You should have no difficulty bending 15mm ply over that distance. I bent 3
sheets of 19mm marine ply for the roof of a new deckhouse. these were over
beams with a 3" rise in the centre. approx 7' wide. screwed using 2" x 8
brass screws at 18" intervals. Screws filled over using automotive
'polyester' body filler. sanded smooth then painted.
Peter
"David Flew" wrote in message
...
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