Coosa board hull (catamaran project)?

In another thread I started a discussion regarding an aluminium construction, so apologies in advance if this seems completely out of the blue. The point is that my personal project is such at an early stage that material selection is still possible, and my goal is to come up with a boat that is light and simple construction.

I came across a design from Angelo Lavranos for a 34 cat made entirely of plywood. The letter from the designer ( attached) call for building upright, glass taping the joints and glassing the outside to protect the ply. No glass on the inside.
Talking with the designer apparently this cat was quite a success and there are many cats still building at this time.

Although I am a fan of plywood construction, I also understand the advantages of composites.

I hah a look at Duflex kits, both Balsa and Foam core and they look interesting, but even more Coosa Board.
Coosa bluewater 26 has same strength and stiffness as marine plywood for the same thickness, and additionally it doesn't rot, and it doesn't absorb water.

My proposed building method would be the following, similar to a plywood construction but taking advantage of the Coosa properties:
- Have Coosa board CNC cut with scarf of puzzle joints
- Panels below water line are laminated and faired on the flat before assembly
- Sand the boards before assembly and epoxy coat
- Assemble panels and kit similar to Duflex system
- Glass tape
- Fairing / gelcoat/ painting etc

This is with the assumption that appropriate scantling is done using not laminated Coosa values.

The main difference from the Duflex panels is that:
- Duflex balsa is laminated outside as balsa is not rot resistant and mere core;
- Duflex foam is laminated as the structural strength there is given by the outside glass layers;
- Coosa board is structural ( cross linked fiber glass layers inside) and doesn't absorb water so it shouldn't need glass above water.
- Coosa board 50% price of Duflex panels

I would love to hear your opinions and suggestions.
Best regards

 

Advice: forget about Duflex. Badly overpriced and subject to rot. Secondary bonding is dubous.
Coosa board - we use it only for transoms, lifting points, etc. Does not make sense to use it throughout the hull.

 

I fully agree with Duflex being overpriced, and this is why I am looking for different options ( Aluminium being next on the list).
Could you expand on Coosa not being suitable throughout the hull? There are many catamarans up to 55feet built entirely of plywood ( Dix 470 Plywood Cruising catamaran https://www.dixdesign.com/47cat.htm ) Coosa could simply replace the plywood.. Epoxy coat, glass taped joints etc..
Is there something I am missing?

 

I don't consider plywood to be a proper cosntruction material for a cruising catamaran... We designed last plywood catamaran in 2007, since then nobody asked for it... It is not rational material selection.
it is easier to build in foam core, including temporary mold. Catamaran structure should be continous, I mean laminate.

 

Thanks for the advice and opinion.

 

What does Coosa say about its use as the main construction material of a hull, bearing in mind there are a few varieties of it ?

 

Perhaps consider making your own full-length prefab sandwich panels on a long table, obviously the systems that use smallish panels are that way because long panels like what I am talking about are a transport nightmare. You may also have less wastage of material

 

There are different densities with different properties.
The Bluewater 26( strongest) is advertised by the producer for structural applications, as being a complete replacement for plywood. They don't specify wether it should be used for the hull or not, they simply say: "just substitute your plywood for Coosa of the same thickness" they also go as far as saying that the same laminating procedure specified for plywood should be used for Coosa. Scarfing can be done as well as puzzle joints keeping 100% strength and stiffness. Epoxy compatible.

Hence why I started this thread, because since Plywood has and is being successfully used for hull construction, I don't see a reason for not using a better version of it, that doesn't rot and is 30% ligther. Not all plywood boats are fully glassed on the outside, some are just epoxied, and the same could be done with the Coosa.
Attached is the price list from VDL composites in EU. Altough more expensive than plywood, is still about half the price of a Duflex panel.

 

How much more expensive than plywood ?

 

Twice as much same thickness plywood, half the price of Duflex.

 

The idea would be to laminate only up to some height above the waterline, and yes I would laminate on the flat before assembly.
Superstructure and above waterline, I would only waterproof with epoxy and paint ( board doesn't rot and absorbs minimal water). Is it a bad idea?

 

I don't know of any reliable data for strenght and stiffness to be able to compare the two. Plus there is the aspect of abrasion and impact, I would expect at least the outside to be fully glassed.
Once one side is glassed there is no reason to not compare it to a full sandwich panel with a "normal" foam or balsa.
I would try to determine the price/square meter and needed workhours of the finished panel, that is including paint and any inside stringers or frames, but for that you need a design with several building options (or do some rough conversion).

 

You have to consider all the properties of the material.

I have never had coosa in the shop under 18-20mm, but I would be concerned with its ability to take an impact without deformation.

And you would need to prefill the pores of it or end up with air pockets.

And then you'd have a hull heavier than conventional foams.

The idea is not without merit, but has many questions. For sure, you need to line up the ply, corecell M, and coosa datasheets side by side and look for deltas.

The cost is hard to justify. Coosa is at least 2x ply. But no rot risk is intriguing.

Probably compare 6mm ply to a 6mm coosa (if they even make one that thin) to a 12mm corecell M..upload the xcel file here..

 

I believe the compressive strength of coosa is about 800 psi vs plywood at 5000 psi vs corecell M80 at 150..so probably some extra fiberglass vs plywood may be needed and on the inside as needed as well..for example, walking areas, etc

 

Attached is a comparison between Coosa and Ply ( 12mm Coosa).
It seems that although advertised as plywood replacement, mechanical properties are nowhere near those of plywood, so it would be suitable only as core for sandwich ( a very expensive rot-free core).