how much fg equals 4mm of wood

[fschutz]

I have read some of the posts, so I am sure there will be an answer and some opinions.I have made a mold, I guess its a plug really, for a 17 foot kayak. My design I made out of marine plywood, but all the changes wood parts are out of the question because of the curves. I also want to to make a few more. my though was 4 0z s-glass wich would be the inside of the hull. 2 layers of 8.9 0z glass then 6 0z s-glass. The deck would be the same. I want to use 4 oz to hold the deck and hull together. There are two bulkheads of 6mm marine plywood already made. I am not sure if 4+9+9+6+4 if enough fiberglass to equal the wood?

[alan white]

Going backwards, a wood hull should weigh maybe 45 lbs to the fiberglass's 60... sampling the two as one foot square sections, the fiberglass is probably about right at 60/45 of the wood's weight, or 4/3, or 1 1/3 more weight.
...For a quick and dirty answer.
That said, light FG racing canoes and performance canoes use some coring to stiffen the bottom and reduce weight, and you could use Kevlar too and equal the wood's weight.
Wood has natural stiffness that makes it more or less self-cored, while fiberglass is denser and needs coring to put distance between the inner and outer surfaces and become as stiff.

A.

[fschutz]

thanks, that makes sense.. kevlar is cool but a little much.

[alan white]

Yeah, I hear you. I'd say talk to one of the guys here who know something about coring small boats. Where curves are tight, coring isn't needed. just the bottom and the deck--- the deck could be braced with coring shapes that are semi-circular section cross-frames. Those frames could use anything for coring--- it's only to give the tape that creates the shape a basis to apply epoxy/cloth to. A split piece of pipe insulation would be great--- light and it follows curves. The bottom could be 1/2" foam core, a long football shape bellied into the bottom with angled edges to allow the glass to flow onto the hull. The stiffer you make thise flattish sections, the lighter your layup can be. Kayaks I see sold these days are mostly way too heavy, either because they're roto-molded polyethelene, or too-heavy fiberglass.
You might get the weight down to about 50 lbs with some clever engineering.

A.

[jimslade]

You could use 1/2 inch nidacore with one layer of 1810 glass on both side's, light and strong. Ask the company for samples. And you would not need any floatation. For a strongback I would go for thin walled pvc glassed in. I did this with a canoe that I converted to an outboard.