Question about weight of materials

[outofthenorm]

This is a question just to fill in some my (vast) missing knowledge:

For a perfectly flat panel of a given thickness, say 1/4", standard 4 x 8 sheet size, how would the following compare, by weight and stiffness:

1) cold molded WRC (or similar) and epoxy
2) fibreglass and polyester resin
3) gaboon/okoume 4-ply plywood

An associated engineering question is this: If you started with a flat panel of each material, then bent it (in one direction) and fastened it to a typical boat shaped form, would there be any degradation of strength or other properties in option 1 or 2? I think I know the answer for the plywood: no degradation. Said another way: can you bend and fasten a pre-lam of WRC and epoxy or one of fibreglass without hurting it?

This is all in aid of some amateur materials research I'm doing for an alternative way to build pre-fab boats with flat panels, so any insight is appreciated.

- Norm

[Lewisboats]

Off the top of my head I don't know about the other stuff but you really don't want 4 ply plywood...odd numbers for structural strength.

[outofthenorm]

Lewisboats: you're likely right. The 4-ply was based on having 1.5mm lams.

- Norm

[kenjamin]

Hello Norm,

Thought you might find this info helpful:

http://www.worldpanel.com/Plywood%20Data1.pdf

I'm interested in prefinished fir ply for my Down and Dirty Double Dory. I think it would be kinda neat, cheap and ethical to build with North American woods. I'm wondering if 1/4" marine fir ply sheathed and epoxied flat would still take a bit of a bend gracefully. Would also like to try the plastic sheet trick to create a mirror finish on the first application of cloth and epoxy. Let me know what you come up with.

Thanks, kenjamin

[David G]

Norm,

I just have a minute before I head over to the shop, but will respond shooting from the hip. You don't specify a radius to your bend, but the short answer is: yes, the 1/4" plywood will bend easier, and farther without degradation than the other two. The cold molded would likely last a bit longer than the polyester, but not a whole lot. The whole point of polyester/fiberglass composite and cold molded composite are to create panels shaped to a mold. If you tried to take flat panels of those composites and bend them, they would certainly fracture. The outside (convex) face would be in tension, the inside (concave) face would be in compression. I'm guessing the failure would be in compression, but can't say for sure without some thinking and/or research.

It's also probable that a very slight bend - well below the failure threshold - would make any of these panels stronger. That's certainly true for the plywood.

Is this an idle question, or do you have some scheme in mind?

"There are too many people and too few human beings" -- Robert Zend

[outofthenorm]

Thanks Kenjamin - that's useful information.

David - I understand your point about molding to a shape. I'm assuming gentle bending well inside an obvious failure threshhold - nothing extreme.

It's not entirely idle speculation. I've started thinking about the problems of creating custom fibreglass one-off boats as a commercial venture ... ie: the need to build a plug to make a mold to make a boat ... and then wondered if some of the stitch and glue designs that used flat panels could be built with pre-fab flat fibreglass or other composites panels- and of course, the easiest pre-fab panel is a perfectly flat one.

It may not go anywhere, but I was hoping someone could help me figure out the flexibility/degradation equation in a general way.

- Norm

[SchoonerRat]

Back in the '70s I built a glass boat that was planked with a foam core material made in the shape of -- well -- planks. After planking, resin and glass coating to outside of hull, pull from temporary framing, roll and glass the inside.

Wasn't my favorite boat building experience, but the boat looked good, and was light and fast. And mostly--no plug, no mold--One off fiberglass.