: I'm confused with conflicting information that I've found regarding
: the plywood necessary for a S&G kayak. I would like to build
: a small kayak for my 8 yr old son.

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.

: I've most often seen
: references to buying BS1088 marine plywood, but I've also seen
: several references to using other plywood since the hull will be
: completely encased in epoxied fiberglass.

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.

: I'm not concerned
: about a little more weight in the material because the boat will
: not be large.

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

: Is the real benefit of the BS1088 the solid core,
: or is there another reason why marine grade is necessary?

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
: would prefer to have more grain and color options (and save
: money) if I can use a plywood other than marine grade.

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