Roof lamination

Hi there,
I am building a 25 foot boat of my own design (don't ask) in beautiful Eastern Canada. It gets pretty cold on the water here so a cabin is desirable for any extended outings for some comfort. I had to laminate quite a few pieces to add strenght here and there and I am contemplating the ides of laminating the coachroof out of marine ply (1/4 over 3/8) to avoid installing beams there. This would give me an extra say 2 inches of headroom. There would be no insulation so forget a foam core here.

This roof would span apprximately 6 X 6 feet with a crown of 2 inches at the most. I do not want to make a sample this size for testing and anything smaller would not really emulate reality.

Note that this boat will stay in protected waters, more like a big bass boat with a cabin on top of it.

Has anyone done this kind of lamination before. Thanks in advance.

Daniel

 

"There would be no insulation so forget a foam core here." - I don't understand this. Are you going on the premiss that plywood insulates better than foam?
Anyway, I don't know if anyone sells balsa/carbon or foam/carbon panels that big but it might be worth looking into. With a 2" camber per 6', supported all the way around some distance in from the perimeter, straight carbon fiber would serve your purpose and you wouldn't have to worry about edge and penetration sealing as much. If you make a quick female mold, you can add visual weight and lots of stiffness by adding a lip.

 

Thanks for your quick answer Mark775,
sorry if i mislead you, i do not wish to insulate the roof.
I would like to laminate 1/4 marine ply over 3/8 marine ply on a male mold temporarily secured inside the cabin under the future coach roof. Once the epoxy has set, i would then remove the temporary mold and have a laminated roof without the usual supporting beams.
My question would then be, can this be done and would 1/4 laminated over 3/8 be sufficient to support normal live load from let say 2 people sitting down on the roof.
Daniel

 

I'm a bit retarded here but I don't follow your molding method. Abslutely 1/4 and 3/8 stuck together in epoxy, with a 2" camber is strong enough over 6'. May I recommend very high quality marine plywood for this? Shelmarine is the best I know - no voids, fungicidal waterproof glue and if you use this product, you can even be sloppy about sealing it (uhhh, I'll get that next year sloppy).

 

If I were you, I'd use only 1/4" plywood to avoid excess reaction to bending (3/8" is twice as stiff as 1/4"). Three layers of 1/4", the inner one being foam for light weight, maybe some solid ply strips 1" wide from scrap between foam areas to solidly connect top and bottom layers.
You'd have 3/4" which is a tiny bit thicker, but the top would be stiffer and loghter and much less prone to reacting to bending (unless your sides are very heavy structures, you might have a problem keeping the roof from spreading).
The added thickness without an inner layer reacting will help to retain the arc as it was molded to be. I would also add an inch or so of extra bend (3" total) in order to allow for bend-back.

 

if your coamings are not rigid, they usually are not, they bow out , over time your roof will almost certainly collapse

 

Spring back is an issue.
I made an assumption that the fore and aft structures would hold the camber.
What am I missing?

 

A forward transverse bulkhead and a pair of partial bulkheads aft will provide the greatest support to retain the curvature, but over six feet it's possible to have some spreading in the middle. This can be prevented by a structural shelf at the top of the sidewall or coaming. It could also be built into the roof itself, a longitudinal set of beams that are flat-bottomed and maybe a few inches wide and an inch or so thick average, thickening the roof to at least an inch and a half near the sides. There are other ways to bolster the midpoint between fore and aft bulkheads like hanging knees of plywood tabbed in, or posts to maintain the vertical position of the coaming/sidewall.
Not aq difficult problem to overcome, but thought must be given to preventing spreading by some means.