You want to refinish your C-22 swing keel the right way, so you’re planning to buy the Swing Keel Refinishing DVD from a certain Catalina parts dealership. It’s been around for over 15 years and is the de facto authority on how to do the job. I’m sorry to break it to you, but if you use the template that’s included with the DVD to fair your keel, it will prevent you from getting the best performance out of your keel.
What follows is a detailed analysis of the template, the geometry of the C-22 swing keel, and the undesirable results the template produces when used to fair the keel during refinishing. If you don’t care about the hydrodynamic performance of your sailboat or don’t intend to refinish your swing keel, this post might not interest you—but don’t stop following this blog. I’ve got some cool DIY projects coming soon that have nothing to do with hydrodynamics. On the other hand, if you’re planning to refinish your swing keel some day and you want to get the most bang for your hard-earned bucks, sweat, and hours of labor, read on to avoid disappointing results.
A template for error
The DVD is advertised as including “A 13% foil template for reshaping the stock keel into a higher performance, more efficient shape.”
This means that the goal of the template is to make a standard NACA 13% foil shape for minimum drag and maximum pointing ability at the average hull speed of the C-22. You use the template to fair the shape of your keel when you refinish it as described in the previous post in the series, Refinishing your swing keel for best performance – Part 3: Fairing.
You cut the template shape out of a piece of thin plywood that is easier to use. You hold the wooden template perpendicular to the keel center line (as shown in the video and in the picture below), to check the keel shape and thickness as you add the fairing compound.
Unfortunately, the template that comes with the DVD is the wrong shape.
The template has a quote printed on it that reads:
“The actual foil percentage if the (keel hanging) angle was 45 degrees, the water length would be 22.62″, which would come out to 13.2%. This is about perfect for a small boat. Generally, a 10% is preferred for higher speeds, but the cross-section is usually too weak. The 13% has better stall characteristics at slower speeds.” Al Gunther
Al appears to have been (and might still be) a Catalina 27 owner that was active in a couple of online sailing forums, not a Catalina designer or otherwise related to Catalina Yachts. I suspect his quote was taken out of context for the DVD or he was speaking generally, not specifically about the C-22. He could not be contacted for comment.
How 45° does not equal 30°
Regardless of the bad grammar, the word “if” is the key.
The first problem with the template is that the C-22 swing keel angle is 30° swept back from vertical as shown in the following drawing, not 45° as stated on the template and in the video.
The bottom end of the keel is horizontal when the keel is lowered. The angle of the end of the keel to its edges is 60°. Likewise, the angle of the front of the trunk where the keel rests when lowered is 15° from vertical. The angle of the edge of the keel that rests against the front of the trunk is also 15°. 15° + 15° = 30° and 90° – 60° = 30°.
Nothing about the design is 45°. Why the producer of the DVD didn’t know that is anyone’s guess. To the Catalina parts dealership’s credit, they didn’t produce the video. It was produced by Robert Trim of Trim Entertainment Services, a C-22 owner who could not be reached for comment.
But why the video or template hasn’t been corrected by now and is still sold that way by people who ought to know better after all these years is inexplicable. When I asked the Catalina parts dealership about the discrepancy, they didn’t deny there was an error, claimed that none of the hundreds of DVD customers have ever asked about it, it wasn’t calculated by them, and it would take them a day of research to figure it out. Their final response was, “If you trying to be that anal about it, buy another boat.” Caveat emptor.
If the math doesn’t convince you, below is a picture of Chip Ford’s C-22 named Chip Ahoy with the keel lowered. If you import that picture into any vector-based PC software, rotate the picture 2° clockwise to level the hull, and measure the angle of the keel, it will be 30° from vertical.
If you think difference of 15° is much ado about nothing, think again. Any keelboat designer will tell you the angle is significant because it determines the water length—the distance that water flows across the keel in its lowered position. The water length is also the length of the foil shape that you want to fair the keel to match. Consequently, the foil shape determines the template shape.
How the template misses the mark
The caption on the template clearly states “NACA 13%” so you might think that you’re looking at a 13% foil template but you’d be wrong as I’ll now explain. This is where understanding the keel shape and its geometry can be confusing. It’s all about the math but hang in there with me while I get my armchair engineer on and you’ll find out why it matters. Read the rest of this section slowly and carefully and you’ll be alright.
The National Advisory Committee for Aeronautics (NACA) is the agency that tests and catalogs standard foils in aeronautics for use by NASA, Boeing, Airbus, almost every professional and hobbyist airplane and boat designer, and even bicycle designers. Imagine the cross-section of an airplane wing and you’ll get the right idea. Since water and air are both fluids, aeronautic flow characteristics also apply to keels and rudders. They simply work in water instead of air.
The names of standard NACA foil shapes include the thickness of the foil expressed as a percentage of its length (also known as its chord). So the width of a NACA 13% foil is 13% of its length. The higher the percentage, the thicker the shape relative to its length, more like a teardrop.
Despite the caption on the template, it is not a 13% foil curve but is meant to produce a 13% foil in the water when the keel is lowered to 30° from vertical. It is actually a 19% foil curve that produces a 16.5% keel as I’ll explain in a moment.
The water length of the keel’s foil shape when in its lowered position is longer than the curve of the template when held at 90° during fairing as shown in the following picture. Compare the three dimensions shown in red. The dashed keel outline hanging vertically is hypothetical only to show the template length at the same angle at the assumed and real water lengths. The keel never hangs at that angle under normal conditions.
So to wind up with a 13% foil in the water, we need to start with a template that has the same length as the keel width (so we can work with it at 90°) and the template (full shape) width should be the same as the width of a 13% foil at 30° from vertical.
Curses, foiled again!
That’s it for the theory. Now let’s look at the reality. If you measure the template (only one half is drawn so that you can measure the keel surface with it) you will find that it is 15.75″ long and 1.5″ at its widest. (Second generation keels are 3/4″ wider at 16-1/2″). Double the width to find the total thickness of the foil and you get 3″. The thickness is the key measurement because that is what is affected by your fairing. The width of the keel and the angle at which it hangs will remain the same regardless of how you fair the shape.
For a NACA 13% foil, thirteen percent of 15.75 equals only 2″, so you might ask yourself what is going on with the 3″ total thickness produced by the template. They can’t both be right now that we understand the relationship of the foil shape to the thickness and the real angle of the keel.
If you recall your high school math class formula for calculating the hypotenuse of a right triangle (), the 30° angle means that the real water length is 18.19″, not 22.62″ as stated on the template, which assumes a 45° angle. This shorter real water length means that the template produces a fatter effective foil than 13%—one whole inch fatter, to be exact, the difference between 2″ and 3″.
In fact, a NACA foil with an 18.2″ chord length (water length) and 3″ thickness is 16.5%, 3.5% more than the design goal. The same thickness foil at 15.75″ long (the template) is a NACA 19% foil. These are the red curves shown below. The template should be a 15% curve to produce a 13% foil (green curve below).
At the end of the day
The bottom line is, if you use the so-called 13% template (that’s actually a 19% template), you will get a 16.5% keel that creates more drag than it should and will need much more (I estimate between one and two gallons) expensive fairing filler to make. More drag means less speed. The keel will do pretty well at slow speeds but will stall sooner at higher speeds than a 13% keel. When keel stalling happens, leeway increases and the boat won’t point as high into the wind. Precisely how much it would affect your performance depends on a lot of variables: boat speed, wind speed, angle to wind, keel algae, and more.
That’s an unwanted handicap for C-22 racers. I can’t imagine how many C-22 racers in the last 16 years have used that template thinking they had the perfect keel shape, not knowing that they were off the mark. If that includes you, you’re probably familiar with Catalina 22 class rule C: Keel:
They shall remain unmodified in composition, size and shape. Refinishing the surface of the keel shall not be considered a modification.
Fortunately, when I contacted the C-22 class rules committee for comment, they replied that they interpret the rule to mean that as long as the profile shape and cross-section are not radically changed, such as by adding wings or a bulb, customary refinishing is accepted without requiring measurements. Race on, DIY keel refinishers!
What’s a skipper to do?
To get a 13% keel using the methods described here, you need to use a NACA 0015 (15%) template.
If you’re already a follower of this blog, in an email newsletter at the end of this series, I will send you a link to download an Adobe PDF file that I created that you can print on your home printer to make your own template. It will have instructions printed right on it. If you’re not already a follower, subscribe right away and you’ll receive the newsletter and link too. But don’t wait too long, there’s no newsletter archive that you can go back to and find the link.
If this hydrodynamics stuff is interesting to you and you’d like to see computed graphs of the differences in lift, drag, and stall angles of these foil shapes, then I invite you to download and read Keel and Rudder Design by David Vacanti that originally appeared in Professional Boatbuilder magazine. There are a lot of other good design resources online too.