Okoume, end-grain balsa, Divinycell, Carbon-Core, Nida-Core, CorePly, Coosa — for novices just getting started with building, repairing or upgrading recreational fiberglass boats, the array of materials available for coring can be daunting.
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Advantages and Disadvantages Of Coosa Board
As with most things, the many options all have their own advantages and disadvantages, but there is still a good deal of “overlap.” In other words, for a given project, there are likely several choices of coring material that will work. Likewise, each kind of coring is suitable for several different applications.
That’s especially true of so-called “Coosa board” — the common term for Coosa Composites’ Bluewater 26 panels — which can be used for many different purposes and truly excel in certain areas like coring transoms and reinforcing decks where hardware will be mounted.
Coosa board is made from high-density, closed-cell polyurethane foam reinforced with fiberglass and is available in several thicknesses, ranging from 1/4” to 2”. It is a rigid sheet and is typically considered an alternative to marine plywood but can also be used for some of the same applications as closed cell foam sheets like Dinvinycell and Carbon-Core.
In determining the best material for a project, it makes more sense to start with your desired outcome and then consider the main factors in play, rather than simply comparing the characteristics of materials. Those main factors are: material characteristics, ability to withstand compression, water absorption, layup schedules, painting requirements, and cost.
Material Composition
What each coring material is actually made from will dictate most of the factors to come. Most people are familiar with plywood, but fewer are familiar with closed cell foam (Divinycell/Carbon-Core) or Coosa board. Marine plywood is a type of plywood that is treated to better handle marine environments, often found on docks and older boats.
Coosa board is similar to plywood in that it is essentially a rigid sheet, but it is made of completely different materials. It starts with a molding process that impregnates high-density, closed-cell, polyurethane foam with layers of continuous strand and woven roving fiberglass. The result is an extremely hard and strong board that has superior structural properties. Generally speaking, Coosa board can be used as a direct replacement for marine plywood; virtually anything that can be made with plywood can also be made with Coosa board.
Coosa board can be used as a direct replacement for marine plywood.
Closed cell foam is similar to Coosa, but it is not reinforced with fiberglass and is not rigid. It is lighter weight, though, and its flexibility is advantageous for certain kinds of projects. On the contrary, Coosa is reinforced due to the multiple layers of fiberglass that are embedded throughout each sheet.
TIP: Cutting, shaping and sanding Coosa board is almost identical to working with plywood, and most of the same tools and techniques can be used. However, closed cell foam is slightly flexible in case there is a need for a radius or intricate work. Eyewear, respirators and other PPE are always recommended when manipulating these materials.
Ability to Withstand Compression
The number one marine use of Coosa board is coring transoms, with over 10,000 transoms successfully completed. That’s largely because of the structural rigidity provided by the reinforced foam, which can withstand up to 800 lbs. of force without crushing. Other applications in the marine environment that Coosa is well suited for include mounting outboards, trolling motors, cleats, rails, etc.
Closed cell foam like Divinycell and Carbon-Core, on the other hand, can be crushed if too much force is applied to them, whether through over-tightening bolts or too much horizontal pressure. Encapsulating them in more fiberglass improves crush resistance but also adds weight. If compression is a big factor, Coosa is the clear winner.
TIP: Instead of using a single 1-1/2” piece of Coosa board for a transom, two 3/4” pieces can be epoxied together to provide a stronger physical structure due to two extra layers of fiberglass. In other words, two sheets of 3/4” Coosa epoxied together form a stronger panel than a single sheet of 1-1/2” Coosa.
Water Absorption
Spend a few minutes on boating forums and social media groups, and you can find thousands of topics regarding waterlogged transoms. The biggest culprit is plywood or other types of wood, which absorb water if their fiberglass “skin” is compromised. Wet wood gets heavy, loses its structural integrity, grows mold and ultimately rots.
Coosa boards absorb virtually no water.
Coosa board and closed cell foam, on the other hand, absorb virtually no water even when directly exposed to moisture. This factor provides longevity and an extra level of security within the restoration and building process, because a builder doesn’t have to worry as much about holes or paint chips leading to water intrusion.
TIP: Even when working with Coosa or closed cell foam coring, always seal all holes with marine grade epoxy or sealant to prevent future issues. Don’t let non-absorbent materials give you a false sense of security; water intrusion still causes problems.
Layup Schedules
Fiberglass work is an important determining factor when considering materials to use. As mentioned, Coosa board is reinforced and is structurally sound on its own. In other words, it’s ready to use for stringer, transoms, bulkhead, and consoles without having to reinforce with fiberglass. (Note that although encapsulating Coosa board in fiberglass isn’t required, it does further strengthen the panel.)
Closed cell foam, on the other hand, requires creating a layup schedule to reinforce the foam with fiberglass cloth for structural integrity, while plywood must be completely encapsulated to prevent water absorption. With that said, closed cell foam is much lighter and should be used when you don’t need the structural strength of Coosa board.
Types of Resin
Secondly, there are three main types of resins — polyester, vinylester and epoxy — all of which stick to Coosa board. If polyester resin is used with Divinycell or Carbon-Core, on the other hand, fiberglass chopped strand mat is needed for proper adhesion. Otherwise, a delamination issue (fiberglass not sticking properly) might present itself. The delamination issue is not present with Coosa board due to the bonding process.
TIP: Coosa board comes sanded from the factory. This means fiberglass and resin will adhere to the material when joining it with other parts of the boat.
Painting Requirements
The project is now entering into the final stages, and the prep work for paint is starting. There are many different types of boat paints and coatings, with gelcoat and two-part epoxy paints being the most used in the industry. The prep work varies greatly between materials used and can be extremely time consuming.
With closed cell foam, once the fiberglass cures, the surface has to be faired, sanded and cleaned to be ready for paint. On the contrary, paint can be applied directly to Coosa board to save time and labor hours. Coosa comes sanded from the factory, meaning that if it isn’t glassed, it can be painted with minimal prep work (Tuff Coat nonskid coating pictured).
TIP: Coosa board is susceptible to UV rays, which causes the material to fade. Coosa that will be exposed to sunlight should always be coated with at least paint or gelcoat.
Cost
There is an old saying: “buy nice, or buy twice.” Plywood can be bought for a fraction of the cost of closed cell foam or Coosa board.
However, plywood needs to be sealed and is vulnerable to water damage, which may lead to more expensive repairs down the road. Coosa would be the top of the line option for transoms, stringers or deck areas subject to compression or heavy weight with the benefits outweighing the cost for materials.
Plywood needs to be sealed and is vulnerable to water damage.
TIP: Boat Outfitters can cut pieces for you in puzzle patterns for connections to save time and material costs.
Conclusion
What is the best product to use? It depends. Anything that has been done with plywood can be done with Coosa, and there are many benefits as mentioned of using the more expensive material.
If saving weight is the main priority or if there aren’t large structural loads, closed cell foam is probably the better option, keeping in mind there is a fiberglass layup schedule involved. The main things to consider are the factors above and hopefully the information is useful to help you succeed at your next project.