Swensons site has numerous kayaks that are old fashioned ply-on-frame which is just what you described. Why go there? There are many thousands of S&G kayks made from 4mm ply with no internal structure except shearclamps and bulkheads. If you're going to go to the trouble to dig up decent plywood why use it on some kind of home-brew design?

: Do you have plans for one yet? Which design have you selected?

: If you have not yet fixed on a plan, let me suggest you look at
: some skin-on-frame designs, and then "skin" them with
: panels of thin plywood. It is a cheap and fast way to build a
: boat your son can enjoy this spring. You won't need any
: fiberglass for these, either. Just epoxy. If you use marine ply,
: you can skip the epoxy.

: For a plywood boat for an 8-year-old boy you could probably save a
: bundle and not buy the fiberglass cloth. The plywood does not
: need the added strength, and two coats of epoxy resin (without
: the added glass cloth) will still encapsulate the wood
: completely. You'll want resin and glass strips (cut from a piece
: of glass cloth, not the expensive, prewoven tapes) for the
: seams. but you should be able to keep down your glass purchase.
: By reducing the amount of glass you also reduce the amount of
: resin needed to cover it, and the time spent in multiple fill
: coats. You also reduce your sanding considerably. And the weight
: will be about 2 to 3 pounds less.

: OK. but take the added benefit if it comes your way :)

: For most of our purposes marine plywood is not necessary. There are
: some designs which call for some bending of the plywood, and
: these designs were based on using certain metric-sized, marine
: plywoods. Anything thicker, or not meeting those specs, would
: crack when folded. Some of the metric-sized panels are larger
: than our standard 4'by 8', and there are designs which make use
: of the extra few inches. You can scarf panels to get longer
: ones, or build a boat a few inches shorter than the original
: design.

: Marine plywood goes back to a time before epoxy. It was developed
: as a material to replace solid wood for planking wood boats. It
: was designed to take the abuse of a solid board, which it would
: replace. With a good-looking veneer face it would be less
: expensive than a solid hardwood board, but be just as pretty.
: Great fro trim above the waterline, as well as planking below
: the water line. Theoretically it should be able to be immersed
: for all of its life without delaminating. In order to predict
: that kind of life expectancy, they test samples in hot water and
: pressure cookers, and such. You pay more than you would pay for
: construction ply, but you should expect to get a (slightly)
: higher quality material.

: Since marine ply is already resistant to the effects of immersion,
: you don't need to cover it with epoxy. If you use a truly
: waterproof glue for the seams (epoxy, urethane, polyurethane,
: phenolic, etc.) you can simply prime and paint the boat with
: marine enamel. Small boats which are not stored in the water can
: use paints designated as "topside" paints, which,
: while the cans state "should not be used for immersed
: surfaces", work really well.

: Is it worth the added cost for marine ply? Not always. but
: sometimes using it can actually make the whole project cheaper
: because you save on construction time and cost of epoxy resin.
: Where the plywood is covered in glass cloth for structural
: reasons you would keep the fiberglass, of course. Is marine ply
: absolutely necessary? Generally, no. At least, not if you are
: going to completely seal conventional plywood under epoxy.

: :I

: Personally I think that the Baltic Birch panels, with their many
: thin veneer layers are strong and pretty. Not much difference in
: price. In fact, some Baltic birch is more expensive, but it is
: easy to find small panels at the local Menard's and Home Depot.

: A lot of decisions about what materials to use were previously
: based on choosing from locally available stuff. If your
: lumberyard didn't stock marine plywood, you didn't have the
: option to choose it. With the internet we can now order things
: from more 'mail order' sources than were ever available before.
: The shipping costs may be high, but people are experimenting
: with more materials--and if you want to send out for some marine
: plywood, then go for it. But if you want to build a boat which
: will last at least 15 years, and maybe 50 years with some care,
: go ahead and use locally available plywood.

: Good luck wiht your project. Hope this helps.

: PGJ