the force needed to bend plywood is a different matter from the amount the
plywood can bend before breaking, and would depend on the amount of
material, ie size of panel, to be bent. obviously a 1 inch strip of
plywood requires less force to bend than does a 2 foot piece of the same
length. I don't have any formula for calculating the force required to
bend plywood but my website contains information on how much plywood can
bend before breaking (www.ncf.ca/~ag384/Boats.htm). the radius of
curvature examples, formulae, and BASIC computer program tell you if the
plywood will bend that much without breaking. However, if the radius of
curvature of your cabin roof is much greater than the minimum radius the
plywood will bend before breaking then presumably the force needed would
not be great.

"David Flew" ) writes:
Hi
I'm looking to replace some rotten plywood on the cabin top. 60's motor
launch.
Original construction looks like 15 mm ply ( probably marine back then - it
wasn't gold plated .. ) with fibreglass on top, and many many layers of
paint. If they had glassed over the front lip ( like an eyebrow over the
slanting windows ) the water would not have penetrated back into the ply.
Please excuse my terminology - probably get this mostly wrong ....
My plan is to cut away all the ply back to the middle of the second "roof
beam" - everything forward of this has some problem or other. Then to fit
a new piece of 15 mm waterproof ply ( "C/D"), pre-sealed with a couple of
coats of epoxy all over. Add a glass sheath to the top and over the front
lip ( I figure the original was polyester, but I've got to use epoxy or else
it won't stick, right?), try to make the join invisible by lots of filling /
sanding / cursing, and eventually painting it all again.

I've got as far as making a template of the shape ( removed the grab rails,
screwed a piece of 3 mm ply to the roof, drilled holes from inside the cabin
next to the beams, scribed around the front edge, removed template, filled
the drill holes with 5 minute epoxy, replaced grab rails .... ), but I have
one concern.

The cabin top has about 1 1/2 inch of camber from centre to edge each side.
It's about 6 ft wide. Am I going to be able to bend the sheet of 15 mm
epoxy impregnated waterproof ply to follow the camber? I can't get clamps
anywhere, but I plan on using lots of SS screws to pull it down onto an
epoxy adhesive bedding, then removing the screws after the glue has set,
and filling the screw holes before I glass over the top. And if I'm
realistically not going to be able to bend the ply, what is "Plan B"


David





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