Coosa board instead of marine plywood for stitch and glue boat?

Anyone heard of someone using coosa board instead of marine ply for a stitch and glue hull? I'm reading "Tolman Alaskan Skiffs" right now, and the whole build is marine ply (only the keel panels are S&G), but after watching video after YT video on replacing rotting plywood transoms, a little seed of doubt has been planted. Why not build all panels, stringers, and transom from Coosa instead and not worry about the plywood getting water damage? Any downsides to this?

 

Coosa is really stiff and works well for flat panels. It does not hold screws as well

 

Any core that becomes compromised is going to be a problem.
I’m not an advocate of core below the waterline.
Solid fiberglass panels might be your best bet.
The Tolman looks like a great boat for Hawaiian waters, reminds me of the local designed Surrat boats of the 70-80s.

 

I have heard of someone doing this and it worked fine. Entire hulls and forms have been built from it.

The real issue is cost —because the Coosa board is about $300+/ 4x8 1/2” sheet blue water 26. I’ve considered doing it as a demonstration on video but haven’t worked up the courage to spend the money.

While it is true that Coosa board doesn’t hold screws as well as plywood— you can get around some of that by applying fiberglass cloth to the surface so the screws have something to help bite into. (If you’re using it for stitch n glue you’re epoxying the hull and parts anyway.) I’ve done first hand experiments —The Coosa board will sheer away destroying itself long before the epoxy tabbed joint fails. It has numerous advantages over foam cores. 30-40% lighter than plywood. Doesn’t need a hot coat. Doesn’t rot or allow bugs. You can even paint it with no need to epoxy or seal the surface for finishing. I talk about it on my YouTube channel and I show real projects.
You be the judge.

Coosa board can also be bent, I have videos showing how I do this for my trawler hardtop video (support columns and hardtop) and my Compac 16 pilothouse conversion. The pilothouse was build using my “screw-n-glue” technique. Built in place by bending panels on deck—Frameless —basically exactly what you would do for a stitch n glue hull.
https://motorcityboatwerks.com/boats/compac-16-pilothouse-conversion/

And I’ve built an ultra light dinghy using Coosa board on watertight compartments and seats. It’s definitely not just for transoms or flat surfaces. Again I have no reservations about using it to build stitch n glue.

I’ve built many things for my boats using Coosa board as a direct plywood substitute beginning in 2015. Once a person has actual experience with it —I think you will love it—the question is money. You want to use it everywhere and I do …but it’s expensive.

 

Did a cursory search of my order history, and im up to 10 sheets of 3/8 blue water 26. Did some for core on a tophouse visor and some as a core on a flybridge venturi. It does bend, but its still stiff and takes a fair whack of force for forming the more complex shapes. Granted the tollman shape isnt exactly a complex hull shape.

My below the water line coosa isnt old enough to give solid feedback on it, maybe in a few years but its still young.

Shop around on coosa, its amazed me the price variation from one dealer to the next. Everyone has slightly different product lines and prices but coosa has been a wild price variation.

 

It can be done. It will be heavier. Despite coosa 26 and say okoume offering similar densities, coosa is less rigid. This difference is made up for in either additional longitudinals or extra layers of glass.

Of course, more glass and coosa and resin costs more, so the coosa boat is going to cost quite a bit more.

A fellow on boatbuildercentral finished an all coosa build and was pleased with the result, but it did come in heavier than the designers stated hull weight for ply. Iirc, it was on the order of 20% heavier, something like 500 against 450 or so.

But I’d say the best approach would be a boat where perhaps the bottom and transom are bw26 or bottom is 20 pcf and the sides are made with vac bagged 4 pcf, or some such, or more engineering and less weight. Of course, this means a composites engineer assists with the materials design,

 

I can’t see how this would be true unless you over built it or are comparing apples to oranges. Whats the rest of the story?

Every single thing I’ve ever built from Coosa board has turned out to be lighter than, faster to build, easier, and better looking than anything I could have ever built out of plywood.

 

The weigh depends on many factors. Plywood with light framer can be made of less weight.
Easier I don't understand. They are both flat panels, and plywood is easier to bend.
Better looking has nothing to do with the core, but the design and finish.

 

Coosa board is lighter than plywood by 30-40%.
You can build with it —without a frame,
my pilothouse project and hardtop are great examples.
I have other examples of boat furniture that looks like it was out of fiberglass but in fact is painted Coosa board.

You could never have built the same thing with the same thickness plywood for the same strength and weight, for the same effort, with the same long term durability results. .


Why is it easier:
1) you don’t have to hot coat coosa board.
2) you can bend the coosa board where you can’t bend plywood.
3) it’s lighter weight so it’s inherently easier to handle for big projects
4) you don’t have to worry about sealing it with resin or fiberglass after-the-fact ( saves time-effort-money)
5) you can sculpt it using a sander or grinder. Something not possible with plywood
6) you can drill into it, cut into it, and you don’t have to worry about exposed core rotting.
7) you can fair and paint without resin which impacts finishing time, saves $$$. My pilothouse was finished Coosa board using just fairing compound and paint

Etc etc the proof is there.

Anyone who’s actually built or restored a boat knows you have to plan early for finishing (boat restoration rule #4) and choice of materials directly affects what an amateur can accomplish. I demonstrate all this in my videos. Lots to watch. I spell it all out.

 

Coosa BW26 and okume ply are almost the exact same density, but okume is 2-4 times more rigid based on flexural modulus data.

And I built a foam core boat and used coosa in places.

As I said, one fellow built a boat that was spec’d ply with coosa and it came put heavier. I think is because he used coosa throughout and it was overkill in places.

 

yeahhhh right.
Something about that didn’t sound right to me. I’m a simple guy and I googled : “is okume 2-4 times more rigid than Coosa board?”


Dr. Google says :

Based on technical data and marine applications, okoume plywood is not 2–4 times more rigid than high-density Coosa board (such as the Bluewater series). While they have different physical properties, they are generally comparable in bending and compressive strength when used as a direct replacement for structural marine components.

• Structural Equivalence: Coosa Board (specifically the Bluewater 26 series) is designed to be a direct replacement for marine plywood in load-bearing applications like transoms and stringers.
• Stiffness/Rigidity: While some users note that plywood can feel slightly stiffer than foam-based composites, high-density Coosa is typically considered a structural equal in practical applications, not inferior by a factor of 2 or 3.
• Weight Advantage: Coosa board is 30–50% lighter than plywood.
• Water Absorption: Unlike okoume plywood, which can absorb water if the seal is broken, Coosa board is impervious to water and will not rot.

 

Coosa Board is what allowed me to build this pilothouse on the Compac 16 sailboat, and build my trawler hardtop extension (90” x 90+”).

It’s significantly lighter, more durable, and was faster/easier to build and sculpt than anything made of similar thickness plywood. Rigid ? —oh yes— I even climbed on it in one of my videos as proof.

 

Adequate strength and adequate stiffness are two different animals.
I replaced some 2x 6 treated handrail on our back deck with some new composite lumber that comes in up to 20' sections and is straight and doesn't warp. It has worked fine but if I was do it again I'd have gone with 2x6 uprights, the 2x4 uprights are adequate from a structural standpoint but you can tell a difference in the stiffness.

 

Ask Google what the density of okume plywood vs coosa bw26 is.

Ask any chatbot what the flexural modulus is for Coosa BW26. 266 ksi

Okume is 435ksi wide way and 725ksi long way…

 

Thank you all for the info, lots of food for thought. So if I were to use Coosa instead of plywood, would the same fiberglass schedule apply, or, with the properties being slightly different, would I need a stronger glass layup to make up for the lack of stiffness? Given that its 30% lighter, shouldn't I add another layer of glass anyway just so that the boat will be equal in weight to the original and keep the same outboard?