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Kevlar is an interesting product.

Can people tell us about their previous experiences/knowledge ?

Also thoughts about going forward as a multihull building material ?

 

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horrid crap that is really hard to work with as it is really hard to cut, difficult to wet out, gets furry when you sand it, and sucks up water. My boat is kevlar and I would never use it again. for anything. ever.

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If you don't know how to use Kevlar, it can be problematic. When properly used with carbon it's the best of both worlds. There is a product that is both a carbon and Kevlar woven cloth.  I won't go into the properties of Kevlar and carbon, but I built my race bike out of 6oz Kevlar and 6oz carbon and it is a world record holder. I also got hit by a truck from behind and the combination of Kevlar and carbon saved my life.

 

https://www.youtube.com/watch?v=6FAzcEPMi14

PL.Coyote.jpg

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I used to be a fibreglass laminator, what basketcase said. 

Good alternatives are carbon, Sglass and maybe basalt depending on the application.

Thats not to say it doesn't have its place, just be aware of the downsides.

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Limited compressive properties, but as others note, used correctly it is unique. My kevlar F40 was washed up on rocks during previous ownership, was repaired, now weighs 10 kilos more with about 1/3rd of the underwater portion of the port hull replaced. Had that been carbon, there would not have been much left to bond to.

The right combination of carbon and kevlar makes for some very strong, stiff, and resilient laminates. Working methods and equipment need to be appropriate, but the nature of the cut fibres means that secondary bonds are relatively strong. Always use compaction in some form, but then you always should for carbon too.

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I'm far from a pro on this, but adding a layer of Kevlar to a glass/eglass/carbon laminate improves the failure modes /radically/. If we're going to bump into anything, I hope there's kevlar in the layup. 

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When building my powerboat with a 100hp outboard engine, I was told a tail of thieves cutting out the transom with chainsaws to get the engine off.     I buried some layers of poorly wetted out kevlar uni into the layup of the top edge of the transom....... lets see what happens when some arse has a go at that with a chainsaw!

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These days I'd never use it for much. As Russell says, it has poor compression strength.

A single layer on the inside of a sandwich panel does improve Puncture resistance.

 

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4 hours ago, Zonker said:

These days I'd never use it for much. As Russell says, it has poor compression strength.

A single layer on the inside of a sandwich panel does improve Puncture resistance.

 

Why on the inside? My Catana has it all over the inside

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Kevlar is very difficult to cut, you need special scissors to get clean cuts. While it has some unique properties making it the number one choice for body armor, these properties are significantly lessened when used in a laminate. However, the biggest downside is that it takes up water to the extent that it blows up the matrix which means pure kevlar laminates usually have a limited lifetime as damage slowly spreads.

I have no experience with hybrid materials, but I would not use kevlar in the outer layer(s).

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Yes, what ProaSailor said. Because Kevlar is strong in tension, you want it on the inside skin as a puncture proof layer.

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Eric Sponberg’s post is worth a read:

https://www.boatdesign.net/threads/general-question-mixing-kevlar-carbon.40016/
 

And particularly this:

“I use Kevlar only as a bullet-proof layer (which it truly is) near the core on the outside skin. It prevents impact of hard objects into the core, but buried deep within the outer skin, it is not subject to miscellaneous abrasion damage. The amount is small enough that it need not be balanced on the other side of the core. I steer away from Kevlar otherwise because it is so poor in compression strength. And boat hull panels do experience as much compression as tension as they bend. Laminates either side of the core should also be mirror images of each other with respect to sequence in lay-up and orientation. You maintain those basics, and you can start your engineering of the detailed laminates.”

 

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Have to work with the stuff quite often and hate it with a passion .

As regards to putting it only on the inside skin ,I'm at odds with that one .

Once the outer skin and core are punctured the inner skin of Kevlar releases and  just becomes one big delaminated piece of crap offering up nothing.

If i have to use it I always sandwich it between outer laminates to try and limit any sort of intrusion from the get go .

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Have not seen it used much recently (if at all) in boat construction, besides small boats that take alot of dock/beach abuse. In this application it is excellent.

 

I have used it in 100% and 50/50 kevlar/carbon for "non-structural" panels that get alot of abuse (floorboards, wash board, hatches, setee inserts) and it has held up very well. As others have said it doesn't do post-cure anything very well, and it difficult to work with.

 

HW

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One of the other issues with Kevlar that hasn't been mentioned here is its ability to actually bond well in a laminate. Basically it has poor inter-laminate adhesion, at best. The various coatings (Silane and Volan) are intended to aide in the base fibers ability to absorb resin, but in the case of Kevlar, it is a band-aide at best. This is well confirmed by cutting a carbon/kevlar/carbon laminate, and noting how dry the inner kevlar fibers are. As noted above, this issue coupled with the poor compressive strength of the material make it a non-starter for most composite construction, with the exception of an abrasion or impact layer as discussed above.

That being said, I wouldn't write-off an older boat simply because it has kevlar in the hull, but I would want an experienced race boat surveyor/composites technician involved in the purchase.

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The Volvo 60's were entirely Kevlar and a bit of e-glass. They survived a few laps of the planet.

I don't see any issue with bonding. Hopefully if you use Kevlar you vacuum bag the laminate because you are using a cored construction.

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8 hours ago, Zonker said:

The Volvo 60's were entirely Kevlar and a bit of e-glass. They survived a few laps of the planet.

I don't see any issue with bonding. Hopefully if you use Kevlar you vacuum bag the laminate because you are using a cored construction.

My understanding where Kevlar “wins” is in helping to protect the core from damage/failure and in the event of sufficient core damage/failure/delamination, a pair of thin Kevlar skins is usually the best “survivability” solution, hence its extensive use in boats like the Volvo 60 which have a much higher risk of suffering  from impact damage in ice strewn waters and from repeated slamming down off liquid mountains.

No question that Kevlar is a pig to work with and repair or modify……

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The only reason the VO60's used Kevlar is the rule didn't allow carbon. As soon as carbon was allowed, they all went to that because it is lighter.

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Thanks to all who answered here (especially the pro's)

I (and maybe a few others) have learned something here.    B)

Now as a final,       what are your thoughts on the carbon/kevlar weave ?

 

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1 hour ago, peterbike said:

    what are your thoughts on the carbon/kevlar weave ?

 

 

As a user only guess while thinking in the way  (+) of Kevlar  + (+) of Carbon makes it "+ +" we  should not forget of (-) + (-) gives  "- -" :rolleyes:

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I don't think carbon/kevlar weaves are smart. They do look nice though. But I think they are relatively costly compared to all carbon or all kevlar wovens.

Carbon has roughly twice the elastic modulus as kevlar fabric. So the usual problem with mixed fabrics is the carbon takes almost all the load and the kevlar is mostly just along for the ride. Yeah it probably adds some "toughness" against impact but it's probably smarter to just add a single layer of Kevlar for puncture / impactness.

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48 minutes ago, Zonker said:

They do look nice though.

only for a little while. it was a big thing with the car guys. yellow, red, blue. neons. problem is it fades too fast. next big thing with those guys id 'forged' carbon. basically carbon short chop strand matt. funny as fuck. looks like granite.

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I’ve always wondered if Kevlar veil might make a good sheathing for plywood hulls. 
So, where you are going for as light as possible and not sheathing with a glass cloth some builders just go with 3 or so coats of resin. What if you just used one soak coat and a layer of veil, it might not be quite as light but it would give the surface some bash ability and protect joins, prevent checking etc and increase service life considerably ?

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Kevlar has very low density. Lighter than resin. So it sort of floats in the layer of resin. It really needs to be vacuum bagged. It is a pain to work with. 

And when you wear into it or scrape it you get a fuzzy surface. It doesn't sand well.

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6 hours ago, Redreuben said:

I’ve always wondered if Kevlar veil might make a good sheathing for plywood hulls. 
So, where you are going for as light as possible and not sheathing with a glass cloth some builders just go with 3 or so coats of resin. What if you just used one soak coat and a layer of veil, it might not be quite as light but it would give the surface some bash ability and protect joins, prevent checking etc and increase service life considerably ?

There are other fabrics, such as Dynel, which can be used in lieu of Kevlar. The good and bad news:

http://www.oneoceankayaks.com/Abrasion.htm

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14 hours ago, Sidecar said:

There are other fabrics, such as Dynel, which can be used in lieu of Kevlar. The good and bad news:

http://www.oneoceankayaks.com/Abrasion.htm

Dynel uses massive amounts of resin, massive. 

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On 7/6/2021 at 12:14 AM, Zonker said:

Kevlar has very low density. Lighter than resin. So it sort of floats in the layer of resin. It really needs to be vacuum bagged. It is a pain to work with. 

And when you wear into it or scrape it you get a fuzzy surface. It doesn't sand well.

I repaired the inside of race car wheel arches that were Kevlar, you can sand them nicely with wet&dry but of course you then have to heat gun them to get the water out ! But it works well. (Enough)

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