Wayne.B wrote:

On 2 Feb 2005 23:34:21 -0800, (teddyareno)
wrote:

Is 1/4 inch (6 mm) plywood, glassed over, adequate for a non
load-bearing, hard canopy for my little cruiser? The canopy will be
about 9ft long by 6ft wide, and slightly curved for water run-off and
aesthetics. How many layers of fibreglass cloth would be needed?

I intend to force the plywood into shape and allow the epoxy and
fibreglass to keep its shape when dry.

I'm trying to avoid rafters as they eat into the avilable height.

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I would suggest reposting your question on group 'rec.boats.building'.

There are a number of people on that group who fabricate fiberglass
parts of various sorts. I agree with K that you are not going to get
the kind of panel stiffness you want with 1/4 inch ply unless you
glass it very heavily, but there are other alternatives such as 3/4
inch foam that might work out with the proper fiberglass layup on each
side. I disagree with K regarding the use of Chopped Strand Mat as a
reinforcing material. CSM is very weak for its weight and is mostly
useful for adding non-structural thickness to a layup. Woven glass
cloth or unidirectional has much better stiffness and structural
qualities.

Well my hull is basically 1/4" ply and is PLENTY stiff. Mind you, the
maximum unsupported area is about 1/4 of yours with a lot of curvature
molded in by Fairey marine when they hot pressed the veneers my hull is
made of. If you epoxy glass a couple of lairs of cloth on each side of
a pre curved ply core of sufficient thickness WITH PLENTY OF
CURVATURE it should be stiff enough to stand on with a 6 foot span. You
*DONT* want to lock in the strain by glassing ply that wont hold its
shape without the glass.
Check out http://www.marineply.com/stagflex.htm
You need a really good bond though, and I wouldn't try it unless I had
the equipment to vacuum bag the laminate.
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.