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

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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On Tue, 7 Sep 2004 00:01:52 +1000, "David Flew"
wrote:
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.

=============================================

I know from experience that 1/2 inch (12 mm) plywood is fairly bendy
and would not be a problem over the large radius you describe. Since
15 mm is only slightly thicker it would probably be OK also. If you
do a test and decide that too much force is required, you could make
shallow kerf cuts in the bottom of the panel with a table saw, and
then fill them later with thickened epoxy.

Thanks
I plan to try bending it over the existing surface - that will give me a
good idea of the forces required. I'll go ahead an buy the ply.
David

"Wayne.B" wrote in message
...
On Tue, 7 Sep 2004 00:01:52 +1000, "David Flew"
wrote:
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.

=============================================

I know from experience that 1/2 inch (12 mm) plywood is fairly bendy
and would not be a problem over the large radius you describe. Since
15 mm is only slightly thicker it would probably be OK also. If you
do a test and decide that too much force is required, you could make
shallow kerf cuts in the bottom of the panel with a table saw, and
then fill them later with thickened epoxy.

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

Wayne.B wrote in message . ..
On Tue, 7 Sep 2004 00:01:52 +1000, "David Flew"
wrote:
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.

=============================================

I know from experience that 1/2 inch (12 mm) plywood is fairly bendy
and would not be a problem over the large radius you describe. Since
15 mm is only slightly thicker it would probably be OK also. If you
do a test and decide that too much force is required, you could make
shallow kerf cuts in the bottom of the panel with a table saw, and
then fill them later with thickened epoxy.

Hi All,

I tried the kerfing approach on some 0.5" plywood.
It wasn't a marine job, just a caravan (aka Travel Trailer in North
America?)
Similar though, wooden frame, glass cloth etc.
Didn't come out all that fair - seemed to me I could still see the
kerf cuts through onto the top surface - the surface seemed a bit flat
between the cuts.
I probably should have used more cuts, and cut them less deep...

Anyway, in the end, I took it off and tried again.

I put the old panel back into place - it was easy to see where it had
been from old paint lines etc. I drilled through it into the wooden
frame in three places, and glued a dowel into each hole, being careful
to glue them only into the frame, not into the panel I was removing.
I then removed the panel and used it as a template on the new panel.
Drilling through the same holes into the new panel meant I could use
the dowels for alignment of the new panel when fitting it back onto
the frame. I left the panel way oversize (as much as I could given
the size of the sheet I was working from), except where it was to butt
up to the next panel (which I wasn't replacing).

When fitting, use the extra length on each edge gave me more leverage
when bending the panel to fit the curve. It took no more than
fingertip pressure to pull it into shape and hold it there. The
dowels made sure eveything was aligned as before.

Although I did epoxy coat the new sheet of plywood before fitting, and
re-did the edges after trimming to size, I didn't epoxy the panel onto
the frame, just sikaflex (which doesn't set hard) and plenty of
screws. If it rotted before, it could (will?) rot again, despite my
best efforts...
The screws were staggered across the width of each frame batten to
minimise the chances of splitting.

I then cut it back roughly to the line marked from the template. I put
the original panel back over the new, again using the dowels to align,
and trimmed the new panel back to meet the old with a bearing-guided
straight router bit. The router was used upside down and overhead -
pretty unpleasant, but managable.


Hope this helps.

Regards,

bookieb.