Coosa Board

Has anyone atempted to use Coosa board to build a CLC design rather than plywood?

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RE: Coosa Board

   Well, according to their own website they have "superior stiffness" which is not a good thing when trying to bend them into the shapes required for CLC boats. Seems like they might make light weight bulkheads and other parts that don't require bending. Even then you probably should ask the experts at CLC before use.

George K

RE: Coosa Board

Being retired during a pandemic leaves one with far too much time, Witness the fact that I just went ahead, on Christmas Eve before 6:00 AM, and did a web crawl to find that:

Coosa board has a modulus of elasticity ranging from .495 to 1.83 GPa (from Coosa Composites technical data page).

Plywood has a modulus of elasticity ranging from 6.96 to 8.55 GPa (from APA  Plywood design Specification).

That indicates that Coosa board is actually far more flexible than plywood. However, it's not suitable for most CLC designs. For starters, it's thick stuff. It ranges from 1/4" to 2" inches thick. That's far thicker than what's called for in CLC kayaks and canoes. Skiffs and larger CLC boats may be able to use the thicker pieces.

But, the stuff is meant to be core material for composite construction, so it has to be covered with glass, carbon fiber or other material for stiffness and impact resistance. Since  Coosa board is not providing the stiffness the way plywood does, the outer layers have to be much thicker than if they were on a plywood core. The boat will be heavier and more expensive than if plywood were used.

It looks like great stuff for larger all-composite boat designs, but not so good for the small light CLC designs that depend on plywood's stiffness and strength.

Merry Christmas,


RE: Coosa Board

just to chime in, laszlo's got it right. 

fwiwi, 1/4 inch is a common thickeness for cedar strips in srip building.  but the modulus of western red cedar is about 7.6 gpa....which puts it up there with plywood....and well above Coosa board.

fwiw, i was involved for a while in larger composite boat construction.....and that is typically where you will see this as an altenative to balsa or other forms of high density foam cores.  balsa runs at about 8 lbs per cubic foot.  the lightest coosa is 12 and western red cedar is about 22 lbs.

all that said, i think it would be an interesting experiment to take a coosa board, and see how it works....and how much glass you need to add to each side to get the properties you want and seeing if you save any weight.

i did one time, in an experiment of trying to get things light, have a bemusing sudden structural failure to a piece i was making....fortunately nobody was hurt....but i am glad i was not at sea:)

RE: Coosa Board

I think it over-glorifies the s&g kayak design process to say that they are designed around some special property of 3 or 4 mm plywood. A panel is a panel, and as long as it can be formed into the designed shape, and has the needed (or acceptable) properties when complete, then it's going to work.

For instance, I've build some things using structural foam, Corecell and such. On it's own, at kayak thicknesses of 1/4" or 5mm, the foam is probably a little too flimsy to stitch into a kayak without drooping. But first add glass to one face of the panels, say the interior, and you'd be in fine shape. Fillet and add another layer once you're stictched, then complete the exterior similar to a ply boat.

But Corecell cost a bunch more than ply, and it doesn't look good varnished. If you're going to the expense to use a foam core, you're probably interested in last nth performance, and not likely to choose a flat panel design in the first place!

My last CH17 I built had foam-cored carbon bulkheads, and a foam cored carbon hatch, made from scraps of material I had at hand. The parts weigh lots less than the ply they replace, which is fun, but doesn't make the boat "better" or faster, in my hands...

RE: Coosa Board

Merry cooking my favorite bread pudding for tomorrow morning....and it's in the i thought i would take a moment to see how this thread is developing.

if you were adventerous and wanted to try to make panels out of coosa....i would see how things come out if you follow nemochads thoughts.   My main reason for you to consider going there is to build a light boat becuase as nemochad points out....the aesthetics of the material are probably going to limit you to a paint finish.   i humbly suggest that a lighter boat (all other thing being equall) is way better than a heavier boat:)

so finding ways to build lighter, for me, has been a real passion.  i have a 32 lb petrel.....which for about state of the art for a stripper without using exotics or vacume bagging.   the boat gets used a lot becuase it is easy to get it on and off the car and down to any beach.  i paddle with a lot of groups through all seasons, and if i could mass produce them at a fair price, i don't know any paddler who would say, "not interested".   i have a queue of friends waiting for me to break my moratorium on buidling a boat for hire.  (i tell them the pressure of building for somebody else would take all the joy out of time doing it....and g-d know's we need to preserve some joy these days)

in my years paddling i have seen more injuries and accidents happening getting 50 lbs + commercial kayaks up and off their vehicles than in any other part of the kayaking experience.  i see a lot of people simply not paddling becuase they can't find someome to help them move their boat safely or just really struggling and frustrated moving their boats around.  so i passionaltely believe that a light boat should be an industry priority....maybe a new kit idea for CLC?  fwiw....i think the magic number is the low 30 lbs range where the problems just really disappears.  my wife's frej is 28 lbs.. she just loves it.

so anyway,,,,that's my 2 cents this christmas eve.  and when i get my workshop back (living in a temporary house this year.....long story)...i am going to build me another one.  you should too:)

anyway....almost time to get the bread out of the everybody on the for the holidays and happy new year.


RE: Coosa Board

nemochad wrote: I think it over-glorifies the s&g kayak design process to say that they are designed around some special property of 3 or 4 mm plywood. A panel is a panel, and as long as it can be formed into the designed shape, and has the needed (or acceptable) properties when complete, then it's going to work.

True enough that S&G doesn't require plywood. It is, after all, just a fastening method.

However (talking about boats designed for plywood), while it is certainly possible to build up the hull shape from composites, the finished boat will be different and the costs and labor will be much more than if it was built from plywood. The special properties of plywood that a design depends on are the ones that result in a reasonably-priced kit that a beginner can assemble with tools that they're likely to already have around with little hand-holding needed from customer support. For that, the plywood monocoque pre-stressed hull just can't be beat, especially on smaller boats.

If the builder is willing to fool with molds, new tools, new skills, etc. plywood is certainly not a necessity. Witness strippers, for example. But changing the materials is always going to affect either the final boat or the build process.

Time to put stuff under a tree,


RE: Coosa Board

   Wow, thanks for the useful information. If I go ahead with this idea the goal is to build a light weight dinghy. The 1/4" Coosa would be nearly 45% lighter than okume at about twice the cost. I like the idea of glassing one side first.

  My only experience with Coosa was making some equipment shelving and a small bulkhead on my sailboat. 

RE: Coosa Board's important you share back if you end up doing it and let us know how it goes.

the only other thing i think worth mentioning, is that i would, unless i was going to be very careful about my engineering, keep the coosa limited to designs with relatively small flat areas.   to laszlo's point,, part of the strength comes from the shape.  so a piece that is curved and braced against another piece such as a multichine kayak or a lapstich boat performs better than a shape with large flat sections.    designs like the peeler skiff with a large relatively flat hull....are probably much more dependent to the strength of thick plywood sections....vs using the plywood simply as a coring material.


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