What is CTBX Plywood Certification? (urgent)

Thanks, Ilan Voyager.

One thing I've learned for sure is that the BS1088 standard requires 1mm ply layers. Because 3mm cannot possibly have 1mm plies and sill be glued together, there is no 3mm that can be called BS1088.

This is why 3mm is always referred to as BS6566, even if it is made with the right glue.

Every 3mm ply I have samples of has a large core and thin outer faces. Vendor A's outer faces are about twice as thick as Vendor B's outer faces, which is what makes A's ply heavier and stiffer. The both have quite thin outer face layers though, and fully thick (1mm) cores.

Upon bending, I find that B's bends easier. Both panels make crackling sounds when bending. When bending to the extreme, B's panel blew out first. A's panel has not blown out, but I think I had it close.

I guess I have to find the densities...

 

The density of Panel B is 510Kg/M^2. I am waiting on the density of Panel A, but it seems to be much more dense. At the same time, my Panel A is the one Charly used in the other thread. The BS1088 stamped Lloyd's approved 3mm.

So... for compounding, do you think I ought to go with the lighter weight panel with the thinner faces that broke in half when I was bending it?

My guy says go with the same stuff Charly used, which is the heavier, thicker stuff that bends very far without breaking, but is stiffer.

I'm close, but need a nudge in the right direction.

 

Breaking News!

I just looked very closely at Panel A, the one that's stronger, heavier, stiffer and that doesn't break when I bend it. The reason is that it is 5(!) layers!

The 3mm Panel A is 5 layers! There is a core that's the same thickness as Panel B, then it has double the outer layers that panel B has.

Is that a good thing or a bad thing??

So Panel A has outer skins that are doubled, while Panel B has a single outer skin. When I bend Panel B to the same radius as Panel A, Panel B snaps.

Which would be best for cylinder mold (and fighting it into a deck jig)?

Which is best for a nice, strong, stiff hull that's lightweight?

I can order today if I can figure this out!

 

I am an amateur and my opinion is cheap, but if the glue is right I would take the 5 ply.

 

Get ready for a temperature drop. Tropical storm near the Bahamas will bring cool northern air south. Use info to your advantage.

 

Thanks, Hoyt. Don't have much to do, actually. No supplies!

Being that there are no other opinions, I may go for the 5 ply.

You know what though?? I got the densities on the two plywoods in this thread. Keeping with the naming conventions, Panel B, the LIGHTER one with less plys is quoted to have a density of 510 Kg/M^3.

Panel A, the one with 5 plies, which is HEAVIER is quoted to have a density of 500 Kg/M^3.

One of these guys is not telling the truth, even if it's an error.

 

Hi CatBuilder,

Five plies for a 3 mm: I've only saw that for birch plywood from Finland and Russia: very strong and bendable, weight around 600-650 kg/m3. What's the wood of your 3 mm five plies? May we see a pic? If the panel is too stiff across (where you have to get the tighter curvature) you'll may be in trouble for compounding.

What the exact thickness of the 5 plies plywood? each tenth of mm counts, as the rigidity goes up at the third power. You should measure also the stiffness by bending for example a 30 cm * 5cm strip to a 2.5 cm deflection with weights, and also find the maximal strength. And make a sample of 3*3mm as you'll have on panels, and measure the 2.5 cm deflection.
Compare it with your other sample, I would even compare with a common 3mm skin door bought a Home Depot or similar.
Why to compare with a skin door? simple, most of the boats made in cylinder mold method were designed to be built with skin door, which could be found 20 years ago in waterproof quality, 3 equal plies in a asian light mahoganny. The panels shape and compounding (and the degree of compounding determines the displacement you'll get...) were found with a rather soft and pliable plywood. If you use a too stiff plywood you'll never be able to get the compounding needed to get enough displacement: in other words the hull, after compounding, will be more V shaped, thus a lower displacement. Also too stiff panels can break in tension or in shear just after the composite keel as the efforts of bending are not dissipated within the plywood, thus concentrating the stresses at the keel zone.

Compounding plywood is to put the panel under "across" tension with straps or ropes. The outer face must not break under tension (in the Stressform method, a layer of fiber is even added outside) and the wood will "crush" in compression in the inner side. It's a controlled "failing" of the panel in compression; if the panel is "soft" enough the stresses of the bending will dissipate within the plywood and won't attain the breaking point at the keel or the outer face.

You ca use a stronger plywood if the panels made with it are not too rigid and can bend evenly. So make a sample panel of 3*3mm and measure the bending you can get: it must be at least 1.5 times the max bending found on the plans.

Also there are a few tricks: use belt straps, the smallest (1.5" wide) used on trucks work well, ropes have tendency to crush locally the wood. Sometimes I had to put 1.5" 1/8 thick aluminum angle at the sheer to reduce the local stresses induced by the belts.

A very useful trick if you expect very high stresses is to cover the plywood of outside at the keel joint with a peel ply tape of 10 to 15 cm. Peel Ply is a very strong nylon cloth in tension and will reinforce temporally this delicate zone by taking the tension stresses at the keel joint.

Last trick is to cut the panels at least 10 to 20 cm too long at the transom: That prevents the splitting of the panels by concentration of the stresses at the edges. After installing the transom you remeasure and cut safely the hull.

 

Glad to read your post, Ilan Voyager. Thanks!

No plywood I can find has 3 equal layers. All plywood available has 1 large core (1mm? More than 1mm?) and thinner "faces." I have not found any plywood that has even layers in 3mm. This only happens at 4mm.

The "5 ply" plywood merely has a "double face layer" compared to the other ply that has a single face layer. These face layers are thin relative to the core, but the "5 ply" has a closer ratio of core thickness to the double face layers. The 5 ply is the same exact plywood Charly has used on his Hughes boat in this forum. He has yet to pour a keel and wrestle it into a deck jig, so I can't say it will be easy.

One thing I can say, from my testing, is that the thicker plywood doesn't break as easily as the thin stuff. Bending both Plywood A and Plywood B the same radius, the lighter, thinner plywood (Plywood B) was easier to bend, but it also broke! The heavier, thicker plywood bent to the same radius and did not break. That was one of my main concerns, as I didn't want to blow out a panel upon compounding.

The heavier ply with the double faces (Plywood A) can be bent into an incredibly tight radius and it doesn't break. However, it does take more effort to bend it and it is clearly heavier.

The heavier plywood can bend at least 2x the maximum bending found on the plan. The heavier plywood is Plywood A in these attached pictures of bending. The thinner plywood, Plywood B, looks thicker in the pictures, but that's just because of a rough cut on the edge.





I will look at Home Depot doorskins to see about the stiffness. Unfortunately, I don't have large enough samples to perform the tests you are suggesting.

This is getting pretty frightening. I have a short time period to build this boat. I am quite bothered by the question of if I will be able to get proper displacement, especially since this is a cruising charter boat, not a day charter. I need rather large displacement.

 

CatBuilder,

You can go with the 5 plies. It's very bendable on a tight radius without problems. No need to make a trial with door skin.

Sure the second sample is not good at all for compounding, the core breaks like glass.

With a little bit of planning and precaution, you'll get very full and round hulls.
Use a jig to get straight hulls without twist. We'll see that at the time.

 

Thank you, Ilan Voyager.

I had ordered the 5 ply this morning. The plan has a specification (and lines and stations) for a deck jig. There will definitely be one. Check out Charly's thread on the panel buckling:

http://www.boatdesign.net/forums/boat-building/kurt-hughes-daycharter-36-a-31846-3.html

He is using this same 5 layer plywood from the same supplier I just bought from. Certainly there are some mysteries to be solved yet. In the Kurt Hughes video, they show people easily carrying the cut out hull panels by the ends. However, Charly's experience says you can't quite do that without buckling. Could be the plywood, could be the epoxy, could be the spread of the epoxy. Either way, it's something to be careful of.

 

Catbuilder, you're welcome.

The deck jib is the primordial component; it will give the definite shape of the hull. It has to stand a lot of stresses (happily in tension) but I was talking about a "hull jig": something to keep the panels well installed with a straight keel while making the composite keel. An undulating keel is a pain and the hulls won't be fair. A strap of peel ply glued with EP before the folding over the keel joint on the outside permits to fold heavily without breaking the panel at the keel joint.

The old video sold by Hughes 13 years ago was properly terrifying with a guy walking inside the 2 panels joined at the keel by some copper wire and trying to get them straight. The whole thing being directly on the floor and resting vertical against a wall. How he didn't break the panels? surely pure luck.

A "hull jig" that you can rotate on each side is a nice plus; instead of coating
a vertical panel with the EP running down, you're coating an almost horizontal surface. Very useful while coating the inside hull.

Panels behave like a bowl of jelly before being tamed by the keel-hull jig. They must be handled with care:
First thing to do for handling and storing the panels is to brace them. A 1*1" stick of wood solidly braced with light duty belt (the cheap chinese ones, you will use them later for folding) every 1.2m to 1.5m. So the panel won't buckle as it becomes rather rigid.

The sticks are longer than the width of the panel so the pane,l when stored, will never touch the ground; You can imagine the consequences of any pollution of the panels with water or some other product running on the floor...The panels are never piled but kept almost vertical on the legs made by the sticks. Always cover the panels to protect them but the tarp or plastic must never touch the panels. Good aeration: humidity and water condensation can make a lot of harm. If needed use a ventilator.

Precoat and finish everything flat; bulkheads etc... The old video with all the uncoated vertical surfaces is a pain to watch. How do you coat and sand correctly vertical surfaces? In fact in my shop we precoated, sanded and finished every plywood panel before cutting, excepted those which will be bent or used for lamination. Even the under-deck was finished before gluing it on the hull: I hate coating or painting anything above me.

On a boat that saves lot of pain, improves the quality and tons of work hours. Do not think that an amateur has not to count hours before they are free. It's false: hours cost to every body. Longer, tiring building time, more money needed because it's longer than thought, more personal problems arising with wifes, children, friends, parents.

A last thing about stringers: do not stay fixed on spruce if it's too expensive or hard to get. Any wood like light pine will do the trick if it bends evenly and it's clear and better quality. You can even use a cedar if the last piece of the lamination "in situ" is done with a stronger wood. For example lamination of a 25*50mm in 3 pieces; 19mm cedar on the hull, 19mm cedar in the middle, 12mm mahogany or fir on top.

We'll see the things in their due time...

 

Great information. Thanks Voyager

Catbuilder, my plywood is three plys...

 

In light of the splitting episode, I am more convinced the 5-ply is the better choice.

 

You're right, Charly. I was looking at the plywood from a side where the grain of the faces runs perpendicular to your viewing angle. There was a long, thin line all the way down making it appear to be 2 layers of facing. It is not, now that I look much more closely and from many other angles. I think it was one of the hollow tubes, broken in half, in just the right spot that looked like a glue line.

It's 3 ply, but with double the size faces of the other stuff I looked at.

 

Three plies but nicely bendable...I was a bit surprised as the lone 3mm 5 plies I know it's an aviation birch plywood made in Finland in 1.2*1.2 sheets and at a price...

I wish to you two, Catbuilder and Charly all the best for your cats. If you have a doubt, or need some help do not hesitate to send me a private mess.