This post is the continuation of Refinish Your Swing Keel for Best Performance – Part 2: Cleaning. I will describe common keel defects, the fairing system I used, sealing a freshly sandblasted keel, applying body filler to build up an accurate hydrodynamic foil shape, and paying extra attention to the shape of the leading edge of the keel.
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Congenital keel defects
Most older C-22 keels are misshaped from the “factory”. The urban legend is that they were cast by one guy on a beach in Mexico. A big MEXICO stamped into the starboard end and the Swing Keel Refinishing DVD confirm that they were made somewhere in Mexico, beach or no beach. The DVD also says that most keels have one flatter side and one concave or flared side, which can cause the boat to point better on one tack than the other. It also explains that most of the foil shape is only in the lower quarter to third of the keel, which increases drag.
That was all true with the keel on Summer Dance. The port side shape was pretty close to the 13% template that comes with the DVD for the bottom quarter of its length but the shape quickly tapered to almost flat at the top. The starboard side was only shaped near the bottom. Over half of that side was nearly flat. To top it off, the keel hung at a slight angle to port, presumably because the pin hole wasn’t drilled perpendicular to the keel’s center line.
Like a seven layer cake
To refinish the keel, I applied a system of coatings that: seals the raw metal and provides a base for the rest of the coatings, shapes the keel to a proper foil, provides a moisture barrier, guards the leading edge against abrasion, and tops it all off with the same bottom paint as the hull received. The complete system with all its layers is illustrated in the following picture.
Start with a waterproof foundation
The same day that I picked up the keel from the sandblasting shop, I hung it by cables from the center beam of my shop so that I could work on the entire keel at the same time without having to flip it over.
To start, I applied West System 105 epoxy using cheap, disposable 3″ chip brushes. For the first coat, I mixed 10 pumps each of resin and hardener per side. I scrubbed it on using a circular motion to work the epoxy into the pits to seal the bare surface.
Then I lowered the keel flat onto a fork lift pallet on my utility trailer to apply a second coat to one side at a time. The reason for this was the pits in the surface. With the keel flat, I was able to fill the smaller pits (up to about 1/2″ in diameter) with epoxy. With the keel hanging, the epoxy flowed out of the pits. The starboard (worse) side took 16 pumps each of resin and hardener for the second coat. The port (better) side took 13 pumps.
After I applied the second coat this way, I wiped it down twice with acetone and then scuffed the surface and knocked down the high points with 80 grit sandpaper. The surface was smoother and ready for fairing. Except for that big cavity, that is. That took several layers of filler to correct.
Houston, we have a problem
To fair the keel for best performance, I turned to the template that is included in the Swing Keel Refinishing DVD. To use the template, you trace the curve onto a piece of thin plywood so it is easier to handle and then cut it out with a sabre saw, jigsaw, or bandsaw. You hold the wooden template perpendicular to the keel centerline (as shown in the picture below), to check the keel shape and thickness as you add the fairing compound.
The goal of the template is to make a standard NACA 13% foil shape for minimum drag and best pointing ability at the average hull speed of the C-22. Unfortunately, the template that is included with the DVD is the wrong shape because it is based on the wrong keel angle. It results in a keel that is 1″ too thick, creates excess drag, and stalls (stops producing enough lift to offset the excess drag) too soon compared to the 13% foil. Describing it is quite technical, so I described it in detail in The Error in a Popular DIY DVD That Will Slow Down Your C-22. Don’t buy the DVD or, if you already have it, don’t use the template until you read that blog post. You could be wasting your time and money.
Bell bottom keels
The maximum thickness of my bare, unfaired keel near the bottom end was 3.5″ thick. That point is also nearer the midpoint of the keel than it should be when compared to the template as you can see by the gaps in the following picture.
Doing the math told me that the keel ends in about a 19% foil curve, almost 50% fatter than the design goal of 13%.
Since removing the extra curve by lots of grinding wouldn’t be worth the trouble, I chose to shape the keel to 13% as much as possible except where it is 19% at the bottom. The extra thickness at the bottom should not make a noticeable difference. I also rounded the leading edge, which was cast to a slight point as you can also see in the picture above.
The top end of the keel doesn’t need to be shaped to a foil where it rotates up into the keel trunk, so I drew a line below the pivot pin at 60° from the edge of the keel (parallel to the bottom of the hull). That is where I stopped the foil shape so it wouldn’t bind in the keel slot when the keel is lowered.
Fair thee well
Out of the wide spectrum of fillers available, I chose USC Pro-Glas Fiberglass Reinforced Body Filler instead of regular polyester (Bondo) auto body filler on the cheap end or marine grade fairing compound on the expensive end. Compared to Bondo, the fiberglass reinforced filler is stronger, adheres better, and is waterproof to prevent the keel from absorbing any moisture that might penetrate to it like non-reinforced filler can. It applied smoothly with a plastic spreader, hardened gradually, was easy to shape with a Stanley Surform Plane, and sanded well.
For the first coat, I wanted to establish a flat, smooth base for the areas that didn’t need to be built up, so I just skimmed the surface to fill in any low spots and to fair the edges. After that cured, I used my template to draw an outline of the area that needed to be built up right on the keel with a Sharpie marker. Then I divided the maximum build-up thickness (about 1″) into 1/4″ layers and drew approximate outlines on the keel like a topographical map.
With the lines for visual guides, I was able to build up the area in multiple layers that weren’t too thick to work with and with minimal waste. Frequent checks with the foil template revealed the low areas that needed to be filled.
Remarkably, it took about one and a half gallons of filler to build up each side for a total of three gallons.
If you don’t feel confident about fairing by eye or you’re running short on time, there are some excellent techniques described in Appendix B of the Final Fairing and Finishing guide available as a free download from West System.
After the filled area was close to the template shape, I applied a thin layer of U-Pol Dolphin Glaze to fill in the smallest imperfections and give a smooth overall surface on which to add the top epoxy layer. The DVD recommends this step but, in my opinion, it’s unnecessary. You should only apply the sanding glaze to a maximum thickness of 3mm. That’s pretty noticeable on a show car hood, not so much on a sailboat keel. You can get similar results with fairing material or epoxy.
In contrast to the filler in which a lot doesn’t go very far, a little bit of glaze goes a long way. Only two cookie-size batches covered an entire side of the keel. Plastic spreaders and rubber squeegees don’t work well with this material, which is very dense. A 4″ steel putty knife gave the best results.
After fairing one entire side of the keel, I flipped the keel over and repeated the process on the other side. When I finished that side, I hoisted the keel up and set it upside down in the sled on top of the trailer so that I could fair the leading edge, which is difficult to do while the keel is laying on its side. With the keel upright, the job was much easier.
The foil template and the casting itself both come to a high-angle point at the leading edge. A true NACA foil shape is rounded at the leading edge.
I experimented with making a 1/4″ plywood template of just the leading edge and using it to screed the filler down the length of the keel but it didn’t work very well. What did worked well was to bend a plastic spreader to the desired curve and, holding it at a low angle to the keel, pull a blob of filler down the leading edge, effectively extruding the filler to the correct shape. After it cured, I only needed to sand it a little to remove the high points and to feather the edges.
This technique also worked well to put near-perfect rounded edges on the end of the keel that swings up into the trunk.
Flatten the trailing edge of the keel to 1/4″-1/2″ wide for minimum turbulence. Don’t fair it to a sharp edge.
With the long process of fairing complete, I finally moved on to the top coats described in Refinish Your Swing Keel for Best Performance – Part 4: Sealing and Painting. In that post I describe: building a fiberglass reinforced guard on the leading edge, applying the last shell coats of epoxy, bonding centering shims onto the pin end of the keel, priming with a barrier coat of epoxy paint, and applying anti-fouling paint last.
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