Building plywood coachroof: Advise please.

rustybarge

rustybarge

Well-Known Member
Joined
9 Aug 2012
Messages
3,665
Visit site
View attachment 28600

Looking at Robins timber marine obeche ply, the 4 mm. Sheet only weighs 6 kg.
Would that be too weak for a 8' by 16' by 4' high hardtop, (bolted to the bulwarks)?
Do composite cross beam save much weight in comparison to solid bent beams?
Spruce is very expensive, is there a cheaper alternative that's light?

Any suggestion gratefully received.

Thanks.
View attachment 28599
 
TQA said:
Do you ever think anyone might walk on it? If yes 4mm too thin. If not should be OK depending on your cross beam pitch of course. SWAG comes up with 15 inches.
Click to expand...

I was thinking of making the forward 4' part of the wheelhouse stronger, just in case of a breaking wave, and going for a much lighter ply for the rest i.e.. more of a light wood canopy...not expecting anybody to walk on it, hopefully.

In the photo they seem to be using 2*2 upright supports.....that would be very heavy, do you think a light wood like cedar would be strong enough.

What's swag?
 
Recently added a full canopy across the back of the cat, 20 foot from side to side and 18 from front to rear.

The original was destroyed during a cyclone, almost inevitable as was only marine grade sun cover.

The replacement made of fibreglass, and some foam areas to provide shape and strength without too much weight, also included all wiring (internal) for lights, speakers, solar panels and antennas, also engineered in some water collection (rain water) as well.

Construction done on a ply platform covered in a release agent, all very simple and 80% of the fabrication completed on the first day.

Easy to lift on with just one person on each side then installed solar panels, lights speakers and so on.

No worry about rot and easy to keep clean, coated with flow coat for a smooth finish.

Good luck and fair winds. :)
 
Building Wooden superstructure like the one in the photo is not a trivial task. Your two problems are making it strong enough and stopping it from rotting. 4mm is nowhere near strong enough. Think minimum of 9mm. Framing can be Douglas Fir a good compromise of strength easy of working rot resistance and cost. Beams can be laminated from strips to the right curve. Ideally the whole outside should be sheathed in glass epoxy.

Do not underestimate the cost and complexity of such a structure. Once you get into the details you will understand why this method of construction is rarely used.
 
oldsaltoz said:
Recently added a full canopy across the back of the cat, 20 foot from side to side and 18 from front to rear.

The original was destroyed during a cyclone, almost inevitable as was only marine grade sun cover.

The replacement made of fibreglass, and some foam areas to provide shape and strength without too much weight, also included all wiring (internal) for lights, speakers, solar panels and antennas, also engineered in some water collection (rain water) as well.

Construction done on a ply platform covered in a release agent, all very simple and 80% of the fabrication completed on the first day.

Easy to lift on with just one person on each side then installed solar panels, lights speakers and so on.

No worry about rot and easy to keep clean, coated with flow coat for a smooth finish.

Good luck and fair winds. :)
Click to expand...


I like your idea, but my only experience of fibreglass is building sailing dingy with ply covered in epoxy. Iirc it was supposed to be super light to lift onto the roof of a car, but by the time I had finished it weighed far took much for one person to lift.

Approx. how many layers of fibreglass matt did they use in your structure......any photos?
 
Tranona said:
Building Wooden superstructure like the one in the photo is not a trivial task. Your two problems are making it strong enough and stopping it from rotting. 4mm is nowhere near strong enough. Think minimum of 9mm. Framing can be Douglas Fir a good compromise of strength easy of working rot resistance and cost. Beams can be laminated from strips to the right curve. Ideally the whole outside should be sheathed in glass epoxy.

Do not underestimate the cost and complexity of such a structure. Once you get into the details you will understand why this method of construction is rarely used.
Click to expand...

I was looking at a similar boat on an austrailian site , the boats are designed for use on the river Murray so no nasty waves, and they used 6mm ply, but as you say maybe that would be far too weak for use at sea here in the uk.

The reason I'm obsessed with weight is that my boat is a catarmaran, and the designer keeps on Asking me how much weight will be on the boat, because that will directly influence the power requirement.

The original design only has a short wheelhouse, I want to convert it into a hardtop.....far more practical with our "summers"
 
rustybarge said:
I was looking at a similar boat on an austrailian site , the boats are designed for use on the river Murray so no nasty waves, and they used 6mm ply, but as you say maybe that would be far too weak for use at sea here in the uk.

The reason I'm obsessed with weight is that my boat is a catarmaran, and the designer keeps on Asking me how much weight will be on the boat, because that will directly influence the power requirement.

The original design only has a short wheelhouse, I want to convert it into a hardtop.....far more practical with our "summers"
Click to expand...

Why not use 6mm and if necessary glue a 4mm sheet on top of you think 6mm is not strong enough?
 
ianj99 said:
Why not use 6mm and if necessary glue a 4mm sheet on top of you think 6mm is not strong enough?
Click to expand...

I think that's the advantage with plywood, you can alter The design as you go along.

Another alternative would be to use a wooden frame with aluminium sheet roof.......I'm sure 0.5 mm aluminium is very light, and it will not splinter with a sharp blow like plywood will. The problem is how do you seal ally sheets so that they are waterproof....rivets, or overlapping joints with sealant ?
 
We used two layers of 4 mm ply years ago on my Dads boat. The coach roof was about 6 foot wide so we layed two 3 foot wide section first so the join was in the middle then layed a full 4 foot wide section over the middle to cover the joint with lots and lots of glue, then added the two 1 foot offcuts of the 3 foot wide sections at the side with more glue and lots of screws. Then covered the lot with canvas set in red lead paint (those were the days when you could get all the good stuff which has now been banned) That was on fairly light roof beams with only 1" camber. never had any problems with leaks and it did flex a bit but was OK
 
Jcorstorphine said:
We used two layers of 4 mm ply years ago on my Dads boat. The coach roof was about 6 foot wide so we layed two 3 foot wide section first so the join was in the middle then layed a full 4 foot wide section over the middle to cover the joint with lots and lots of glue, then added the two 1 foot offcuts of the 3 foot wide sections at the side with more glue and lots of screws. Then covered the lot with canvas set in red lead paint (those were the days when you could get all the good stuff which has now been banned) That was on fairly light roof beams with only 1" camber. never had any problems with leaks and it did flex a bit but was OK
Click to expand...


Thanks for that info. It looks like 4 mm would be far too light, and would just collapse in gusty conditions.

Here's a very simple canopy on the same design, but with steel uprights and a foam core roof, not nearly as pretty!


View attachment 28634
 
TQA said:
If weight is a concern a 3/4 or 1" SS tube frame with a Sombrella awning laced to it will do the job. Common solution in the Caribbean.
Click to expand...

It looks like a really simple solution, and steel is extraordinarily strong, in fact I may be considerably lighter than timber uprights and a ply roof.

Will it handle gusty conditions without ripping?
 
john_morris_uk said:
When I last rebuilt a coach roof I used two 6mm sheets of ply epoxied together. They were fairly easy to curve and made a strong structure which could be walked on.

I think on a boat, if it can be walked on its likely to be walked on...
Click to expand...


Hi John,

What sort of cross members did you use, ( longnitudinals?) ?
Was it marine or exterior ply?
Did you epoxy the exterior surface, or just paint it ?

Any photos?
 
rustybarge said:
I like your idea, but my only experience of fibreglass is building sailing dingy with ply covered in epoxy. Iirc it was supposed to be super light to lift onto the roof of a car, but by the time I had finished it weighed far took much for one person to lift.
Click to expand...

Common mistake I'm afraid.
Poly resins need one part cloth to three pats resin, By weight.

Epoxy only needs a one to one ration of cloth and resin.

Adding extra epoxy will actually weaken the strength of the whole job.

Good luck and fair winds. :)
 
I was replacing the coach roof on an existing structure so the cross members etc were already there. The spacing was about 12 inches between cross beams from memory. The sheets were epoxied together and screwed down with s/s screws. I left an overlap all round which was taken off with a router. I epoxy coated the whole structure and then painted with two pack paint. It ended up as a very strong watertight structure...

I am deployed to Cyprus at the moment, so no pictures available.
 
You must log in or register to reply here.

Share this page

Top