Carbon fiber cockpit coaming

     I'm building a carbon fiber (5.7 oz  2x2 twill) cockpit coaming for a Shearwater 17 hybrid.  Any recommendations as to how many layers of carbon fiber to use?

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RE: Carbon fiber cockpit coaming

Gregg - I did the same thing on my SH17 and had very good results (It's rediculously strong).  I have pictures with a narative of the carbon coaming layup at  That part is toward the end of the pictures. 

I guess I finished with 6 layers of 6oz 'glass, 3 layers of 5.7oz twill carbon fiber and 1 layer of 4oz 'glass (10 layers total).  I put them down in two separate layups.  The following is how I applied my two layups:

First Layup - The first layup formed the coaming but did not join the coaming to the hull.  When the first layup had dried, I removed the coaming from the boat and trimmed and sanded it to shape.  The glass and carbon layers in the first layup were applied in the following order:

- 3 layers of 6oz 'glass

- 1 layer of 5.7 twill carbon fiber

- 1 layer of 6 oz 'glass

- 1 layer of 5.7 twill carbon fiber

- 1 layer of 6oz 'glass.

 Second Layup - The second layup both joined the coaming to the hull and added the "show" layers of 'glass and carbon fiber.  These layers all wrap from the top of the coaming around and under the hull to "clamp" the coaming down.  The layers were applied in the following order:

- 1 layer of 6oz 'glass

- 1 layer 5.7oz twill carbon fiber

- 1 layer of 4oz 'glass

A few suggestions - Definately cut all the glass and carbon on the bias.  The layup will be very difficult without that.  I'd really recommend putting at least a single layer of glass on top of the carbon "show layer" since the carbon is not at all resistant to abrasion.  

 Have fun with the project.  It looks great when complete.


RE: Carbon fiber cockpit coaming


Is the purpose of the CF (rather than wood) coaming decorative or functional? The reason I ask is that the plain wooden rim is strong enough and stiff enough that the kayak can be picked up and swung around by it. So if all you're looking for is the function, plain wood is plenty good.

If you're looking for decoration, then a single layer of CF on top of the wood with a layer of 4-oz. glass for abrasion protection is also all that you need.But if you want a high-tech composite just to say you have one, read on:

In general, you'll need a layer of glass on each side of the layup not only for abrasion protection, but also to contain the carbon fiber threads in case the layup breaks. CF is strong but brittle and will fling sharp little needles when it shatters unless contained. That said, a coaming is not likely to snap the way an overstressed mast would because of the support of the coaming spacers (risers)  underneath, so you can probably get away with only the top layer of glass.

CF is expensive and stiff, so a good approach to follow is to have the CF on the outside with a core of "inferior" material sandwiched between the CF. This material is in the neutral plane of the layup and only needs to resist compression. If you're looking for inexpensive and light, this could be Okoume. Expensive and light can be some king of closed cell structural foam (must be compatible with the epoxy). Inexpensive and heavy can be glass cloth.

A rule of thumb I've heard is that a single layer of uniaxial CF is about as stiff as 10 layers of biaxial glass of the same thickness. Sandwich layups also get stiffer as the core spacing increases. So if I was doing a CF coaming, I'd build up the risers from wood (as per the plans) and make the lip a layer of CF, a layer of wood (as per the plans), a layer of CF and a final layer of 4 oz glass and leave it at that. It'd be very stiff, though heavier than plain wood.

Have fun and don't use the spousal scissors to cut the CF,



RE: Carbon fiber cockpit coaming web page, from Ross Leidy, is a great place for getting information on all sorts of cool modifications like that one. 

As for how to lay it up, you shouldn't need more than a couple or three layers of CF and sandwich that with a layer or two of glass.  No point in making the coaming so thick and heavy that you would have been better off with wood.


RE: Carbon fiber cockpit coaming

When you build a CF mast from a kit, the inner and outer layers are glass.  It seems from the above that at least one reason might have to do with containing possibly splintering carbon fibers.

But is there a second reason?  My hunch when I first heard about these kits (after being initially surprised that glass was added at all) was that it must have  to do with allowing the CF to "share" the stress with the glass.  It would grab just enough stress, because of its higher stiffness, to earn its keep as expensive high stiffness/weight stuff.  But the glass would grab some because of being in the skin, resulting in a mast with lower modulus (as a structure) but higher ultimate strength.


RE: Carbon fiber cockpit coaming


The kit that I'm familiar with uses uniaxial CF and biaxial glass. Because of the differences in fiber directions the glass contributes only 16% of the total lengthwise stiffness but almost 100% of the circumferential stiffness. And it catches the splinters when the mast breaks.



RE: Carbon fiber cockpit coaming

Thanks.  If lengthwise stiffness means resistance to bending and it's only 16% then I think my first guess was wrong. The CF is carrying most of the stress and the glass isn't contributing a lot, except in failure mode. Regarding circumferential stiffness, does it mean resistance to twisting?

RE: Carbon fiber cockpit coaming


Thanks to all for the advice.  After I put on the final layer (4oz. glass) should I put on a layer of peel ply?  I know this will leave behind a finely tectured surface.  Do I need to sand this smooth or should I just go over it with a coat of epoxy?  Then I would have to sand the epoxy smooth.  My intention is to clear coat the finished piece & have the carbon weave show through.


RE: Carbon fiber cockpit coaming

I just filled the weave as usual, sanded it smooth and applied varnish.

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