What is CTBX Plywood Certification? (urgent)

Thank you very much. Your posts in this thread have been a tremendous help to me. The same plywood Charly used is on its way to me. The mold is half way built. There are quite a few scarfs to make and then it's time to make some hull panels.

 

AT Ilan Voyager:


If I want to build a catamaran with little panel bending (eg: Kohler design ), will ctx ply work as well as marine. I live in the caribbean and every $ saved is appreciated. Will be making my own rig as well.

I have the feeling that as plywood gets thicker, then equal size voids are less of a problem because the void percentage falls and there are more plys to distribute loads past the void. Am I correct in thinking this?

 

Sorry mate! I just have seen this thread for the first time.

The day you opened it (and the following two weeks) I was on a sea trial and did not contribute here regularely.

But luckily Ilan chimed in and provided advice in a quality I would not have offered. My experience in Ply is by far below his first hand knowledge, and I have definetively NO clue about Hughes method.
We use ply only on bulkheads (not always) and cabinetry. Everything strucural is veneered or laminated, even our transoms are cold moulded.

So, congratulations, you have had the very best advice possible here!!!

Richard

 

Thanks, Richard. I am very happy to have you guys to help steer me through the bumpy road of a first build. Materials sourcing was one of the most difficult parts. Now that I'm actually building, things are going well.

I was surprised at Ilan's depth of knowledge with both the CTBX and the Hughes methods.

Thanks, guys.

 

You have been lucky...I had been very busy and no time for leisure.

What's the size of the boat? and what are the scantlings? Do you have study plans to measure the bending? Is it a hard chine hull?

The problem of voids is double:
-possible source of rot.
-structural. A void is a solution of continuity where the stresses will concentrate. A small void in the center core does not jeopardize the integrity of the structure, the same void in a outer ply is a big problem.

The true problem is not truly the norm by itself, it's the real quality of fabrication of the panel. Some CTBX are far superior in general quality to APA fir "marine" plywood, unhappily some CTBX have no place on a boat.
So you have to buy samples and test the plywood. Marine has the great advantage, when you can't see and choose the panels yourself, that it can be simply ordered... A travel to France to see the plywood, make the tests, order and ship the panels may be more expensive than the savings on the plywood...I had just to cross the street to buy the plywood for my shipyard so I used CTBX because I could choose practically each panel.

First remember that CTBX is a voluntary norm, it can cover a wide array of quality. Stay on okoume, the other woods are too heavy.

Some plywood makers are very careful and practically the lone difference between the marine and CTBX panel will be the thickness of the plies, some are less careful, the difference of quality may be too great.

You should look at Joubert in France. They produce a structural CTBX in okoume the oukouplex exterieur, and also the oukoume bardage. You should get further information from them, specially the strength data. Joubert has excellent and old reputation.

I won't rewrite this thread. Yes CTBX can be used on boats but with precaution in your case. The gluing is marine when done with resorcine/phenolic resin called in France Class III.

But I must warn: the use of CTBX in the peculiar method (cylinder mold) of this thread was for 3mm plywood to be glued with epoxy, resulting in a 9+ mm panel with 9 thin plies, and with epoxy glue lines every three plies. Plus a lot of fiberglass outside, and a lot of structure inside.

So if you plan to use a CTBX you must consult the NA of the boat and the construction probably will need more fiber and epoxy. If the boat is more a composite wood epoxy (core in wood and structural skins in fiber epoxy) this alternative can be used.
If the boat is more a classical plywood where epox is used instead of resorcine, and just a waterproofing with epox and a bit of fiber, you must calculate with great attention and care 2 things:

-structural. You must get the same strength with the same weight. If the bare boat gets too heavy, you'll have to sacrifice amenities to get the same displacement fully loaded. Or you will enter in the infernal circle of the too heavy boat needing stronger rigging, thus getting heavier again.
Multihulls must be lean, over-weight reduces dramatically seaworthiness.

-final cost. Epoxy and fiber are more expensive per kilo than plywood. A cheap plywood with a lot of epoxy costs more that a marine plywood with a few drops of epoxy. Calculate also the amount of work hours for each method. You may be surprised...Boats cost per kilo: an heavy boat made with inferior materials can result more expensive than a lighter boat made with the good materials.

A good and easier (less calculations!) solution is to use marine plywood where needed, and CTBX for all the remaining. And a cruising boat has tons of things inside where a good structural CTBX has its place with no problem.

Also keep in mind that the plywood is just a small part of the total price of a boat.

 

Thanks Richard (Apex1) for the kind words... And I'm happy to see that the construction is going well for CatBuilder.

 

To each saint his candle Mate!

None of us is the "know it all" expert. And after more than 40 years around boats of all kinds (I guess you have more), we know that we don´t know much. But we know what we know.

Yes, nice to see he got started. And we are part of it.....

Regards
Richard

 

Ilan Voyager.

Thanks for the useful reply. I have 3 yrs university in mechanical engineering and can work with stress/strain engineering mathematics. I will be doing tests and detailed comparisons for both strength and costs.

You do have a huge amount of useful information and experience that i"m sure everyone appreciates very much.

The design I'm looking at has hard chines and narrow hulls. Very large bending radii.

Joubert occume is my present 1st choice.

 

I have almost 40 years in the field...and like you I know what I know, and better I know that I know finally very little in the diverse and immense field of boats and ships.

But about plywood and composite wood-epoxy I have a first hand experience as builder and engineer from the pram to the mine hunter, so I feel qualified in this domain as well in practice as in theory. Besides I love this material (with its limitations), it is one of the best for a small shipyard or amateur builder.

That's one of the purposes of this forum; to share knowledge.

Saludos cordiales. Ilan.

 

You're welcome. With such a background you won't have any difficulty to make the comparison of strength and final cost.
I feel (just smelling the wind, an intuition) that a "mix" of marine and CTBX is the solution. Surely the bottom of the hulls and some other "delicate" parts will require marine for the quietness of mind and general security. The other parts can use a good CTBX, if the engineering is done in that way.

A suggestion: do not tell to Joubert what use you are planning of the CTBX, when asking engineering information. They make very good marine plywoods and will try to sell it to you for 2 reasons:
1-financial. As everything labelled marine (or aircraft) the price is higher.
2-liability. They will never write that their CTBX can be used on boats. They'll leave the responsibility of the use to you. And that's normal.