Builders' Forum Archives
Re: Hanging from the roof
Posted by Laszlo on May 8, 2005
Rob, not a problem. Go for it.
Rotomolders are just plastic, which is, well, plastic. That is, it can and will deform under stress. The long polymers that they are made from have only an OK resistance to the various forms of stress (tension, compression, shear and torsion), so a prolonged period of stress in the same direction will cause the plastic to flow and deform. In fact, the main reason you can get away with that stuff for use in a boat is that when it's on the water, the water itself is what's providing most of the resistance to much of the load. But the labor costs to build them are really cheap and they resist gouging pretty well.
The CLC boats are a composite construction of epoxy, glass and plywood. None of the components can resist all the stresses well, but each resists a particular kind superbly and they work in a partnership to resist all the forces.
For example, epoxy by itself resists compression very well (it's hard) but it has real problems with tension (it's brittle). By embedding a layer of glass fibers (which have tremendous tensile but very little compressive strength) in the epoxy, you end up with a composite that has the compressive strength of epoxy and the tensile strength of glass.
The wood comes in because the epoxy/glass composite it very flexible. Plywood provides a light stiff core that keeps the epoxy/glass from flexing by turning the flexing force into a tensile force which the glass can resist (The epoxy bonds the glass to the wood so that when the wood bends the glass would have to stretch, which it resists). If we didn't use the wood, we'd have to add lots and lots of glass/resin mix to resist the bending. This is exactly what the rotomolders do, add lots of plastic, which is why they're so heavy compared to our boats. It's also what most commercial boatbuilders do, add glass matte and lots of polyester resin. This is why glass matte has no place in composite stitch and glue construction.
Finally, our epoxy molecules cross link to each other in 3 dimensions, whereas the ABS plastic that the rotomolders use don't have as many mutual bonds. So our plastic is less plastic.
The result of all this is that our boats are stiffer and lighter than rotomolders (albeit more expensive in materials and labor). When you hang one empty from the ceiling it doesn't produce as much load, and what load there is is much more efficiently resisted.
Hang 'em high!
In Response to: Hanging from the roof by Rob Johnston on May 7, 2005
- Re: Hanging from the roof by Karl on May 8, 2005