A few days ago in the store I was surprised to feel how big the difference was between the compression resistance of a piece of foam insulation sandwitched b/w two flesible layers of weatherproofing compared to same foam without the insulating layer. The one with the layers felt sturdy and the layers yielded very little to pressure. That is because the outer layer was puncture resistant (to my finger at least) and was not stretchy so the only way to yield was to slide from its position on the surface and it could not do that since it was glued to the foam below. Of course, a sharp edged object would have no problem penetrating. Peel off the layer and it exposes a fairly fragile piece of foam that easily compresses with fingertips.

I expect a similar effect will be there with a foam core paddle. The foam alone is weak but when sandwitched it supports the outer shell to some degree. It won't be very puncture resitant but it will have better structural properties than without foam core. I will have a reinforced stringer inside to provide a good amount of structural integrity.

I do not have much experience with composite materials but I pondered the question of layering "mentally". Folks who have done carbon paddles from a plug (hollow core) have used 6-8 layers of carbon cloth that was pressed together in one shot. I am not sure that is any better than a single layer heavier layup for this application. I think it is done mostly to allow the fabric to comply better with the mould and easier wetting. Multiple layers only have benefit if there is something between them to provide "space" for additional rigidity is my thinking. Or if the orientation of the fibers matters for strength in a given direction. Looking at my Valley boat carbon/kevlar layup - it is mostly single layer with up to one or two layers of reinforcements where needed. I'm doing the same - hoping it will be strong enough to not break on impact any more than a wood only paddle (and hopefully a little more abrasion resistant and a lot lighter).

I was very pleased with the strength of a single layer of "medium" weight fabric I used as an outer shell for a wood paddle. I also experimented with it folded in two and it was more than strong enough to provide structural integrity - I had to put significant effort to break it with my fingers by bending. To save on "manufacturing steps" I decided to go with a single heavier weight layer this time. But that is directionally oriented as I need it and I hope will be strong enough. If it turns out it feels flimsy, I guess I will need to lay up a second or third layer on top of it, which should not be a big deal to do.

As for the fracture, for the foam to fracture, I think my paddle's outer shell and internal stringer will need to fracture first and I would not care about the foam at this point -;) I'm a little worried that I will not have kevlar in that particular paddle but I'll give that a try. The experiment will cost me about $60-$70 in materials, most of it for the carbon cloth. If it breaks I can put it back together and reinforce it with a few more layers without loosing too much of the original shape...

: You will be far ahead if you plan it like there would be no core,
: paddle full of air. The insulation board has very low impact
: strength, the material will often fracture just below where it's
: bonded to the epoxy when stressed. Makes a nice light core to
: lay up upon otherwise.

: Bill H.