NidaCore vs Coosa Board: Marine Applications

Plywood? Balsa? Coosa? NidaCore? Divinycell? For do-it-yourself boat repair and improvement projects, the numerous options for coring materials can quickly become confusing for those just learning about fiberglass fabrication. Here’s a quick guide to the basic core options and a comparison of two especially popular choices with significantly different characteristics — Coosa composite board and NidaCore honeycomb panels.

In fiberglass fabrication — such as boat building, repair and modification — various kinds of panels are used between two fiberglass “skins” to achieve a lighter, more rigid finished product. The result is basically a sandwich with a thin slice of fiberglass “bread” on either side and a thicker layer of core material in the middle.

A well-engineered and well-built panel of cored fiberglass weighs far less than an uncored solid fiberglass panel of equivalent rigidity. On the other hand, panels cored with certain materials may be significantly more brittle than solid glass panels or panels cored with denser, heavier materials.

Every core material has a unique set of characteristics that make it best suited for particular applications. Some of the common characteristics that differentiate one core from another include:

There are dozens — maybe hundreds — of brand name products available for coring. The vast majority of them, though, fall into one of a few basic categories. Certain specific products may be superior to others for certain applications, but their basic composition is similar.

As long as it’s completely encapsulated in a waterproof coating and all penetrations with screws or other fasteners are carefully sealed, quality marine-grade plywood is actually a fairly good coring material. It’s on the heavy side, but it’s inherently strong, relatively affordable, and holds screws better than any other core material.

Like plywood, end-grain balsa will eventually rot if exposed to moisture. It has to be fully encapsulated, and penetrations must be sealed. End-grain balsa is actually a flexible “sheet” composed of small squares of wood bonded to fabric. That means it’s particularly well suited to coring curved hull sides. It’s lighter than plywood and roughly comparable in price. It has no inherent “bending” strength but excellent compressive strength.

They have relatively poor compressive strength and screw holding properties, so it’s commonly recommended that areas around fasteners be reinforced either during construction with an area of denser core like Coosa or after the fact by hollowing out the area around the penetration and back-filling it with epoxy putty. Foam core is available in various densities with “5 lb.” foam being most common in boat building.

Like foam core panels, polypropylene honeycomb panels put an emphasis on light weight. Honeycomb is even lighter, though. A typical honeycomb panel weighs about 65 kilograms per cubic meter, versus about 80 kg/m3 for 5 lb. closed cell foam panel. Also like foam core, honeycomb is relatively flexible but lacks compressive strength and doesn’t hold screws well. Honeycomb has a couple of unique characteristics — both positive and negative — that will be discussed in more detail below. Generally speaking, honeycomb costs less than foam.

Coosa is significantly lighter than plywood but heavier than most other core materials. Offsetting that is the fact that Coosa doesn’t technically need fiberglass for reinforcement. It can be used for decks or similar applications with no more than a coat of paint. It’s also on the costlier end of the spectrum but, again, that can be offset by the fact that less fiberglass and resin are needed because the core is so strong by itself.

With the basic coring options covered, let’s look in a little more depth at two of them: Coosa composite board and NidaCore honeycomb. Both are excellent materials, but not always for the same applications. They each have different sets of characteristics that dictate when and where they should be used.

Coosa Composites manufactures a wide range of composite panels, but the term “Coosa board” is commonly used to refer to a specific product: Bluewater 26 panels, which are made from closed-cell polyurethane foam reinforced with continuous strand fiberglass and woven roving and have a density of 26 lbs. per cubic foot. Coosa also makes Bluewater panels in 20 lb. and 32 lb. densities as well as lighter-duty Nautical panels, which have less fiberglass reinforcement.

Although they are very different materials, prelaminated NidaCore and Coosa Bluewater 26 do share a few things in common.

Neither Coosa nor prelaminated NidaCore is subject to rot or other deterioration as a result of exposure to moisture. Coosa’s manufacturer says that encapsulation in fiberglass is not necessary to prevent water absorption. Prelaminated NidaCore, though, must be sealed along its unfinished edges to prevent water intrusion.

Both Coosa Bluewater 26 and prelaminated NidaCore are toward the top end of the coring price range. Prelaminated NidaCore costs 40% to 50% more than Coosa, but assuming you’ll fiberglass or at least coat the Coosa, those materials will partially offset the cost difference. Prelaminated NidaCore already has a glossy white gelcoat finish on both sides.

Coosa Bluewater 26 weighs 26 lbs./ft3 or about 416kg/m3, which is around 30% less than plywood. A 4’ x 8’ sheet of 3/4” Coosa weighs about 53 lbs. In comparison, a 4 x 8 sheet of 3/4” prelaminated NidaCore weighs about 36 lbs.

The fact that the Coosa isn’t glassed means the weight difference is even greater. Although the manufacturer says fiberglassing Coosa isn’t necessary for strength, most boat builders and DIYers still add a layer of glass to each side.

Coosa Bluewater 26 boasts good compressive strength of around 1,015 psi. That’s nowhere near plywood — which has a reported compressive strength of 2,000 psi to as high as 7,000 psi — but still plenty strong to withstand the compressive force of outboard engine mounting bolts.

Definitive measurements of the compressive strength of prelaminated NidaCore are hard to find, but the bottom line is that it’s significantly lower than Coosa. The core by itself has a compressive strength of 174 psi, meaning that the majority of strength comes from the fiberglass skins. For through-bolting anything that will be subject to significant forces — leaning post feet, for example — reinforcement of the core is required.

Coosa doesn’t hold a screw as well as plywood, but it does so much better than either NidaCore or lower density foam like Divinycell. Because the “inside” of prelaminated Divinycell is mostly a vacuum, screws essentially have only the 1/8” thick fiberglass skin to bite into.

Just as with through-bolting, NidaCore should be reinforced anywhere screws will be used to hold anything that’s subject to more than minimal force. That can be done during construction by cutting out the area of the NidaCore where screws will be installed, replacing it with a “puck” of Coosa or wood, and glassing back over the puck. After the fact, NidaCore can be reinforced by digging out the core material around the screw hole and then filling it with a “plug” of epoxy putty. This is often done by chucking an Allen wrench into a power drill, inserting it into the screw hole, and running the drill to tear out the polypropylene honeycomb structures surrounding the hole.

NidaCore — both prelaminated and raw — has a couple of noteworthy quirks that don’t really fall under the general characteristics used to compare core materials. It’s important to keep them in mind if you’re deciding between NidaCore and Coosa.

Both prelaminated and raw NidaCore panels have rough, unfinished edges where the honeycomb structure is exposed. This isn’t an issue in applications where the edges won’t be exposed, such as cockpit soles. But for projects with exposed edges such as hardtops and hatches, achieving a clean finish on the edges of the NidaCore requires extra steps.

There are several approaches, each with its own advantages and disadvantages:

• Dig out the inner structure along the edges and backfill it with putty. Then sand the putty to the desired edge profile, wrap with fiberglass, sand and fair.

• Dig out the inner structure along the edges and glue a strip of structural foam into the resulting opening. Then shape the foam, wrap with glass, sand and fair.

• Use a router, grinder or hand tools to shape the edge of the NidaCore itself. Then wrap the edge with glass, sand and fair.

All of those different characteristics between the two materials mean they’re each good for certain applications. Coosa is heavier, has better compressive strength and holds screws better; prelaminated NidaCore lacks Coosa’s compressive strength and screw holding but is significantly lighter weight than even unglassed Coosa.

Coosa is the hands-down choice for coring transoms. Strength — especially compressive strength — is critical here and a much bigger consideration than weight.

This is a toss-up. In large parts like decks, weight savings is an important consideration. Plus, NidaCore’s unique sound-deadening ability is attractive in a deck, especially with noisy machinery underneath. And NidaCore’s rough edges are a non-issue, since the edges of the deck or sole are hidden.

On the other hand, a NidaCore deck will need to be reinforced for extra compressive strength with a denser material anywhere a cleat, T-top or hardtop, leaning post, pedestal seat, rocket launcher, etc. is mounted. Coosa is heavier but doesn’t require reinforcement.

NidaCore is the way to go for most wall applications. It provides thermal insulation and sound deadening, plus the higher you go on a boat, the more important weight reduction becomes.

NidaCore is also the choice for hardtops, which need to be light and rigid. Keep in mind that the rough edges will have to be finished, and you’ll want to reinforce mounting points for outriggers, antennas, etc. with a denser core material.

Hatches are a toss-up, depending on their size, location and use, with the edge going to NidaCore. The lighter weight of NidaCore is a big advantage, but remember that fasteners for hinges and supports will need to be reinforced and edges will have to be finished. Coosa, again, is heavier but easier.