Strength laminating Coach roof vs single sheet ply

[Rigadog]

Question: Would laminating 2 sheets of 1/4" ply on a cabin top roof be stronger than a single 1/2" sheet? I'm thinking of having the lower layer be Okume and the outside run of the mill Doug Fir exterior.

It seems like it would be stronger since I would be gluing the compound curves together and could offset seems.

Thanks

[Rigadog]

Re: Strength laminating Coach roof vs single sheet ply

Wait, 28 people read this and no one has an opinion?

[Stiletto]

Re: Strength laminating Coach roof vs single sheet ply

Wait, 28 people read this and no one has an opinion?

Well, here's my opinion, which I've briefly tried to verify on google without success.

In general the strength of any thickness of ply will increase with the more plies it has.
In your situation it will be easier to bend a single thinner sheet to your cabin roof without as much springback, then the second layer, properly fixed, will do the same.
This will be stronger, whether it is necessary is another matter.

[ADonald]

Re: Strength laminating Coach roof vs single sheet ply

Wait, 28 people read this and no one has an opinion?

do you really believe that there’d be many people who’d have true data on this question, except the usual internet bull. I reckon the only people who’d be able to give an answer may be the likes of Gougeon’s, some designers etc most of whom have more sense than to come on here.

For what it’s worth my Grey Seal cabin top, that has a fair curve to it, was laid up with two opposing layers of 1/4” ply and it’s feckin strong. Dunno if single 1/2” would be as strong without destructive testing but I don’t reckon it’d be possible to bend 1/2” to this radius. BTW the hull is planked with 1/2” and those curves were bordering on no-go. One thing I do know is that once a panel is bent to a curve it’s way stronger than flat, or at least resistant to overall flex.

[nrs5000]

Re: Strength laminating Coach roof vs single sheet ply

I don’t know if it will be stronger or not, but it will be a lot easier to bend the 1/4” ply. If I recall correctly stiffness increases exponentially as a material gets thicker.

[wizbang 13]

Re: Strength laminating Coach roof vs single sheet ply

heck yea it' l be stronger

[Slacko]

Re: Strength laminating Coach roof vs single sheet ply

I was one of the 28.
As others have said, it will certainly bend easier if 2 layers.

[long time lurker]

Re: Strength laminating Coach roof vs single sheet ply

You might try foam core for strength and light weight. Stiffness goes up as a cube of thickness, weight increases at a much much lesser rate.

Frame the roof and use spacers to make sure the frames are dead parallel. This will make it possible to rip the EPS foam on the table saw and have it fit perfectly. Laminated frames are the strongest and most stable. Screw the frames in from underneath but don't glue them. Put blocking between the ends of the frames, or cap the ends with full length pieces just like a rim joist in house construction, so you have solid wood all the way around the perimeter. the frames should be the same height as the thickness of the foam you intend to use so it will be flush throughout inside and out.

Once ththe frames are all in place with 2 or 3 temporary stringers screwed to their undersides to keep them fair and properly spaced screw and glue one layer of ply to the top. Fill the screw head depressions/countersinks with thickened epoxy, fair it well enough that glass cloth will lay tight and glass it.

Remove the entire assembly and flip it over on sawhorses. Support it well, you may even want to re-apply the stringers on the sheathed side. Rip out the foam on the saw and bed it in thickened epoxy applied with a notched trowel between the frames. Apply epoxy to the edges of the foam and the frames too, not just the faces, you want every surface to be bonded and sealed, everywhere. It should fit snuggly enough that it doesn't try to straighten once you press it in place. Apply the interior ply in epoxy over the foam and frames, also bedded in epoxy. Remember, glue starved surfaces are not allowed, use the nothched trowel and err towards too much epoxy. You can insure contact with bricks or bags of sand .

Glass the inside and fair it while its on the horses and easy to work on. The inside glass will add a lot of strength since it will have to stretch in order for the roof to deflect under load. That's where glass really comes into it's own. There are 2 ways to make it stronger, make it thicker or add add glass to the surface under tension. If you're planning to party on that roof do both.

Foam core doesn't conduct much heat, it's a good insulator against heat loss to the exterior and will keep the cabin cooler on a sunny day.

Paint it a light color to protect the epoxy from UV and so as not to get too hot in the sun. Epoxy begins to soften at about 150F so white or a light grey is best.

[long time lurker]

Re: Strength laminating Coach roof vs single sheet ply

[QUOTE=ADonald;6859602]do you really believe that there’d be many people who’d have true data on this question, except the usual internet bull. I reckon the only people who’d be able to give an answer may be the likes of Gougeon’s, some designers etc most of whom have more sense than to come on here.

There are a great many who know exactly what they're talking about right here on this very forum. Believe it.

[Toxophilite]

Re: Strength laminating Coach roof vs single sheet ply

Wow! Welcome to the wooden boat forum! ADonald...last name Trump by any chance?

do you really believe that there’d be many people who’d have true data on this question, except the usual internet bull. I reckon the only people who’d be able to give an answer may be the likes of Gougeon’s, some designers etc most of whom have more sense than to come on here.

For what it’s worth my Grey Seal cabin top, that has a fair curve to it, was laid up with two opposing layers of 1/4” ply and it’s feckin strong. Dunno if single 1/2” would be as strong without destructive testing but I don’t reckon it’d be possible to bend 1/2” to this radius. BTW the hull is planked with 1/2” and those curves were bordering on no-go. One thing I do know is that once a panel is bent to a curve it’s way stronger than flat, or at least resistant to overall flex.

[Scary Borg Craft]

Re: Strength laminating Coach roof vs single sheet ply

[QUOTE=long time lurker;6859642]

do you really believe that there’d be many people who’d have true data on this question, except the usual internet bull.

Plenty of people here have built boats from scratch.

I used 12mm ply in a single layer for the last cabin top i built in ply. The butt joint was made over a deckbeam, and the top had a light glass covering for wear.

I did not see the point in using 2 layesr of 6mm for 2 reasons. First a 32sqft glue area is a big thing to deal with solo, with the risk of voids. Secondly the 12mm sheet took the camber without issue. The 12mm sheet i was using had more consistent layers than the 6mm stuff. I would only use the 6mm if the bend was too tight for the 12mm. The difference in strength between the two with the extra glue line will depend on who laid it up and with what resin and filler. It would not keep me awake at night, and i would not want to do twice the job..

[ADonald]

Re: Strength laminating Coach roof vs single sheet ply

If my last name was Trump it’d be changed quick smart, biggest oaf that was ever born.

my comments were actually in reply to Rigadog’s haste in expecting a reply. Prob badly written but true I believe regardless.

Scary B C , have you fully read my answer? The voids were nil partly proved by cutting out the companionway and the drilling of relief holes, in 2nd layer, every 100mm square. When I knock on 2nd layer there’s a solid thud. The second layer being bent over the first conformed very well as expected. The epoxy was “combed” with a notched trowel. All up a very good way to do this type of structure thanks to Mr. Oughtred. A friend who built a closely related design commented that he had a tough time bending 3/8” ply to the cabin-top.

Sorry RD still no answer to your original question.

[wizbang 13]

Re: Strength laminating Coach roof vs single sheet ply

two pieces of ply lammed together in a curve is far more than just a bit stronger than a single piece bent and glued to beams.one is adding strength, the other is almost dead weight in comparison

you may be able to omit the freakin beams
If you put one quarter ontop of three eights , it makes practical sense

[Scary Borg Craft]

Re: Strength laminating Coach roof vs single sheet ply

[QUOTE=ADonald;6859711]I

Scary B C , have you fully read my answer?

Yes, but when you go on to say you dont know if 1/2in will bend to the radius, what radius are you talking about? None has been mentioned.

A friend who built a closely related design commented that he had a tough time bending 3/8” ply to the cabin-top.

A deck that has double curvature is not going to get planked with a flat sheet without torturing. Adrian should have known better.

[bark-eater]

Re: Strength laminating Coach roof vs single sheet ply

I've got a small cabin top to replace. The original plywood was probably 5/16" and heavily kerfed on top and the grooves filled with polyester filler (now sand) and a layer of glass on top, again with polyester resin. I'm pretty pretty sure I can rebuild in exactly the same manner with epoxy and have the work last as long as the rest of the boat. What I am tempted to do is put down a thin layer of some sort of bead board and then a layer of glass then a layer of thin non woven core, maybe another layer of glass and then a thin layer of ply over the top. Relative to the original question about laminating sheets of plywood, it seems to me that adding a layer of fully saturated core between the 2 layers of plywood would reduce the chances of voids. the material I was going to try is 2mm Spherecore SP which I would order from Sweet Composites.