Tortured plywood thickness/layers

[filiperosa]

I am looking for plywood to build a trimaran hull in tortured plywood according to The Gougeon Brothers on Boat Construction. The designer refers as 3 layers of 3mm plywood making a total thickness of 9mm.

I was looking for plywood here in Portugal and Spain and I could find 3mm plywood with doubt qualities.

I was able to find a firm that makes 100% Okume phenolic WBP with very good quality and price. I already bought from them in the past without any problem. The problem is that the minimum thickness they do is 4mm. If I keep the 3 layers it becomes 12mm thickness making too heavy and difficult to bend to the desire form.

It is possible to use 2 layers of 5mm or even one layer of 9mm instead of the 3 layers or this would give me problems in the bending or other structural problems? What about 2 layers of 4mm making a total thickness of 8mm instead of 9mm? Any other better solution?

Thanks,
Filipe

[rwatson]

yes, tricky situation.

My limited experience with tortured ply inclines me to think you run the risk of not being able to get enough 'torture' if you go thicker than 3mm. 4mm ply is not nearly as flexible. I would expect the Gougen boys would have recommended 4mm as a minimum if they thought that it would go around the bends.

You could contact Joubert in France ( just over the border ) for quality marine ply http://www.joubert-group.com/contact.php

- but they do not advertise a 3mm Marine ply in their range of products.

http://www.joubert-group.com/gamme_entree.php

JOUBERT
Saint-Jean d’Angély

Rue Lafaurie
17416 St-Jean d'Angely Cedex
France
Tél. +33 (0)5 46 32 20 37
Fax +33 (0)5 46 32 26 15


Facing that - there is plenty of precedent for cutting ~ 1mm V grooves in 4mm sheets to make the stuff bend around corners - but you need to ensure very thorough filling of the gaps ,and it is an extra layer of complexity in the build.

In the face of a plywood shortage, do the plans provide a foam and glass construction option ?

[filiperosa]

Hello,
Thanks for the reply. Yes I have also details for the core version
Regards,
Filipe

[gonzo]

You won't be able to get the proper shape with two thicker layers.

[filiperosa]

Thank you, so it is pointless to order 4mm plywood. I found 2,4 mm external (wbp) hardwood Plywood without knowing the type of wood. Would this suit me? If yes it would be better 3 layers making 7.2mm or 4 making 9.6mm nearest to the 9mm and the epoxy between them making more flexible?

[Charly]

Hi Felipe,

Boulter plywpood stocks 3mm Okume. http://www.boulterplywood.com/

It is good stuff, and is made to British Standard 1088, even though it is advertised as 6566 (exterior), the actual ply is stamped 1088 (marine). The way I understand it, according to Boulter, 3mm ply cannot meet true BS 1088 standards, because the three individual layers cannot be thick enough to meet the specs, and still be only 3mm in overall thickness, so... it is advertised as 6566. But everything else about it, the glue, the absence of voids, etc., meets the 1088 guidelines, so... it is stamped 1088.

It may, or may not be too stiff for your particular application. Best to check with the designer.


I am currently building a Kurt Hughes 36 daycharter cat with the same product and epoxy using the "cylinder mould" technique. The designer recommends 6566, which is what I thought I was getting. When my plywood arrived it was stamped 1088, which I thought of as an "upgrade". The 1088 did cause some problems at foldup, though not insurmountable. The end result is probably superior to 6566, but it is stiffer plywood, and takes more care when bending. I tourtured mine a couple of times until it "confessed"

[lewisboats]

Originally Posted by filiperosa

Thank you, so it is pointless to order 4mm plywood. I found 2,4 mm external (wbp) hardwood Plywood without knowing the type of wood. Would this suit me? If yes it would be better 3 layers making 7.2mm or 4 making 9.6mm nearest to the 9mm and the epoxy between them making more flexible?

I suspect that that is a birch plywood which is significantly heavier than Occume and also quite a bit stiffer. With the reduced thickness the stiffness might equal out but the weight wouldn't... not with 4 layers. Is the wood very white with almost no real grain showing and only small pin knot or dark wood spots? Purchase a piece and let it get wet outside...if it turns black in short order then it is most likely Birch. Well sealed it isn't bad but you have to keep the moisture out. It is really only a little less durable than Occume.
Is it possible for you to get veneers themselves and basically laminate your own plywood as you go? You will not have any issues with it bending to shape and it will be stronger as you can orient the grain direction to make a superior laminate.

[filiperosa]

Thanks. So if I understood properly it would be OK to use 3 layers of 2.4mm with stiffer wood. I will try to get some samples and test them.