Plywood versus Coosa Board

[CatchAll]

I'm doing the transom on my 1995 campion explorer 185. Is it really worth it to use coosa board? Other than coosa's water resistance, is there any other advantage (strength, ease of use, etc)? I asked a couple of fiberglass guys and they say coosa is a no brainer because it doesn't rot and will last forever. On the other hand, the factor plywood lasted 28 years and if the factory would have sealed the plywood ends under the transom cap it could have lasted a lot longer.

 

[Brando]

coosa is a no brainer if your keeping the boat seems expensive now but once you go through all the work you’ll be happy to never have to do it again.

 

[brutus]

coosa is a no brainer if your keeping the boat seems expensive now but once you go through all the work you’ll be happy to never have to do it again.

Small price to pay , when you consider the work involve in redoing the transom. Plywood if the glass work is done right will last a long time,

 

[ship happens]

Coosa is alot more forgiving than foam core. I recently had a go around with Coosa board and it's probly most best material I've seen ever, when it comes to that type material. Expensive but worth every penny. It will save alot of labour on certain things. In regards to a transom, that's a different cause. 3 times or more the cost on a sheet of material and you'll need a thicker glass job imo. Also I would use epoxy and not poly resin but that's just me. Material costs gooes waaay up

 

[Foxsea]

Coosa is alot more forgiving than foam core. I recently had a go around with Coosa board and it's probly most best material I've seen ever, when it comes to that type material. Expensive but worth every penny. It will save alot of labour on certain things. In regards to a transom, that's a different cause. 3 times or more the cost on a sheet of material and you'll need a thicker glass job imo. Also I would use epoxy and not poly resin but that's just me. Material costs gooes waaay up

If you completely seal the plywood, no issue. I repaired the transom on a 17 foot Malibu last year, using plywood. Definitely use West Systems epoxy, as "ship happens" suggested. It's bond to both clean, dry wood and fiberglass is far better, it's much stronger and also nicer to work with.

 

[Brando]

to each his own if I’m keeping a boat any length of time and doing that labour myself I’ll go composite 2k in materials is nothing in boat land..

 

[ship happens]

100 percent what brando said. However it's all in what your building. Push pull effect from a stiff material like Coosa is hard to beat. I have a boat that's foam core. Better do it properly that's all I have to say. De- lamination is the biggest problem. That's why expoxy works so well. It goes deeper with a slower curing time.

 

[Fritz]

I'm doing the transom on my 1995 campion explorer 185. Is it really worth it to use coosa board? Other than coosa's water resistance, is there any other advantage (strength, ease of use, etc)? I asked a couple of fiberglass guys and they say coosa is a no brainer because it doesn't rot and will last forever. On the other hand, the factor plywood lasted 28 years and if the factory would have sealed the plywood ends under the transom cap it could have lasted a lot longer.

Is this the boat for life? If it isn’t, just use plywood.

Also if you look up the tech specs on Coosa it is not as strong as plywood.
It gets its strength from laminates between layers so the weight savings are moot. It will also eventually absorb water as Coosa states on their site.

These are merely my opinions and I do not tell people what to do but you should be aware of the structural concerns with a material change. The online propaganda for Coosa shows a piece of Coosa being beaten with a sledge hammer on a concrete floor and a piece of wood (which was not plywood oddly?) to show that Coosa didn’t break. If you hit a rubber strap on a concrete floor and it doesn’t snap does that mean it’s stronger than plywood?

If money was no object I would use Coosa…………..but if that were the case I’d probably buy a new boat to!
lol

 

[CatchAll]

Coosa is alot more forgiving than foam core. I recently had a go around with Coosa board and it's probly most best material I've seen ever, when it comes to that type material. Expensive but worth every penny. It will save alot of labour on certain things. In regards to a transom, that's a different cause. 3 times or more the cost on a sheet of material and you'll need a thicker glass job imo. Also I would use epoxy and not poly resin but that's just me. Material costs gooes waaay up

Are you referring to the lack of compression strength? That's one of my concerns. I'm bolting my outboard through the coosa, so there is risk of crushing the coosa when I cinch it down. I guess I can make the skin extra thick or use aluminum plates to back the bolts on the inside.

 

[Foxsea]

Are you referring to the lack of compression strength? That's one of my concerns. I'm bolting my outboard through the coosa, so there is risk of crushing the coosa when I cinch it down. I guess I can make the skin extra thick or use aluminum plates to back the bolts on the inside.

There is more stress on the transom than anywhere else in a boat. Compressive strength and flexural strength are important considerations. All areas with compressive loads should be backed with solid lamination, throughout. All penetrations for fittings as well.

3/4" D.Fir, structural plywood compressive strength exceeds 7200 psi. That's 7 times greater than Coosa. End gain balsa core has a compressive strength of 1800 psi. It's a great core material for decks unless poor maintenence or repairs allows it to be exposed to oxygen and get wet. Sealed, laminated plywood is a best practice for transoms. Wood can only rot when moisture and oxygen are present. Cured epoxy bond strength with dry wood exceeds the strength of the base material.

 

[CatchAll]

One thing about these old campion explorers is that they built the transom in three separate segments. The center 30" is three layers 1/2 inch plywood. The outside sections are about 24" wide and are only two layers of 1/2" plywood of lesser quality than the center section. The outside sections flare forward about 3" inches from where they meet the center section. The joint between the outside and center sections are just a butt joint without any overlap. They did lay some fiberglass between the layers that runs the width of the transom and holds it together. I wonder if a the butt joint of coosa would be as strong as plywood when epoxied together, or if that layer of glass is all that is needed to hold the three sections together?

 

[Foxsea]

One thing about these old campion explorers is that they built the transom in three separate segments. The center 30" is three layers 1/2 inch plywood. The outside sections are about 24" wide and are only two layers of 1/2" plywood of lesser quality than the center section. The outside sections flare forward about 3" inches from where they meet the center section. The joint between the outside and center sections are just a butt joint without any overlap. They did lay some fiberglass between the layers that runs the width of the transom and holds it together. I wonder if a the butt joint of coosa would be as strong as plywood when epoxied together, or if that layer of glass is all that is needed to hold the three sections together?

Ideally, plywood splicing will be with an epoxy-glued scarf joint at 7:1 minimum: easily machined with a simple jig and a power plane. Butt joints create a weak joint in any material and are especially prone to flexural failure.

 

[ship happens]

The strength of the material inside the inner and outer glass layups is only needed when it's delaminated. Push pull.

 

[Foxsea]

The strength of the material inside the inner and outer glass layups is only needed when it's delaminated. Push pull.

Not really. A failed bond means the layers will slide past each other so the composite stiffness is entirely lost. A composite material relies on a rigid, inner and outer stressed skin, separated by a well-bonded core web, with reasonable compressive strength. The design concept is similar to I-beams: 2 flanges separated by a stiff web.

 

[ship happens]

Not really. A failed bond means the layers will slide past each other so the composite stiffness is entirely lost. A composite material relies on a rigid, inner and outer stressed skin, separated by a well-bonded core web, with reasonable compressive strength. The design concept is similar to I-beams: 2 flanges separated by a stiff web.

I don't think you understood my reply. However it probly wasn't explain very well.

 

[Fritz]

There is more stress on the transom than anywhere else in a boat. Compressive strength and flexural strength are important considerations. All areas with compressive loads should be backed with solid lamination, throughout. All penetrations for fittings as well.

3/4" D.Fir, structural plywood compressive strength exceeds 7200 psi. That's 7 times greater than Coosa. End gain balsa core has a compressive strength of 1800 psi. It's a great core material for decks unless poor maintenence or repairs allows it to be exposed to oxygen and get wet. Sealed, laminated plywood is a best practice for transoms. Wood can only rot when moisture and oxygen are present. Cured epoxy bond strength with dry wood exceeds the strength of the base material.

Thank you for that!

A much better description of what I was trying to convey.

 

[Fritz]

One thing about these old campion explorers is that they built the transom in three separate segments. The center 30" is three layers 1/2 inch plywood. The outside sections are about 24" wide and are only two layers of 1/2" plywood of lesser quality than the center section. The outside sections flare forward about 3" inches from where they meet the center section. The joint between the outside and center sections are just a butt joint without any overlap. They did lay some fiberglass between the layers that runs the width of the transom and holds it together. I wonder if a the butt joint of coosa would be as strong as plywood when epoxied together, or if that layer of glass is all that is needed to hold the three sections together?

I think in the beginning of this post you answered your own question. It lasted for 28 years..............

I highly and thoroughly recommend you download the West Systems Boat Repair Manual. It is a free download off the West Systems site.
I do also very much encourage the use of West Systems epoxy for it's fantastic secondary bond strength. Also be aware though that this will change your selection of laminates to those that are epoxy friendlier like 1708 Biax. and in my own preference I prefer doing pre-wet out lay-ups as opposed to the "dry" method. My other preference is to do a transom from the inside as opposed to the outside but some instances will dictate otherwise. In my own limited experience, I believe I would not have been able to do my transom unless the aft sections of the stringers were also being removed. I was very lucky to have had sound advice from a Shipwright also when I did my project that killed my notions of making it "better".

Coosa is an amazing product but I am of the opinion that you could unknowingly create other issues for yourself if you are not careful.

In reality anything you put in the ocean has a finite life.

Good luck with your project Man