Tips for Working with COOSA Board

[raahell]

Hi CF people. Been a year since I last posted. Back then I was building a new rudder in steel.

This year I decided to replace the hollow steel “swing keel” on the same vessel (a Caroff 36’ multi-chine steel monohull sailboat). Why, you ask? Well, the keel was at the end of its 40 year life. I’d tried a repair in the Spring refit by pad welding areas that had thinned out. When I took the boat out at end of season, however, she’d taken on water again. So I decided it was time to get radical.


I could have built an identical keel in steel. The shape was simpler than the rudder, so I knew I could do it. But I didn’t want another hollow keel, filled with oil. The original plans by the French designer, Gilbert Caroff, described a solid keel. I didn’t want to ever have to worry about it leaking again so I decided follow the original plans which came with the boat.

I considered mahogany (Caroff’s specified material) and marine plywood but then discovered Coosa Board. Light-weight, easy to shape and - most importantly - zero water absorption, even after prolonged immersion. That last characteristic was the clincher, even considering the CAD$470 sticker price for a 4' x 8' x 1/2" sheet. "Do it right, do it once, save dough" is my motto.

So I'm not asking any advice here, like I did with the rudder (and thanks again everyone who contributed to that project). I'm just posting pics and providing some suggestions for anyone considering building something for their boat from Coosa board.

My technique would be particularly useful to anyone building or rebuilding things like rudders and swing keels, dagger-boards etc. that are blade shaped and needing to be strong, thin, light and impervious to being immersed for long periods. But a lot of it could be applied to redoing a transom or any other kind of bulkhead, boxes, supports, etc..
First, a description of the design parameters and construction. The exploded view tells most of the story, but here’s the details of why and how:


1. My new swing keel had to match the original swing keel's thickness of 2-7/8" thick, in order to fit into the keel box, match up with the guides, etc.. I opted to build it up from 5 layers of 1/2" “Bluewater” Coosa board and encapsulate that in 1/8" thickness of GRP.


2. All the layers would be bonded together using marine epoxy, with a bit of colloidal silica thrown in to improve the structural bond.

3. A hole needed to be built into the top of the keel where the pivot pin would go through, which allows the “swing” function in my swing keel. The hole needed to be 2.625” in diameter to match the Ultra High Molecular Weight Polyethylene (UHMWPE) pivot pin’s diameter. I opted for stainless steel pipe with the correct interior diameter to provide a corrosion-proof hardened sleeve surface that would face the UHMWPE. This pipe would then be epoxied into a hole in my Coosa Board assembly.

4. I needed to ensure that the finished keel would not float. Otherwise the keel would never swing down! Also, having some weight at the end of the keel improves the sailboat’s counter-heeling moment. The original design specified 30kg, if using mahogany. Due to the Coosa board's lighter density, however, I did some specific gravity/volume calculations and came up with 45 kg of lead needing to be added. I managed this by cutting an identical rectangular hole in the middle three layers to give me a 1-1/2” deep by L x W volume for the lead ingot.

The ingot would then be encapsulated by the final outside layers of Coosa, plus the 1/8” GRP layers.

5. I also had to make the lead ingot to the specific shape. I detail that in the tips section.

6. Final touch was the attachment of a hard point for the line to raise and lower the keel (line goes to a deck winch). I could’ve done this in different ways including building it right into the Coosa Board assembly. In the end I opted to have a SS316 bracket made up by Stainless Outfitters in Barrie Ontario, which would be screwed onto the completed Coosa Board GRP assembly. I did the design and drawings for that using CAD.

7. I found the Coosa Board and all the materials I needed for the job at Noah’s Marine Supplies in Mississauga. BTW I’m not associated in any way with any of the companies I’m listing here.

OK, here’s my tips;

COOSA BOARD IS EXPENSIVE!
You don't want to make mistakes laying it out it or cutting it with your jigsaw. So don't! I used a free, easy to learn CAD software (Sketchup) to design the hell out of my Coosa Board keel assembly before I cut anything. Then I took the completed CAD file to a water-jet cutting operation and they cut all the pieces for me out of the Coosa Board sheets I delivered to them. Yes, it cost me money to have someone else do this work. But I got parts that were shaped and dimensioned perfectly. I didn’t have to loft any drawings and suffer from the mistakes that I might make in that process. I also didn’t have to suit- and mask-up for a day or two to protect my skin and lungs from the itchy and (probably) carcinogenic fibreglass-impregnated high density foam that is Coosa Board. There’s value to all of that that nobody should discount. BTW I used the same CAD file/water-jet cutting process previously for my steel rudder and it worked so amazingly well that I didn’t have to think twice about using it for the keel project.

COOSA BOARD IS EXPENSIVE!
Didn’t I mention that already? Each of the five layers that make up the assembly are roughly 5’ long by 2’ wide and no matter how I tried to lay these out in CAD into a 4’ x 8’ surface area, I always ended up needing three sheets and having lots of wastage. So finally I redesigned two of the five layers to be made up in four pieces, with the length of them able to fit into the 4’ width dimension of the board. This saved me one sheet, or $500!


TREAT SEAMS CAREFULLY
Having divided two of the pieces into four to save money, I now had the problem of those seams becoming weak points in the assembly. You need to be careful about seams in your Coosa Board assembly (or any layered assembly for that matter) as this becomes a natural line of failure when subjected to a large bending or torsional force. The manufacturers of Coosa board recommend doing seams in a wave-form instead. I opted for an insert which breaks the straight line into two short lengths, as you can see in the pic. The insert is shaped with large curves as well to avoid more straight-line seams.


As you can see in the exploded view, these "split layers" are capped and bonded together by the continuous centre sheet and the two end sheets, so I'm pretty sure the strength of the assembly has not been compromised in any significant way by doing this.

ACCOUNT FOR THE EPOXY THICKNESSES
If you’ve got seams as a part of your design, you need to allow a gap between all those mating surfaces, enough to allow a layer of epoxy to complete the bond. I designed a 0.010” gap into mine. The precision of water-jet cutting allowed me to maintain that consistent gap and it worked out really well.

SAVE TIME AND MATERIAL THROUGH DESIGN
Instead of taking a reductive approach (making a big block and sanding down to get to the desired shape, I took the additive approach. This makes a lotta sense when you consider that it uses less of the very expensive Coosa Board and more of the less expensive epoxy to fill in the voids. Honestly, it also takes less time to add epoxy than to endlessly sand down the Coosa Board. Way less mess and itchiness too! You get a more accurate profile as well. So what’s not to like about the additive approach? Yet I see so many people wasting time and materials sanding down instead of building up.


The image shows this approach very well. For example, to allow the taper of the keel towards the trailing edge I stepped back each outer layer and added slots and wedges (also cut from Coosa Board) to not only give a little extra strength, but also act as spaced guides for my additive fairing work. I used the same “stepped back” approach for the leading edge bull-nose. The outermost edge of each layer gave me a good visual guide for fairing and shaping, later in the project.

TIPS FOR PREPARING YOUR WORK AREA AND TOOLS - When it came time to bonding it all together, I made sure I had a perfectly flat work-surface. Then I clamped the Coosa Board sheets together, one at a time using heavy steel rectangular channels and clamps to compress the Coosa Board layers down onto the worktable. This spread the compression load over the entire length and middle of the boards where the clamps would not reach.

Before gluing anything, I recommend doing a dry assembly to work out how you’re going to clamp everything. After I'd done that, I also opted to pre-drill lots of countersunk pilot holes into the Coosa Board, sized for #8 screws. Roughly one screw every 8 inches, all over the board.

Then it was time to do the “wet” assembly, once I'd removed the clamps and flipped over the top board. I wetted the lower board with epoxy (more details on that below) and re-clamped the pieces down as hard as I could, I was able to quickly screw down the whole thing with SS countersunk head screws to further assist the even distribution of epoxy. Perhaps that’s overkill, but for the extra bit of money the 200 screws cost me, it’s worth the peace of mind. Boatbuilder’s brass screws would’ve worked fine as well. Coosa Board accepts screws and hold the thread quite well. Also with the epoxy pressing up through the holes once you clamp the parts together, the screws are being bonded into place once the epoxy dries. This is another advantage of pre-drilling the holes, to allow this to happen. Good luck trying to get any of those screws out once the whole thing cures!

Don’t rush the process. Let each layering cure before proceeding to the next layer. So for my design using 5 layers, the whole assembly took 4 days.


CHOOSING YOUR EPOXY
For something structural and critical like a keel I wasn’t going to cheap out on the gluing medium. Marine epoxies were my only option. I’m used to working with West System epoxy but found the amine blush to be a pain. I then discovered East System 5:1 marine epoxy. It claims no amine blush whatsoever so I decided to give them a try. Also, they’re a Canadian company so I’m not paying the duties of the imported West System. I wasn’t disappointed! I had no problems whatsoever with blushing. East System has also copied from West System by offering calibrated pumps for the epoxy and hardener, so you don’t have to measure the proportions. That was the final clincher to switch to East System.

I used West System colloidal silica for the bonding layers that I had left over from previous jobs. I mixed to something a bit more runny than mayonnaise. Don’t ask me the proportions, I just go by feel on that sort of thing. Be generous and apply the slightly thickened epoxy evenly over the entire surface with a spatula. You don’t want air gaps in your assembly.

LOCATING, LOCATION, LOCATION
I planned two locating holes in my assembly to be able to insert "locating pins" to ensure each layer would be precisely positioned relative to each other. One hole was already there – the pivot hole. At the other end I added a ½” hole to my CAD file which was cut during the water-jet process. When assembling I found a wine bottle that fit the pivot hole snugly (now there’s a good justification for drinking) and used a ½" bolt as the pin for the other hole. These prevented the layers from sliding out of alignment while I was squishing the parts together using the clamps. Once the entire assembly was completed, I just filled the smaller hole with thickened epoxy, as it was not needed anymore. Don’t forget to remove these locating pins once the assembly is glued and screwed in place. Otherwise you’ll never get them out as the epoxy cures!

HOT LEAD
Lead can be a bit of a pain to source these days. Fortunately my boat’s ballast is made up numerous 15 kg lead ingots and I had some spares left over from my refit. So I repurposed three of them to be my keel ballast. An observer of my work-in-progress suggested melting these lead ingots directly into the cavity into my Coosa Board assembly. Problem with that idea is that I’d likely might melt right through the Coosa board, or at least alter its properties, and screw the pooch on the whole job. I made a form instead by tack welding 1-1/2” x ¼” steel bar onto ¼” plate, forming an open sided box. The interior dimensions of this form would give me an ingot that would fit loosely into the void in my Coosa Board assembly. I melted the ingots into the form using an ox-acetylene torch.


Lead melts at a pretty low temp so it didn’t take long to melt 45 kg. Once it cooled it popped out of the form easily (it shrinks as it cools, of course). I then put down a generous layer of silica-thickened epoxy into the keel’s void and dropped the lead into it. Its tremendous weight squeezed the epoxy up and around the ingot, filling the voids between the ingot and cavity walls, securing it firmly into place once the epoxy hardened. The final layer of Coosa Board sealed the deal and I made double sure that area would never split open by doubling down with 1-1/2 long screws every 3” all along the perimeter of the void.

CARING ABOUT FAIRING
I filled in the deeper voids with East System thickened to peanut butter consistency using colloidal filler. To help keep the epoxy from running off I always re-positioned the keel to make the surface being filled as level as possible. Even with the thicker consistency, it will run over the few hours it takes to set. Even though I was using fast hardener, I was working in a cool (OK cold, this is Canada after all) space so each application took a day to harden.


Once I was down to 1/8” of void or less to be filled I switched to thickening the epoxy using micro-bubbles fairing compound. This stuff is lighter and easier to sand, but it’s not structural, which is why I used the colloidal filler for the deeper voids.


PREPPING FOR FIBREGLASS
Once I had the assembly completely faired, I had to prepare it for the GRP overlays. This meant rounding any sharp corners. GRP layups don’t like tight edges and recommend at least 1/8” radius. I used a router with a ¼” semi-round bit to quickly take down the edges in a consistent fashion. The Coosa Board is lovely to shape and work with. Then I sanded the whole assembly with 80 grit sandpaper to get a good profile for the GRP/resin to adhere to, blew with compressed air and vacuumed. I wore a proper half-face respirator, fitted goggles and gloves while doing all this, of course.

CHOOSING AND LAYING THE FIBREGLASS
I’m no expert on GRP lay-ups and I wanted something that would go down easily and smoothly over the relatively flat surfaces of the keel shape. I opted for 12 oz bi-directional fibreglass fabric from a wide roll for the main surfaces and 4” wide strips for the edging. At first I was cutting it with a regular pair of scissors until a buddy loaned me his electric scissors, which were amazing. It left very clean cuts with no frayed edges. I pre-cut and identified each layer and carefully rolled them up until I needed them.
When it came to application, I used the same East System epoxy but with no filler. I pre-wetted the Coosa board, then applied a layer of fabric, then wetted the fabric on top as well, spreading and pressing it all down with a plastic spatula and finally with a fibreglass spiked roller to press out any remaining air.

On inside corners where the fibreglass fabric was likely to come away and leave an air gap, I added some dabs of thickened epoxy just to those areas, before laying the fabric, to better retain the fabric in place. This worked very well.

In the end, due to extra thickness of the epoxy between each Coosa board layer and the double wetting of the fabric, I only needed two layers of fibreglass to get to my desired width. If my core was mahogany or marine plywood I would’ve fretted about whether that would offer enough protection from water getting through and swelling the core. But since the Coosa board is impervious to water absorption, the GRP layers were really just a little bit of insurance and strengthening, so I was happy with the result.
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Those are my tips. I hope someone is able to benefit from them and I’m happy to answer any questions.

So how did it all work out in the end? Well, after additional steps of getting the bracket fixed into place using with 8 x ¼” SS screws and “ain’t goin’ nowhere“ 3M 5200, followed by painting (Amercoat commercial marine coatings) and anti-fouling, I was finally able to manhandle the keel into the keelbox, raise and pin it, with the help of a few other sailors at the club. Got it done with two days to spare before launch! I was pretty happy to watch it rotate smoothly into the full down position on the way to my slip. Can’t wait to try it out on my first post-keel replacement sailing trip, once I get the mast on!


Now that I’ve completed the keel using Coosa Board, I’m kicking myself for not discovering this amazing material when I was planning my new rudder. It would’ve been much lighter and probably have taken less time, as well as one less hollow void to worry about under the waterline. Oh well, hindsight is 20-20, as they say.

[MikeHoncho]

Can I ask how much was the cost for the water jet cutting? I've always wondered how much it is to just take something in and have it done like this. thanks and great write up.

[raahell]

Originally Posted by MikeHoncho

Can I ask how much was the cost for the water jet cutting? I've always wondered how much it is to just take something in and have it done like this. thanks and great write up.

As for most things - it depends! These conpanies start with a set-up charge which, in the case of the company I used, is $150. One set up chanrge for each type/thickness of material, so it pays to keep your materials to a minumum. Then it depends on how long it takes for the machinbe to cut the paths. That depends on the total length of the cutting x the hardness of the material. Coosa Board was easy to cut through so I paid a bit over CAD$300.

For the steel rudder, which included lots of intricate parts, lightening holes and even the parts for a steel welding table with 1/2" diam holes in a grid every 4", I paid nearly $1600 for that. So as you can see it varies a lot, but is more-or-less directly proportional to the size and complexity of the job.

Noah's Marine, where I bought the Coosa Board, also does cutting right on site, though with a different method than waterjet. They're probably competitive and saves the trouble of lugging sheets of material to another location.

[Bill O]

Good job.

Similarly, made a new NACA 0010 rudder for our boat out of Coosa a few years ago. https://phoenixketch.blogspot.com/20...er-part-2.html

The high density urethane board (HDU) is easily machined w/normal wood working tools. In general, it is nice stuff to work with except for the dust it creates while tooling. Especially w/Coosa (f/g impregnated HDU), a good dust mask is mandatory to prevent inhaling airborne f/g particles.

[Jim Cate]

Excellent post... informative and complete, understandable to any careful reader!

Would that all posts were as good.

Jim