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Old 09-08-2017, 12:47   #1
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Kevlar reinforced GRP?

I have been mildly surprised to find very little information on the interweb about using kevlar to reinforce GRP in hull construction. I know Jeanneau have done some boats with some kevlar reinforcement, but it is by no means widespread, and there is not that much information about the pros and cons.

So why not? Seems on the face of it an attractive solution for a reasonably rugged cruising boat - the kevlar would resist penetrative sharp objects and the GRP would provide good resistance against blunt impact - a bit like safety glass in car windscreens. Possibly cheaper than aluminium, without the aesthetic challenges of aluminium, and with no long term corrosion risk.

So what are the pros/cons? Is it difficult to ensure there is no delamitation in the long term? Are the costs still prohibitive?
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Old 09-08-2017, 13:33   #2
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Re: Kevlar reinforced GRP?

The only disadvantage is price. I believe Bavaria was the first from the mass production shipyards to use it on the bow area, on the area that would be more subject to the contact with submersed or floating objects.

Very expensive boats are today made with many different fibers (combined) that are used according with their properties on different parts of te boat to maximize their potential. Kevlar is normally used on the hull areas more prone to hit objects.
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Old 09-08-2017, 13:52   #3
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Re: Kevlar reinforced GRP?

IIRC Minaret has commented on this subject in the past. Some digging might scare those knowledgeable posts up.

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Old 09-08-2017, 14:04   #4
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Re: Kevlar reinforced GRP?

Kevlar is good solution. It adds a lot of impact resistance to fibreglass.

However, it is difficult to work with and repair. It does not sand, and does not wet out well.

I think a significant layer of Kevlar (preferably with epoxy resin) is a great addition, adding valuable impact resistance. My only concern is I suspect some manufacturers add just a minimal layer of kevlar, which especially with polyester resin can add more problems. The Kevlar looks good in the advertising brochure but the danger of this sort of token addition is that the real world the impact resistance will only be marginally better, but the kevlar makes repair more difficult and delamination more likely.

It would be nice if manufacturers published details of the layup. In this way it would be clear if the kevlar is real attempt to significantly improve the impact resistance of fibreglass.
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Old 09-08-2017, 14:09   #5
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Kevlar reinforced GRP?

I believe you can get a very similar outcome with just a little more glass, the penalty of course is weight.
Carbon fiber has been for years a great marketing tool, all kinds of fake carbon fiber laminates are put onto things to look cool, carbon fiber sells.
Best flight helmets are a mix of fibers, same for motorcycle helmets, often carbon fiber, Kevlar and glass of course.
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Old 09-08-2017, 14:14   #6
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Re: Kevlar reinforced GRP?

Here are some of Minaret's past comments on Kevlar:

"Kevlar does not provide strength. It is the least stiff of all the fibers used in laminating. If you build a laminate of solid Kevlar, it will be floppy like a noodle. It is only good for point loading, ie you can't make holes in it. It's one of the more common misleading things I see printed in glossy brochures. It is also almost impossible to make repairs on. Not a good thing. The vast majority of boats I have worked on with Kevlar in the layup, it has been strictly a sales gimmick."

(Crosstalk about Kevlar goes on for a couple of pages after that. . . .)

"I have done a lot of construction in carbon/kevlar, and I can tell you that kevlar and other aramids are almost never used in a laminate by themselves. Kevlar provides stellar resistance to abrasion and point loading, but very little stiffness. That is why it is almost universally used as an outer skin laminate only, and is also why it is almost always combined with carbon fiber. Carbon compliments it quite well, because where carbon is stiff and brittle kevlar is flexible and impossible to poke a hole in. Particularly for a cat, this would be a terrible construction material to use on it's own. Most of the boat needs to be a traditional laminate or better to make up for the lack of stiffness in the kevlar lam. Laminating in kevlar also requires special methods. It is almost always vacuum bagged in epoxy, with the kevlar buried between plies of S-glass or carbon. The reason is that you can't sand or grind kevlar-if you do it turns all fuzzy and becomes a real nightmare to work with. It's tricky stuff to use in a big build. Try talking to some builders who are currently using it instead of asking a manufacturer who just wants you to buy more product."
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Old 09-08-2017, 14:35   #7
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Re: Kevlar reinforced GRP?

Kevlar its mostly abrasión resistance , then a boat made of kevlar sándwich composite is dificult or imposible to repair if is seriously damaged , simple as if you try to grind kevlar the result is a smooth Surface,, i see a kevlar race boat totaled when holed in a local regatta .. for local áreas of the hull could be ok like the bow section in front of the FWD trailing edge of the keel , or in rudders, and also Carbon its best suited for boat construction , i cant see any reason for use Kevlar this days... Cheers.
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Old 10-08-2017, 16:13   #8
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Re: Kevlar reinforced GRP?

This one seems interesting, I don't know if it can be reasonably applied on sailboats:

Hitting a log or some sharper marine garbage might just inflict that "point damage". But running aground on a sand bank I'd be glad to loose some glued protector not parts of the hull/keel.
Useful as a fireproof bottom paint...
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Old 11-08-2017, 05:11   #9
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Re: Kevlar reinforced GRP?

in 84 or so I saw a 33-35' Crowther cat in Hemmant, QLD, being built in foam/epoxi/kevlar. seem to remember the guy was called Geoff Cruse
exterior-skin layup was triaxial kevlar (I'm 99% sure)
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Old 11-08-2017, 06:23   #10
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Re: Kevlar reinforced GRP?

The outer skin of my boat, forward of the keel, is made of Kevlar laminate.

According to the designer, Bill Dixon, the purpose is to increase resistance to a hull breach in case of collision (and there's a watertight bulkhead in the bow as well).

AFAIK Kevlar laminates provide no other significant benefit other than resistance to penetration. I believe what Minaret wrote.
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Old 11-08-2017, 09:07   #11
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Re: Kevlar reinforced GRP?

Kevlar was a stopgap solution to a problem dreamed up to help people get around class rules that prohibited carbon and not really practical experience.

Kevlar is pricey, absorbs moisture, and has poor compression strength. It is awful to try and build with because it floats. The only benefit it offers over glass is impact resistance. Designers had to make the laminate significantly thicker because of how poor compression Kevlar offered.

If you're concerned about impact, build a foam filled bulkhead or a steel boat.

The only boats built with Kevlar for a period of time were done so because carbon was banned as a cost containment/class design rule.

Basically it is crap and has no business on a boat anymore because carbon is so much better in performance AND also cost.

As for hybrid laminates that try to weave carbon w/ Kevlar or glass, it is a silly idea because it is inefficient. The higher modulous material ends up carrying the load - so why have the lower modulous material?

As for actual robustness? Kevlar works in a vest or puncture resistance tire because the fibers are strong and relatively flexible and absorb the impact. But that only works when the fibers aren't laminated in epoxy. Built a boat with Kevlar in epoxy, take a sledge hammer to it - yah the fibers aren't breaking but the hull is still broken and sinking.
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Old 11-08-2017, 12:34   #12
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Re: Kevlar reinforced GRP?

I always thought Kevlar hulls were just a marketing gimmick, playing off the general public's familiarity with Kevlar's use in body armor.

Kevlar has many strange properties, some of which might make it a less than desirable boat building material. A level III-A vest made of Kevlar will stop a .357 magnum round, but an ice pick will easily go right through it.
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Old 11-08-2017, 12:46   #13
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Re: Kevlar reinforced GRP?

In aviation (gliders) kevlar is only used to take energy if you chrash. It also keeps the carbon from exploding in 10.000 pieses. Kevlar has no other or bether carateristica than fiberglass
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Old 11-08-2017, 13:25   #14
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Re: Kevlar reinforced GRP?

Hunter reinforced many models with Kevlar from the stem to the keel sump.
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Old 11-08-2017, 13:43   #15
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Re: Kevlar reinforced GRP?

Originally Posted by gumme View Post
In aviation (gliders) kevlar is only used to take energy if you chrash. It also keeps the carbon from exploding in 10.000 pieses. Kevlar has no other or bether carateristica than fiberglass
That is a good example of what I already said. Today top boats are built with a variety of fibers, including kevlar and carbon used in different places and according to different physical needs.

Today top shipyards and top Architectural cabinets include on their staff composite engineers that are the ones responsible for the composition of the different fibers used, working closely with structural engineers.

We are still at the beginning and for now only top brands mostly specialized in performance boats use them extensivelly but the future will go that way.

The use of a out layer of kevlar on the bow areas is already very common today as a way of increasing hull resistance to shock and perforation. It is no publicity gimmick, it is a way of making a better boat.
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