: Hi there, can anyone tell me if there are practical alternatives to wooden
: strips ie foam? I have designed and built an 8 chine S&G which was very
: successful and would like to build a surfski (long narrow racing sit on
: top), and wonder if it would be lighter and faster to build with a ply
: stringer,shaped foam blocks glued on, and glassed. I believe surfboards
: are made this way.

A lot of surfboards don't even have the plywood stringer. They are just a slab of foam shaped with an electric planer or a belt sander.

: Production surfskis are made with moulds which doesn't
: seem practical for a one off. Thanks for any input. Andrew

Well, there are two types of molds: male and female. What you are going to make is a one-piece male mold (also called a plug) which you are going to leave inside the fiberglass you apply. It is only a bit more effort and material to make a 2-part mold which opens up so you can put the glass and resin on the inside. Once the resin cures you open the mold, remove the parts, and go to finishing them.

If yo make this mold from weak materials it will be god for just one boat, but fairly fast to construct. If you use more durable materials it will be more expensive and take more time to construct, but it can be useful for short production runs. Got friends who want boats like yours?

Either way, building the mold from foam is well covered in James Moran's book, "Building Your Kevlar Canoe: A Foolproof Method and Three Foolproof Designs". before you rush to buy this book, though, read this commentary Ed Stiles and Bob Bowers, who built from the book. It includes probably all the info you need on making the molds in his comments.

One thing Bowers says which I differ with is this: He says you can't use styrofoam building insulation as epoxy will dissolve it. That is not so. The cheaper polyester resin will dissolve styrofoam. However, in this project the styrofoam strips don;t come in contact with the resin. They are first covered with drywall "mud" (NOT plaster!) which is very soft and easily smoothed with a coarse bath towelwrapped around a stick. A damp towel won't kick up as much dust. Of course sandpaper works faster, but way more dust. After that the mold is painted or varnished, then it is waxed. Buried under the drywall compound, varnish and wax the original foam is unlikely to come in contact with an resin. Whatever, styrofoam works with epoxy, and with a couple coats of paint as a barrier, it would probably work with polyester for one boat.
that review is at: http://www.bwca.cc/tripplanning/equipment/canoes/kevlarcanoebuilding.htm

If you are using foam strips be sure to compensate for the difference in thickness as compared to wood strips. Make your forms smaller.

Hope this helps

PGJ