Coosa Cored Fiberglass Composite - Bending Strength?

I did some searching here and elsewhere, but have not found much information. So I thought I'd throw it out here for any real-world experiences. . .

In my upcoming build of a combo 'Hull Extension/OB Bracket' my plan is to use 1/2" Coosa (Bluewater 26) panels and glass them both sides with 1708 biaxial cloth. . . probably several layers of 1708 on exterior surfaces.

BW26 is fairly strong by itself, and glassing the top & bottom should make the bending strength (bending moment) increase substantially. I'm just wondering if some folks have any real-life examples/tests/etc. of how strong glassing Coosa makes it? I'd say most of the strength would be in the fiberglass layer, based on engineering principles . . .

My plan for the project is to use 1/2" Coosa as the core and 1708 as the lamination layers for much of the structure. The transom will be 1.5" Coosa glassed-up on both sides to a total thickness of about 2".

T. I. A. for any info on this topic.

Ref: my outboard conversion thread.

 

The bending moment is the force applied.
Are you asking about bending strength of a flat panel? A hull extension is more or less a box, so the shape and reinforcements have to be taken into account. Having different thickensses of fiberglass inside and outside makes an unbalanced cored laminate, which is usually not ideal. Also, the strength of a composite (a material made of different components) is dependent on all of them. A very stiff skin may shear off the core and fail. In general, a design starts by calculating the forces involved. In this case, they would be the weight of the motor, thrust in forward, reverse and turning, acceleration forces from slamming etc. Otherwise, if there is a similar boat with a Coosa core extension, it can be used as an example of a design that is successful.

 

Wow!!!. That's totally wrong. Review the basics of physics.
The rest of the statements... better to forget them.

 

In my experience, Coosa is not as stiff as plywood, but stiffer than say a 12 pound Corecell.

Flexural strength coosa bw26 is 4848 psi. But I believe that is only a measure to break. And you are asking about rigidity.

I’d say you are going to need to test.

You can create a test panel fairly easily. A composites engineer may have the data, but not likely to volunteer it here.

To test the piece, glass each side of say a 2” wide strip 24” long with 1708, one layer. Oven cure it or in the sun for a day and then set it on blocks say 20” apart and put a known mass and calculate the deflection.

I might be more inclined to get 3/4” Coosa. A single laminate with local reinforcement would be pretty strong. Gonzo brings a point about box beam and that would also make it pretty strong with a single layer each side.

 

Keep in mind that I do have an engineering background, and familiar with the considerations of weight, forces, and making things strong & light. I'm just more looking for real-world examples, which I have not found much in terms of glassing Coosa (or similar).

Although . . . "Coosa core extension" might be a useful search term.

Speaking of Coosa and the potential for the fiberglass 'skin' to de-laminate from the core, . . . There are some examples of this happening that I have found in boating forums. Not sure if it is a workmanship issue, surface prep, or other causes?

I do plan on using Vinyl Ester resin, which has been a good choice on my other fiberglass projects.

 

Thanks for the continued responses . . .

I do plan to use gussets 'galore' to stiffen-up the box structure (although my 3D images in the project thread do not show that level of detail). . . After all a triangle is the strongest shape.

As FG mentioned, I may want to do a test of the material. Glassed vs. Bare. I have seen some videos of testing bare Coosa BW26 vs Plywood (the plywood eventually won, but the strength of the Coosa was fairly impressive).

 

You can ether make a test blank, or get a book like this and calculate it. We used Tsai for our composite studies.

Amazon.com https://www.amazon.com/Composites-Design-Stephen-W-Tsai/dp/0961809027/ref=sr_1_2?dib=eyJ2IjoiMSJ9.X3dVCVGSXjKsfZ-xGUZteJb5BWayJuqFhJx7IbRLUJM9pCLSbcOG2pjnF09V4IeugY04027C4ix0F8H1m8QgNcFv8LGo8APmv0ZtChB7EQAsGeenbWn4aCgO3D1hLqLg-d9BImwgNwjURuT4SNnr97snI38Z6ravGYQsTlUDxbDyiyDfuGJvAYU69wi7Y2nbITksMIMiR7RR9TxDFQUGeRNxzwBUnZoOnHk05ugNvNo.ZQCoWe4XEEGzFXk1xFA2VZhQ1_OKQu3Ejh5r55wZJjE&dib_tag=se&qid=1756315796&refinements=p_27%3AStephen+W.+Tsai&s=books&sr=1-2

 

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Coosa probably has an uptake recommendation or could be precoated.

For the best bonding, precoat(hotcoat) with epoxy resin and then rewet before glassing.

 

Having used lots of coosa over the last couple years, to get dust free surface is a bit harder than divinycell foam. Combination of a compressor spray nozzle and a shop vac with a bristle nozzle seems to work. Seems like its surface bond is a smidgen more difficult to get perfect, it's a bit less forgiving than divinycell. Wet coat is the best way to get good adhesion. Went through a dozen sheets this last winter, probably do about the same this winter so getting a bit more familiar with the material.

Definitely shop around distributors, on my coast the prices even in case quantity varied much more dealer to dealer than the lower density foams.

Only other quirk is how heinous the dust is, all glass grinding sucks but coosa is one notch higher in my opinion.

Id probably do a similar thickness transom but would do 2x 3/4 sheets.

I like to wet the sheets with 1.5 oz csm straight to the coosa then lay my combo mat csm up, ending with csm on the top that gets peel ply. Have gotten my best bond and surface finish this way.