One of the most significant advances in fiberglass boat building was the introduction of composite coring materials to replace plywood and balsa coring. There’s a wide range of composite coring materials available, ranging from honeycomb materials like Nida-Core, to closed cell PVC foam sheets like Divinycell, to reinforced polyurethane foam panels like those made by Coosa Composites.

What is Coosa Board? 

Commonly referred to as “Coosa board” or just “Coosa,” Coosa Composites’ most widely used panel in boat building and repair is Bluewater 26, a tough, structural sheet that is used as an alternative to plywood yet is 30% lighter, absorbs virtually no water, and won’t rot or delaminate. While honeycomb and PVC foam coring materials are not considered “structural” — meaning, basically, that they have to be laminated between layers of fiberglass for strength — Coosa board is strong even without any additional reinforcement. That makes it a great choice for transoms, stringers and decks.

Coosa is a high-density, closed-cell polyurethane foam reinforced with continuous strand fiberglass.

Originally introduced to the boating industry in 1999 as a replacement for plywood, Coosa is a high-density, closed-cell polyurethane foam reinforced with continuous strand fiberglass. Even though Coosa has a higher strength-to-weight ratio than wood, doesn’t absorb water and doesn’t rot, it can be handled and manipulated similarly to wood, down to the same saw blades, sandpaper and routers. All of these traits are why I chose to rebuild my boat with it. 

Is Using Coosa Instead of Plywood Worth It?

My project boat was a 1993 18-foot foot C-Hawk center console. C-Hawk is a popular North Carolina-built boat, similar in design to Parker, Jones Brothers and May Craft. These boats are utilitarian fishing and work boats. C-Hawks are wood cored and sparsely finished, made to run the shallow, but potentially rough waters of sound-side North Carolina and coastal Virginia.

For my style of shallow-water, light-tackle fishing and open-water diver duck hunting, the C-Hawk 18 perfectly fits my needs. Prior to purchasing my boat, I knew the deck had some soft spots and would need to be replaced within a few years. I planned to fish it as long as I could until I needed to replace it. I got two years of phenomenal fishing out of the boat before it became apparent that the transom would need replacing in addition to the deck. So in spring of 2023 I began my rebuild. 

Deciding on Coosa

In the planning stages of the rebuild, I first considered using Coosa. Then prices skyrocketed during the pandemic, and I leaned back toward replacing the rotten transom and deck with the same marine grade plywood the boat had been built with originally. After all, properly encapsulated plywood can last for decades, and wood has been used to build boats for 10,000 years. Who am I to end tradition?

Upon cutting open my boat and exposing the rotten coring, though, it was apparent why wood is losing popularity in preference to composites. My entire transom was rotten and had the consistency of wet cardboard.

The wooden coring of the deck was falling apart, with the fiberglass being the only thing holding it together. The stringers had completely fractured in some places. As I cut the stringers, water leaked out. It was clear they had been saturated for years. Every piece of wood in my boat had some level of rot. 

Then, as I prepared to buy my replacement coring, a buddy of mine contacted me and let me know that he had half a sheet of Coosa leftover from a rebuild he had done on his Contender 31. With this material in hand, and a picture of my rotted-out transom ingrained in my mind,  my dad and I decided to spend the extra money and rebuild the entire boat with Coosa board. 

Raising the New Deck for Better Self-Bailing

After rebuilding my transom and stringers, it was time to rebuild my deck. An important part of my rebuild began when I was cutting out my old deck. When removing the rotten deck, I left a two- to three-inch lip around the edge of the deck where it met the gunwales.

Had I been maintaining the original deck height on my boat, this lip would’ve given me plenty of surface for my glass to tie into. Since I was going to raise my deck around an inch, I just rested my new deck boards on top of this lip.

Why Raise the New Deck?

This would give me just under an inch of added height on my new deck. When rebuilding an older boat, consider raising your deck to keep your scuppers out of the water.

Many older boats were designed to self-bail with the lighter two-stroke outboards they were originally rigged with, but heavier four-stroke engines put the scuppers underwater.

That was the case with my C-Hawk, which was not self-bailing at rest with its newer four-stroke outboard. Raising my deck an inch and getting rid of all the rotten wood weight, though, would allow my boat to bail at rest.

Choosing the Right Coosa Coring, Fiberglass and Resin

Before you can begin replacing your deck with Coosa, you need to know which Coosa product to buy. Coosa comes in different thicknesses, fiberglass strengths and densities. Coosa makes two lines of panels: Bluewater and Nautical. Nautical panels are built with only continuous strand fiberglass, while Bluewater panels are built with both continuous strand fiberglass and woven roving. Bluewater panels are stiffer, while Nautical panels are lighter weight.

Bluewater panels come in two different densities: 20 pounds per cubic foot and 26 pounds per cubic foot. Bluewater 26 is recommended as a replacement for plywood and when spanning more than 14 inches unsupported. As a general rule of thumb, you should use Bluewater 26 panels of the same thickness as the plywood they are replacing. Since my deck originally used 3/4” plywood, I used 3/4” Bluewater 26. 

As a general rule of thumb, you should use Bluewater 26 panels of the same thickness as the plywood they are replacing.

Choosing Resin and Fiberglass Layup

After you choose your Coosa, you need to choose both your resin and fiberglass layup. I chose to go with polyester resin and 1708 biaxial fiberglass.

1708 is a very popular choice for boat builds, and polyester resin is a great choice with 1708, since the styrenes in the resin break down the binders in the fiberglass, improving the wet-out and chemical bonding of the glass. Polyester is also less expensive than the alternatives of vinylester or epoxy resin. 

Templating for the New Deck

Now that I had my fiberglass, Coosa and resin in hand, it was time to begin measuring the boat for replacement panels. I decided to make panels that were the full width of the deck with joints running across the boat from one side to the other.

Although it may have been a more efficient use of the Coosa to run the deck boards parallel with the stringers, I didn’t want to have them join together over the unsupported length of the fuel tank.  

Accurate Measurements

I measured the width of the deck every 4” on my bow panels to accurately match the curves and every 6” towards the stern, where the sides were fairly straight. I then scribed these marks onto 1/4” lauan sheets to verify the fit and use as a template. I recommend test-fitting your measurements with lauan or something similar before transferring the shape onto Coosa. A mistake on a $20 piece of wood is a lot easier to stomach than a mistake on a sheet of Coosa that costs 10 times as much.

In total, I ended up with four deck panels. The bow panel was roughly 23” from the forward edge to the aft edge, and the rest were 48” each. I built a little bit of play into the bow panel of my deck. I wanted the deck panel that mounted against the transom to have the tightest fit for maximum strength. 

Cutting the Coosa Deck Panels

Once I had my lauan cut into the correct template, I traced the shape onto Coosa and cut it out. Coosa cuts similarly to wood, with minor differences. For one thing, it eats up jigsaw blades quickly; I found that carbide blades held up best.

Cutting Coosa also creates a very fine dust that can be irritating when it gets on your skin. I recommend wearing long sleeves and a respirator, in addition to your normal PPE. If you do get some Coosa dust on your skin, simply wash it off with cool water.

If your deck boards are a little too large just sand down the edges with a dual-action sander. Coosa sands very easily! If they’re slightly too small, remember that you can fill small voids with thickened resin.

Glassing the Deck Panels 

With my Coosa cut out and fitted, it was time to decide on my fiberglass layup. I chose to put one layer on the underside of my deck and two on top.

The old deck of the boat did not have any fiberglass on the underside, resulting in it rotting out from the bottom up. I would do one layer on the top and bottom before the boards were installed and one after they were installed.

Wetting out fiberglass is much easier with the boards on sawhorses than in place in the boat. Having full access to all sides of the board made it easy work. 1708 drinks a lot of resin! Keep track of how much resin you use on the first layer of fiberglass so you can pour exact amounts on your second and third layer.

Remember those lauan templates you used to cut out the Coosa? They have another use as a template for your fiberglass. Try to keep your overhang minimal, as any excess glass is going to have to be sanded down or cut off. 

Bedding the Deck Panels with Thickened Resin

After you have the top and bottom glass laminated to your Coosa, I recommend yet another test fit to make sure everything is fitting as it should. Once it’s time to bed the glassed panels in thickened resin, the clock is ticking and you don’t want to have to do any last minute sanding.

Try to keep any height differences between the boards to a minimum. This usually involves sanding down high spots in stringers, but make sure your fit is good or you will have a lot of fairing and sanding ahead of you.

Bedding Your Deck Boards

Once you have a perfect fit between all your deck boards, the moment of truth arrives: bedding your deck boards. I bedded my deck boards in resin thickened with fumed silica. Fumed silica is a popular thickening agent for resins and creates a strong bond that can be very difficult to sand. It can act as a filler in any voids and be used to create any radiuses needed for tabbing.

Working with polyester resin is a time-sensitive matter. Once you activate your resin with MEKP, you only have about 15 to 30 minutes, depending on catalyst ratio and temperature. Since I had some pretty high temperatures, I enlisted the help of a friend to avoid the resin curing prior to having the deck board in.

The deck boards would need to sit on resin on both the lip of the existing deck and the stringers. We split these by port and starboard. My helper would have the port side and I would have the starboard. To fully prepare ourselves, we didn’t catalyze the thickened resin until we were inside of the boat ready to lay down resin.

Applying Weight

Once we applied a generous amount of thickened resin to each contact surface, we laid down the board and made sure everything was even. Any resin that squeezed out of the sides was used to fill any voids in other places, and any excess was used to make a radius for the 1708 tabbing that we would use later.

After laying down each deck board and cleaning up any extra resin that squeezed out, we applied weights to the deck board to make sure it bedded evenly. For weights, we used buckets of water and batteries. As the resin holding the deck boards cures, avoid walking on them. Even after they have cured, try to minimize disturbance until it is time to tab them in. Although the resin is very strong, it can be brittle and doesn’t offer as much strength without being tabbed in. 

Tabbing In the New Deck

After bedding your deck boards, it’s time for tabbing. Tabbing is the process of using a small strip of fiberglass to bond one surface to another. In this instance, I was tabbing the deck to the sides of the hull. I used 8” wide strips of 1708 for my tabbing.

1708 is an incredibly strong material, but it doesn’t like sharp angles. Because of this, it’s important to create a radius prior to tabbing in your deck. This radius softens the bend the fiberglass has to make, allowing it to have a much much stronger bond with the two surfaces. I used more thickened polyester resin to make my radiuses.

Tabbing is the process of using a small strip of fiberglass to bond one surface to another.

Everything in Place

I recommend having everything in place prior to starting to tab a section. For tabbing, this means having a section of glass measured and cut, an appropriate amount of unthickened resin ready to wet out the glass, an appropriate amount of thickened resin ready to create a radius, and all tools required.

The first step is to create the radius for the length of tabbing. Typically I use a body filler spreader or a plastic spoon to make the radiuses along the seams. Try to keep them as uniform as possible. After creating the radius, begin to wet out your precut tabbing. Take your time. It’s a good idea to let your radius resin begin to gel slightly before tabbing everything.

Removing Air Bubbles

Once your tabbing is wetted out, place it along the seam of your deck and hull sides. It’s best to try to split the tabbing with half on one of the materials and half on the other. In my case, I split the material between the deck and the hull side.

Similarly to fiberglassing the deck boards, you want to remove any air bubbles using your fin roller. This is when having a still slightly wet radius can help, as it’s easy to manipulate the radius to work out any air pockets. Be sure to tab the entire deck to the sides of the hull, as well as tabbing each deck board to the other.  

Similarly to fiberglassing the deck boards, you want to remove any air bubbles using your fin roller.

Adding the Final Layer of Glass

After you have your deck tabbed in, it’s time to put on the last layer of fiberglass. I recommend staggering the joints of your fiberglass to the joints of your deck boards. In other words, two pieces of fiberglass should not meet in the same place as two pieces of Coosa.

To do this, I went from the bow to the stern with my 50” wide pieces of fiberglass. Because my bow deck board was only about 23” forward to aft, the first 50” piece of glass spanned the first joint between deck boards and ended near the middle of the second deck board. This ensured all of the seams were staggered for maximum strength.

Prior to laying your glass down, you may need to hit some of the tabbing areas with a little sanding to knock down any loose pieces of fiberglass that may have stood up. Even the slightest bump or stray piece of fiberglass can cause an air pocket in your layup. Similarly to how I did the first two layers of glass, I rolled this out with a solvent resistant foam roller and followed that up with a fin roller.

Fairing the New Deck

Once you’re done with your fiberglass work, it’s time to fair everything out. Fairing is the process of making a surface smooth by sanding the high spots and filling the low spots. On my boat I used a fairing filler to fill the low spots and my dual action sander to sand any high spots.

For a perfectly smooth finish, I would’ve used a “long board sander” which ensures that the surface is perfectly even all the way across. I chose not to do this since I was going to be putting non-skid across the entire deck. Fairing helps to blend your tabbing into the hull sides and mask any minor blemishes. It can be a tedious task, but the results are worth it. 

Coosa Deck Takeaways

Following the fairing process, your deck is finished and ready for paint! Although this entire process may have condensed into a relatively short story, fiberglass work is a tedious process. Here are a few important things I took away from the process that may be beneficial to you if you decide to do this on your own:

  • Wear your PPE! This includes a respirator, safety glasses and gloves. The fine dust of Coosa isn’t good to breathe, nor are the fumes of polyester resin or MEKP. 
  • Measure twice and cut once. Coosa isn’t cheap, and cutting mistakes can be costly.
  • Always give yourself a margin of excess in the amount of resin you prepare. When wetting out fiberglass or making thickened resin, I would much rather waste an ounce of resin than have to sand off and redo fiberglass work because I ran out of resin and it kicked before I could pour more. 
  • Be prepared when working with resin. Have a course of action clearly laid out with all of your needed tools and materials. You do not want to have to find tools while the clock is ticking on your resin’s curing time!

Rebuilding any boat can be a challenging but rewarding process. Rebuilding with Coosa guarantees your project will last for many many years. 

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