Thread: Marine Ply vs Standard Ply test

Originally Posted by blue sailor

I am not sure what you mean by "encapsulated, and particularly if sheathed". Are you referring to epoxy and fiberglass or something that is done by the manufacturer? I was concerned that the epoxy would not bond to the putty and since it is not to BS 1088, there would be core voids that would impact the integrity of the canoe . . .

I'm assuming you'll use epoxy and 'glass fabric sheathing to encapsulate the boat parts. Epoxy will stick to the filler, though many like to pick it out and replace it with their own (I do sometimes). There will be very few if any voids on a BS-6566 sheet (Aquatech). I buy this stuff by the pallet full (55 sheets at a time) and I can count the voids in the whole lot on one hand usually. Hydrotech (a BS-1088 sheet) will have no voids and prettier faces, but this (again) is a margin call, as both sheets are usually quite good and under epoxy and/or 'glass fabrics a moot point.

Go down to the supplier and look at both sheet types and see for yourself. Pick the one you want.

thanks PAR. I will go ahead and get the aquatech. I am planning on picking it up this friday.

thanks for all your help.

Hi all - great thread. A plywood question that bugs me: Why are the plies in marine plywood generally the same species / strength all the way through?
MIK mentioned beam theory in plywood stiffness - why aren't boatbuilding plywoods assembled like I beams or trusses, with heavier, stronger plies on the faces acting like beam flanges, with lighter, weaker plies on the inside acting as webs? A bit like the balsa core composite panels the cashed-up multihull types use..?

For instance, a square metre of 6mm gaboon at 430kg/m3 weighs about 2.58kg...

If we make a square metre of 6mm 5-ply with outer faces (say) Hoop Pine at 530kg/m3 and 3 inner plies of (say) QLD Paulownia at 330kg/m3, we're looking at about 2.46kg per square metre of 6mm panel for something with a similar section stiffness to the gaboon. (I have no idea how you'd figure material stiffness for a composite material).

Does the answer have to do with cross-grain strength if the outer plies align? Bending behaviour? Peel failure?

Howdy,

It might be possible to optimise plywood by playing with different timbers for different veneers. but I think the main restriction is the complexity of doing that.

The factories need to keep it simple.

In recent years as builders voted with their feet for cheaper and cheaper plywood some of the "gaboon" did have paper thin gaboon faces but interior veneers of poplar. Also low density but not very strong and a lot cheaper.

The other thing is think if you want to make plywood.

This is how it seems to have worked to me over some period.

4mm was a standard size. 3 plies/layers

Subjected to heavy sanding .. that gives you 3mm.

Add two veneers and you have 6mm with 5 plies.

A cheaper 6mm is made of 3 plies

Heavily sanded it becomes 5mm

Add two veneers and you have 8 or 9mm ply.

It seemed to work something like that. See how it means you can have two standard thickenesses of veneer and mix and match to get a whole range of finished boards of different specifications.

I would DEFINITELY love to see ply with gaboon face veneers and paulownia interior veneers. It would be light and have nice faces.

But I don't think it will happening soon. The factories want to standardize and pump it out.

That's all little bit more than an educated guess from seeing quantities of plywood stored in racks over the years.

MIK

I've custom ordered plywood on a few occasions and you can get it "constructed" anyway you like. The same or similar species are used simply to provide two things; uniform physical attributes through out the panel and rot resistance.

Lets say you do elect to get a hardwood faced, softwood core panel. You'd think this would make a stronger panel and on a technical level it would be. In reality, for this type of "sandwich" construction to work, the "center of gyration" needs to be examined. Sandwich composites work, because of the distance between the two outer structural skins. The farther these skins can be from the center of gyration, the more effective this construction method becomes. Simply put, in most plywood, you just don't have enough distance to make any meaningful gain in panel stiffness, so why bother.

There are other considerations, such as one species taking to bending loads differently than the others used, setting up shearing load along the glue lines and other physical attribute discrepancies, that lead to the differing species fighting with each other within the panel under load. There are other considerations, but these are enough to warrant a closer look into true sandwich panel construction, rather than plywood. Weight is also a big driving force behind these types of panel build methods. A solid wood panel is at an obvious disadvantage in this regard. This said, I know of more then one company that uses balsa core with hardwood faces, to build composite panels that are exceptionally strong, stiff (and costly). This would be the better approach and from a technical stand point and could be considered an "engineered plywood" panel, though typically separated and called a composite panel, if for nothing more than marketing purposes.

That's what I like about you Paul ... i get a few "a-ha" moments!

Good argument about the "radius of gyration" or "second moment of area"

Also interesting for lightweight structures it makes sense to go for the lightest ply possible ... the greater ply thickness is always worth more than higher strength but heavier ply.

Never heard of anyone building a lightweight boat out of thin heavy plywood

So the more dense plywood on the faces argument doesn't make sense ... it's always better to go thicker with lighter plywood.

MIK