Ply Covered with Epoxy Info Request

[Houseboat Man]

Hi All,

New to the forum and made a mess of my first thread, posted it in the wrong section.

Anyway I am now in the right section, maybe mods might delete the fist one.

I need to know a bit more about the minimum plywood thickness that I can build a cabin from and coat it in epoxy and cloth.

The boat is 18ft 6inch and as it stands I am going to purchase some epoxy some thin ply and some thicker ply and use the thicker ply for the uprights and beams and the thinner ply for the walls and roof then coat the whole lot with epoxy and cloth.

I know that sounds rough, so any suggestions re. ply thickness , cloth and epoxy would be much appreciated.

Going for a functional workboat type arrangement .

Thanks

 

[lewisboats]

So long as you don't plan on walking on it you can use 1/4" with solid wood framing. If you frame it up strongly enough you can might even be able to walk on it. Crowning the cabin roof will also help stiffen the plywood sheathing and allow drainage. Plywood will not give you proper framing unless you use it edge wise and fillet it in, it may be heavier than solid wood and will have to be at least 3" deep (cutting down on head room). You would have to use at least 3/8" thick but 1/2" would be better. I would recommend you frame it with solid wood and skin it with ply.

 

[Houseboat Man]

Thanks for the reply,
Alright, 1/4 inch ply for the skin, with epoxy and cloth over it? On both sides? and solid wood framing. 2 inch by 1 inch for the all the framework uprights and crossbeams with a distance between them of 12 inches?

I'm not sure about crowning the roof, I would prefer it but time and money and a lack of real skill etc, would be nice though. Maybe a slight crown. Always like to see boats finished if you know what I mean.

Pretty much now I am thinking a bunch of 2 x 1 's and 1/4 inch ply the 2x1's flat against the walls and on their edge for the roof ( unless I crown the roof )

Do you think 2x1 's is overdoing it?

 

[rasorinc]

Crowning a piece of wood is quite simple using 1" x 2" lumber. Lets say to roof is 7' wide and you want a 1" crown. Get an 8' x 2 x 6 and anchor the opposite ends then you could put 3 to 5 frames on it stacked with the 2" side vertical. Put a piece of scrap 2 x 4 or 6 over one end and clamp it down tight. In the center put a 1" or 1.5"narrow block under the roof rafters then put another scrap block over the other end and push it down and clamp tight. In 24 hours, release it and see the crown amount. If not enough, re-clamp for another 24 hours. Thats all there is 2 it. A 1x2 rafter will let you put a !" piece of flotation foam between them giving you an R-4 rating which prevents condensation.

 

[PAR]

You'll want some crown in the roof, preferably quite a bit. Besides shedding water, it makes the piece much stronger, with no additional weight gain. Across say 6', you'll want at least 3" - 4" of crown, with 6" being much better.

The same is true of the walls. If they have some curve in them and lean inboard slightly (5 - 10 degrees) then they will be considerably stronger. This means you can use lighter stock (which saves money and weight) for the same strength and stiffness, compared to a similar design with a flat roof and walls.

To further this train of thought, with curved panels, you can tape the seams and skip the majority of the solid wood framing stock.

Lastly 'glass cloth doesn't add any real strength or stiffness, unless it's quite thick, so a sheathing of light cloth, only serves to offer abrasion protection. So, unless you plan on adding a 1/4" thick skin of goo and fabric, the structure needs to rely on the plywood, its shapes and the joints/frame for it's strength and stiffness. Think light weight as weight or an excess of it, is a small craft performance and efficiency killer.

 

[Yellowjacket]

I don't quite agree that adding cloth and epoxy doesn't add strength or stiffness. Actually, if you are going to add an epoxy coating to protect the wood, you can actually improve the strength and stiffness of the end product for not much more weight than you would add by just using enough epoxy to properly seal the surface. Here is a table that was produced by Duckworks in testing fiberglass over plywood and cedar strip planking.

The test panels were 12 x 12 inch tested in a three point bending machine, so it's probably representative of what we are talking about here.

I've added columns for a stiffness to weight and strength to weight ratio, but the basic data says that adding a layer of relatively light glass cloth improves the strength of thin plywood by about the same amount as Increasing the thickness of the plywood to get about the same weight. You can get about the same strength to weight and stiffness to weight out of 7/32 ply with 4 oz glass on it than bare 1/4 ply, and if you go thicker with the ply the effect of the glass is even more pronounced. The obvious the question is, why would you want to bother with the glass? Well, the answer is that if you were to add just epoxy to seal the surface you would be adding almost the same amount as if you use the 4 oz glass, and essentially get no strength benefit from it.

So, the answer is, that you won't make it any stronger than if you just used more plywood, but if you are going to waterproof it with epoxy, then using 4 or 6 ounce cloth probably makes sense, not only will it make the finished surface more durable, it will indeed make it stiffer and stronger than a thinner plywood, or, to look at it the other way, it will be about the same weight and stiffness as if you had used thicker plywood, but it will be waterproof and more abrasion resistant, so there actually is a payoff from using relatively thin layers of glass over plywood. It's not that different in strength or stiffness, but it's waterproofed from the start, and that's not a bad thing.

Also, using 4 oz of glass over plywood if you are bright finishing it makes it a lot less prone to checking, so there's a durability factor there too.

I've included the file as a spreadsheet too, if you want to play with the numbers or put them into different units, but you get the idea.

 

[PAR]

There is a measurable increase in strength and stiffness, but not one that's significant with sheathing schedules typically employed, which for the most part are on one side. This is why I say a 1/2" sheet of plywood gains no (appreciable) strength or stiffness with a light sheathing (typically one side and less then 6 ounces). This is especially true with thicker plywood than thin. A 4 ounce skin on one side of a 1/2" sheet isn't significant enough to really consider. A 4 ounce sheath on one side of an 1/8" sheet is, but this would be a very small and/or light hull shell, where strength to weight ratios, need special attention anyway.

This said, there are advantages to it and most know my opinions in regard to sheathing plywood. Waterproofing is greatly improved with sheathings and abrasion resistance too.

As to the original poster's concerns, a taped seam type of build will produce a light, strong end result, with a minimum of pieces and fitting.

 

[Houseboat Man]

The update on the plan is as follows.
The roof will have as much crown up to 6 inches as I can achieve.
1/4 inch ply with epoxy, not sure about cloth at this stage.
2x1 hardwood for framing.

 

[Houseboat Man]

Crowning a piece of wood is quite simple using 1" x 2" lumber. Lets say to roof is 7' wide and you want a 1" crown. Get an 8' x 2 x 6 and anchor the opposite ends then you could put 3 to 5 frames on it stacked with the 2" side vertical. Put a piece of scrap 2 x 4 or 6 over one end and clamp it down tight. In the center put a 1" or 1.5"narrow block under the roof rafters then put another scrap block over the other end and push it down and clamp tight. In 24 hours, release it and see the crown amount. If not enough, re-clamp for another 24 hours. Thats all there is 2 it. A 1x2 rafter will let you put a !" piece of flotation foam between them giving you an R-4 rating which prevents condensation.

I am not certain which way the 2x1 is going to be bent and then used.
Is the 2 inch face the one that gets laid face down to be bent?
Then when that is installed as the roof beam you have two inches for the roof ply to sit on and only one inch to bang your head on?

Or the other way round?

 

[PAR]

Dimensional stock (1x2's) can be bent "standing up" be it's very difficult, without a jig. The usual course is to bend them on the flat. This is much easier, but they'll want to unbend, so have to be "contained" with a perimeter frame, of sufficient stoutness to prevent them from spreading it. The common way around this is heat, steam or laminated bent beams. These will take the new "set" and remain bent, without much spring back. This permits a lighter perimeter frame.

Using beams is pretty old school, though most folks can get their heads around it. If it was me, I'd make a 1x2 perimeter frame and insert foam in the crown desired, which would then be skinned with a layer of 1/8" plywood on each side. No beams to bang your head on, built in insulation so the sun doesn't cook your brain and it's light weight. The use of bead board on the inside skin, would look good and have a traditional feel to it. This technique would handle more weight on it, than a 1/4" plywood on 1x2 arrangement too.

To make something like this I would build a male jig to the roof crown, then place the inner layer of plywood down (slightly over size, for the fasteners), lightly tacking to the jig. Next, the perimeter frame would be glued and screwed in on the plywood, followed by the foam, all bedded in epoxy and filler. Lastly, would be the outer skin of plywood, glued and screw to the perimeter frame, over the same epoxy bedding. Pop it off the jig and you've got a beamless roof with a built in crown and solid wooden edges around the perimeter that could be rounded off and varnished (if desired). To get fancy, you can rabbit the plywood edges into the perimeter frame for a clean look and one that protects the plywood end grain.

 

[Houseboat Man]

Using beams is pretty old school, though most folks can get their heads around it. If it was me, I'd make a 1x2 perimeter frame and insert foam in the crown desired, which would then be skinned with a layer of 1/8" plywood on each side. No beams to bang your head on, built in insulation so the sun doesn't cook your brain and it's light weight. The use of bead board on the inside skin, would look good and have a traditional feel to it. This technique would handle more weight on it, than a 1/4" plywood on 1x2 arrangement too.

T

What do you mean by foam, is that any type of insulation, or is it high density foam like what is used in surfboards? I am confused here. Is it a product that you have readily available in the states?

And also what do you mean by bead board, is that the panelling that looks like pine for about 4" and then has a thin groove?

 

[michael pierzga]

Foam core plywood. Flat panels can be purchased . Cambered cabin tops must be shop made. 3 or 4 mm ply skins over something like 20mm foam. Thick foam core is nice because it allows you to bury electric lights and cable in the ceiling or walls

Ask your supplier for suitable foam density scored.

Foam core cabin tops are a very nice detail. Clean, strong and good insulation

 

[PAR]

The foam you'll want is at least 5 pound density, closed cell polyurethane (not pre-scored unless you want compound curves) and 19 - 25 mm thick will do. Again, foams are expensive, though there are options, such as EPS or the "blue board" stuff at the big box store. These aren't structural foams, but if you're not going to use your roof as a dance floor, not so much a concern., particularly with a stout perimeter frame and the odd beam or two.

Bead board is plywood that's been grooved to look like old school wainscoting. It can be had in various thicknesses and usually has a WBP adhesive.