Which Kevlar fabric?

[dogbrain]

Hi All – I have a question for all of you who have used kevlar fabric in a build. I bought plans for my next build which will be a JEM Northwind, aka Kruger Seawind just like Deerfly built last year. A good friend has a Kruger and I'm convinced. I’m taking advantage of some of the things Deerfly learned with his build including his fabric tests. This boat will be for big waters like ocean, big lakes, big rivers. I want to get rid of my sea kayak, so this will replace that for me. I’m currently at the point where I need to get rid of a boat to get a new boat.

Since I don’t really plan to ever have to portage this canoe very far, I’d like to build it strong without adding any unnecessary weight. I’ll use 1/4” strips and plan to sheath with 5oz kevlar inside and out on the hull with 4 or 6 oz. S-glass over over the outside for a layer to sand. Outside of the hull will be painted white. I’ll use kevlar underneath the deck, which will be constructed using 3/16” strips and possibly finish the top clear, probably using 6 oz e-glass.

My question is regarding which kevlar to choose. It seems that 5 oz kevlar comes in plain weave, twill weave and 4 harness satin weave. Does anybody have a good explanation of the difference in these in terms of strength, ease of wetout or any other important factors worth considering?

Should I add the sandable layer on the inside of the boat (bottom of hull and under the deck) ?

I’m in the planning stages at the moment and will probably won’t actually build the boat until this winter, but I’ll need to make more strips, cut forms and do other prep work as time allows into the fall.

Thanks, Mark

 

[Jim Dodd]

Sounds like one tough hull !

I'm working on a composite hull, for a friend. This AM, I finished wetting out two layers of 5 oz Kevlar.

S-glass. I had trouble wetting out their 9 oz. 6 oz was great !


Bought mine from Sweets, conformed to the tumblehome, and wet out great ! Style 285

https://sweetcomposites.com/

Sweets are great to talk to ! They can recommend to you what fabric to use !

Jim

 

[Jim Dodd]

Looking back at your post.
your question on inside layering, and adding a sandable layer, on top of the Kevlar, I'd say no.

As you said you are not sanding it, and the Kevlar will be fine. I'd fill the weave, to make it easier to get things in and out .

Jim

 

[muddyfeet]

Good short read on weave styles: https://netcomposites.com/guide/reinforcements/woven-fabrics/

For the purposes of a kevlar/wood core canoe, I'm not sure fiber orientation matters much for strength/stiffness. The only thing you probably don't want to do is lay a satin weave inside and out with the warp fibers running in the same direction (both fore/aft). Satin composites should be layered with alternating directions in the stack. Or you could just pick a plain or twill weave for both inner and outer and not worry about fiber orientation.

The biggest difference in canoe layup is probably drapeabillity: how easy does the fabric smooth around compound curves and corners. Twill or satin is better than plain in that respect. The more drapeable it is though, the more unstable and delicate the weave is when cutting and moving dry pieces: (a twill will tend to come unwoven more than a plain). I overcome this by taping fabric along cut lines with 1/4" tape before cutting. This eliminates any fraying. You do have to either make sure the tape is cut off before laminating, or on a trimmable edge of the part, though.

 

[dogbrain]

Looks like Sweets style 285 is the 4 harness satin weave. Good to know it wet out went well Jim. Thanks for all the good info.

 

[dogbrain]

For the purposes of a kevlar/wood core canoe, I'm not sure fiber orientation matters much for strength/stiffness. The only thing you probably don't want to do is lay a satin weave inside and out with the warp fibers running in the same direction (both fore/aft).

HI Muddyfeet. I'm not sure I understand what you said here. Even with a wood core between them you think that the orientation of the warp fibers would affect strength? Are you suggesting laying pieces of the cloth across the width of the hull on the inside? That's what I did on my last build.

Mark.

 

[muddyfeet]

HI Muddyfeet. I'm not sure I understand what you said here. Even with a wood core between them you think that the orientation of the warp fibers would affect strength? Are you suggesting laying pieces of the cloth across the width of the hull on the inside? That's what I did on my last build.

Mark.

Yes. The canoe has inherent stiffness in a bow/stern direction just because of its shape. The weaker axis is side-to-side: so things like ribs/core-floor construction try to stiffen this axis to prevent oil-canning. So make sure at least some fibers are oriented running side-to-side. So any weave fabric will be fine, but be aware that something like a 7-harness satin will have 85% of the fibers running one direction. It probably won’t matter much though, with a multiple ply layup and wood core construction. The major thing you’re looking for in weave selection is drape-ability and ease of handling/wet out. Most people physically lay the fabric in a bow-stern direction (along warp) on the outer layers to prevent seams or overlaps. Laying fabric side-to-side on the inside has the advantage of saving fabric/cut it more efficiently with less waste.

 

[dogbrain]

Muddyfeet, thanks again for the details. I'll likely use the 4 harness satin weave kevlar, 5oz. This is what deerfly used. It's described as having good drapeability and good wet out characteristics. I plan to lay the fabric cross ways on the inside to save some fabric and also take advantage of some extra strength at the overlaps.

Mark

 

[goonstroke]

I also used that 285 satin weave from Sweets on a restore job (new underbelly) and didn't have any trouble with it, and I'm much less of an epoxy jock than most of the guys here. It is slightly more inclined to unravel when cutting it longitudinally, i.e., vertically in the image below [I think that's the warp direction, as opposed to the weft direction, IIRC], but that's managable. It does drape noticably more easily than plain weave.

 

[dogbrain]

Thanks Goonstroke, What did you use to cut the kevlar?

 

[goonstroke]

I used a brand new pair of decent quality regular scissors (Friskars I think). They were OK, but if I was doing a lot of large scale kevlar work I'd invest in something better. Once those scissors lose their edge cutting the cloth becomes a three hand job.

 

[Jim Dodd]

Cutting Kevlar is not fun.
Bought a inexpensive shears from Express Composites a couple of years ago, and they work. Blue handle.
Just checked their site, and I don't see them. you might ask !
https://www.expresscomposites.com/pub/media/logo/websites/1/logo.jpg

Alan has gotten a regular pair of scissors to work, by doing a course sharpening on an old pair. It's the roughness, that makes them work.

Looking at my shears, there is tiny serrations on the blades.

 

[pblanc]

I have used the style 500 plane weave 5 ounce/square yard Kevlar fabric from Sweets and have been very happy with it. I personally can't see a good reason to use a fabric with anisotropic tensile strength characteristics.

I agree with the others that there is no reason to cover aramid on the inside of the hull or underside of the decks.

I use the cheapest, crappiest scissors I can find to cut Kevlar cloth, typically the $3 per pair jobs from Harbor Freight. They work for a while and then I throw them out. Sure, more expensive scissors will work better and probably last longer, but they will still wear out more quickly than you would like.

 

[Jim Dodd]

I have used the style 500 plane weave 5 ounce/square yard Kevlar fabric from Sweets and have been very happy with it. I personally can't see a good reason to use a fabric with isotropic tensile strength characteristics.

I agree with the others that there is no reason to cover aramid on the inside of the hull or underside of the decks.

I use the cheapest, crappiest scissors I can find to cut Kevlar cloth, typically the $3 per pair jobs from Harbor Freight. They work for a while and then I throw them out. Sure, more expensive scissors will work better and probably last longer, but they will still wear out more quickly than you would like.

I stand corrected ! I too used the 500 ! They were out of 285. ( just checked my invoice) The 500 worked great as far as wetting out,and conforming to shape.

I've tried cheap scissors. What I bought from Express composites at the time was only about $12. They were blue handled and relatively short bladed. Used them on 4 canoes so far. Worth the money to me.

 

[dogbrain]

I have used the style 500 plane weave 5 ounce/square yard Kevlar fabric from Sweets and have been very happy with it. I personally can't see a good reason to use a fabric with anisotropic tensile strength characteristics.

I agree with the others that there is no reason to cover aramid on the inside of the hull or underside of the decks.

I use the cheapest, crappiest scissors I can find to cut Kevlar cloth, typically the $3 per pair jobs from Harbor Freight. They work for a while and then I throw them out. Sure, more expensive scissors will work better and probably last longer, but they will still wear out more quickly than you would like.

Pblanc, I didn't see where others said there's no reason to put aramid on the inside of the hull or underside of the deck. My understanding is that kevlar performs best under compression, which is why you would want it on the inside, and why I'm using it there. Care to elaborate?

Here is what I read regarding using something other than plain weave:

Plain
Each warp fibre passes alternately under and over each weft fibre. The fabric is symmetrical, with good stability and reasonable porosity. However, it is the most difficult of the weaves to drape, and the high level of fibre crimp imparts relatively low mechanical properties compared with the other weave styles. With large fibres (high tex) this weave style gives excessive crimp and therefore it tends not to be used for very heavy fabrics.

Twill
One or more warp fibres alternately weave over and under two or more weft fibres in a regular repeated manner. This produces the visual effect of a straight or broken diagonal ‘rib’ to the fabric. Superior wet out and drape is seen in the twill weave over the plain weave with only a small reduction in stability. With reduced crimp, the fabric also has a smoother surface and slightly higher mechanical properties.



Mark

 

[dogbrain]

Just re-read pblancs comments again, and see I read it wrong. He's talking about putting another layer over the top of the kevlar inside the hull. Sorry

mark

 

[pblanc]

Yes, that was my meaning. My impression is that what you are planning represents something of an overbuild, but if you want the boat to be really strong and don't mind the extra weight, by all means proceed as planned.

If you do choose to use Kevlar on the hull and deck interiors, just fill the weave for the areas you want to be smooth. You might want to save weight by not filling the weave with resin on the areas where it doesn't matter. You don't expect to be sanding these interior areas and they will not be subjected to much in the way of abrasive forces so there is no reason to cover the aramid with fiberglass. I personally have not had any difficulty with wetting out the plain weave Kevlar or getting it to drape on concave surfaces in the 5 ounce/square yard weight.

As for the strength properties of aramid fibers, you actually have it backwards. Aramid fibers are much stronger in tensile strength than when they are subjected to compression. Aramid fibers are actually weaker in compression than those of fiberglass, and that is why it is best used on interior rather than exterior layers.

Since a boat hull and decks tends to be rather rounded on the external surface, and since you expect impact forces to come from the outside of the hull and not the inside, if the hull or deck is impacted, the outer layers of the layup will be subjected to compressive forces and the interior layers to tensile forces.

 

[dogbrain]

I'm still learning about the terminology. In my post I repeated something I read without making sure I understood the meaning. Thanks for clearing it up for me.

mark