Glass both sides?

I’m about to build a new cover over the forward cabin on my boat.
It’s currently covered by canvas over aluminum frames, and it’s stretched out to the point where it leaks whenever a cloud comes over, or a bit of spray blows up.
Plywood with fiberglass is the medium that I am comfortable working with, but due to its size, and other limitations, I won’t be able to flip it over to glass the inside, so I’m debating the wisdom of glassing the outside and just painting the wood on the inside.
Any thoughts and insights appreciated.

 

The last ply boat i built only had glass on the bottom, but everywhere else got 3 coats of epoxy. Sold it 5 years after it was built, still had the same paint on it.

 

Well, the big question is whether you are keeping the aluminum frame for some sort of frame. Plywood alone won’t support itself well over that span. So, you need some transverse members, aka beams. For my 8’ rooftop, I laminated 3 pieces of redwood 1/2” thick each to a crowned shape. I screwed up and did not go plus 10% for springback, so I lost a bit of my crown, but had enough, so was okay. The plywood doesn’t need anything on the bottom. My spans between beams is 11”, but I designed the thing for 100 pound foot loading.

Mine is ply bottom, 12mm foam bonded above that, with a nice hexcel 17oz woven on top. But I think you could do 10oz woven here as well and just over plywood. The beams will allow for insulation or wiring, etc. my headliner is 4mm okume, but you don’t need one

 

I’m thinking the aluminum bows will not be useful, they have a large radius in the corners, are not very accurately bent, and have very little crown. Canvas is very forgiving!
There will be one plywood bulkhead, and several beams, (spacing yet to be determined) possibly cut from 2x dimensional lumber to the desired crown.
The bow is quite rounded, was looking at using this product to achieve the radius on the short vertical bow portion. Looks like it will soak up a lot of resin, but much easier than bending multiple layers of thin plywood.

 

Wish I'd taken some pics a few years back, but a fella had an aluminum framed canopy very similar in size. He ended up putting some kind of temporary battens to keep the contour and coated the whole shebang with a heavy heat shrink wrap. Followed it with a layer of chop and after it kicked he did a couple layers of combi mat. They craned it off and pulled the heat shrink like a make shift male mold. He then faired it a bit and added paint, then dropped it all back on the aluminum frame.

It was one of those crazy ideas that worked. Guess he got the idea from a local legend that made complex frp tunnels out of airplane cloth stretched over spruce forms. All I know is when he started I'd thought he was nuts, but ate my words when it was done.

 

Does anyone have experience with glassing over the bender board?
The product I’m looking at is fairly rough surfaced, and will probably require some filling before glassing.

 

That looks like a great application for a couple of layers of glass and epoxy over a polystyrene foam plug, then tear out the plug, clean out the remains of the polystyrene with solvent (gasoline?), sand inside and out, and paint. It's not unlike the system used to make composite fuel tanks for homebuilt aircraft--the solvent will dissolve the foam but leave the cured glass and epoxy.

You'd want to incorporate some reinforcing ridges in the plug or even simpler make the plug out flat sheets of foam to create a faceted shape where the folds will serve as reinforcements. Alternatively you could do make the faceted foam sheet serve as a core, do a single layer of glass on the outside, flip it (it will still be quite light) to glass the inside, then flip it back to finish the exterior layers. That way the foam will serve as insulation and reduce condensation.

It all seems very doable.

 

If not glassed on the inside I would still give it a good epoxy coating before painting.
A horizontal surface like that will develop condensation that over time will start lifting the paint.

 

“It all seems very doable”
Until you consider that this is being built on a boat in the water, the piece is over 10’ wide and 12’ long.
It’s not going to be flipped over, thus my question about painting the inside, as it will not be glassed.
Can mold/ mildew preventative additive be used in epoxy paint?

 

Epoxy won't mold
But dust collecting on it will mold.
No anti-mold additive for Epoxy.
Yesrs ago i frequently used a Zinzer paint for bathrooms that had anti-mold qualities
A smooth wipeable surface will be the easiest to keep clean.

I would pre finish the plywood before installation.

 

Are you locked into making the two corners round? Can’t you make them flat panel and just put a light radius for woven glass on them? The round front doesn’t even match the hull lines of the windshield firther back.

 

The corners will be square with enough radius to glass over without creating problems.
It doesn’t bother me that the cabin lines don’t match the windshield, this is a very utilitarian vessel, function first!
There is a nice coaming already in place that determines the shape of the forward end.

 

I’m developing a plan, now fishing for ideas on how to build compact yet strong beams without much protrusion into headroom.
Fallguy, would you be willing to share any pics/ specs of putting your beams together?

 

How about using laminated wood strips that form a hat section?

 

I used redwood because I had it. It was old redwood, so had to precoat it.

I tested the strips to a deflection standard to determine their thickness. I don’t want to go into the calc here, but basically I setup the roof for 100 pound foot load from someone walking gently on it.

I made 1/2” thick strips by ?2.5” wide. Then I made a laminating station with plywood box to keep it flat, covered in plastic. I made spacers so I could make multiples.

The top piece keeps them flat. Need to account for springback by overbending 10%. I did not.

I tried to do this well and had trouble with pics.



see them in situ; everything was built above