abrade cleanly) so it's often covered with a layer of glass so you can sand the glass without making it fuzz up.

No "special resin" but I'd use epoxy only. Kevlar is weak in compression compared to glass but stronger in tension. So Kelvar on the outside of hull panels would actually not be very good, as hull panels tend to be in compression on the outside skin due to water pressure.
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Old 28-01-2007, 20:50   #4
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I would be sugesting that the kevlar would actually be better on the inside laminate of a cored hull.

It has no real impact strength, you can poke a sharp stick through a foam kevlar hull easily, a layer of glass will usually be required over the top for abbrassion/impact resistance.

I also would have thought that epoxy would be more suitable for kevlar's strength, just like carbon and epoxy are a better match.

It would be a waste of time and $$$ using polyester resin IMHO

I hate working with Kevlar, it's a bitch to get neat when sanded, turn's into little yellow furballs, so definetly glass over it if using outside the hull.

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Old 28-01-2007, 22:44   #5
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I think you would be wasting your time Scott. The main reason for kevlar use in a Glass hull was to give extra rigidity in high load areas. Kevlar has very little stretch along with great tensile strength. So it helps to "stiffin" up load areas where say chain plates meet the hull and where the tensional load of the keel is "pulling" the side of the hull down.
As a fabric, it has excellent abrasion resistance. Hence the issue with it "furballing" when sanded. As a single layer, it would give you absolutley no impact protection at all.
For the racing hulls built entirely out of the material, which is rare today due to Carbon fibre, the real reason was that it could make a very light hull because of the material weight being less than glass cloth and they could get away with less layers due to it's tensile strength. All up making a much stiffer and lighter boat. It is only when multiple layers of Kevlar are used that the impact strength is obtained. Epoxy resin was the usually resin of choice with the racing hulls due to issues of working with the cloth as has been stated. It gave a far greater working time, was able to be vaccum bagged and Epoxy has a greater impact resistance than Ester.

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