Chris didn't hammer the idea home enough. He and I got it from the same source, a naval architect.
It is:
There is no weight savings using foam core until you get to hulls of about 25 ft. This is because foam has no resistance to penetration. To overcome this many layers of fabric and resin are necessary in and out. The resin load negates any weight benefit gained from the foam core.
I would add that it is possible to build a lighter hull but, you will have to accept the hull will be more prone to injury and will, therefore, have a shorter lifspan if used too hard.
Next, Kevlar is never used on the exterior of a hull except on racing hulls which usually have a short lifespan. Kevlar in not especially good at abrasion and when the yarns are torn loose it is almost impossible to get rid of the ragged ends. The stuff is so tough it cannot be sanded and it cuts with extreme difficulty.
Next, I haven't checked on this lately but I think Kevlar requires more resin content to reach saturation. Ask your NA friend.

: Chris - if you know something the rest of us don't about foam core, could you
: fill me in? I am planning on prototyping a kayak this spring using
: Divinycell H80 foam in strip form (variable width, ~1" by 3/8"
: thick). Build method will be the same as a wood strip-built kayak,
: possibly with some of the forms retained and glassed in place. I'm looking
: for a fast build and the ability to fair it quickly and aggressively
: during testing. I may not glass the outside right away, waiting until the
: shape is optimized. The finished hull would get glass (or kevlar or
: carbon) on the outside. I don't see how this is functionally incorrect -
: am I missing something? If so, please advise before I blow the time and
: money. I've been running these ideas past a colleague who is a working
: naval architect (mostly sailboats), and he hasn't steered me away from
: foam core yet. I agree wood is probably best, but not always called for.

: Thanks, Carl