Keel Thickness

Should you go for a “thin keel” or just fair your keel? The first thing you need to be aware of is the “keel rule” in the 1999 Amendments to the Constitution.

I have been sailing with thin shape on Aliens Ate My Buick since 1998 and find it a little faster downwind and not any slower upwind, though it is harder to point and slower getting off a tack. In some areas and conditions, the thicker keels may have an upwind advantage. The thin keel is trickier to drive upwind than a fat keel too.

Here is some information, opinion, and advice, no warranty, to anyone considering a keel change. Forgive me if this is stuff you know already; maybe some of it is new to some people.

Well-sailed boats like Lunchbox and Barnstormer (fat keel at the 99 Nationals) are fast with a fat, asymmetrical keels. Barnstormer is a particularly interesting case in point; next to the other gleaming white expensive keels in the Shilshole yard, the class champion’s keel is an ugly, fat, lopsided (but lovable) mess of patches and cracks. (Barnstormer has since remodelled to a beautiful, thin keel.)

The class minima were taken from different boats — the bottom two stations come from Roadrunner/Wildfire, but the third station up was thinner on Splash Tango so the third minimum does not make an existing keel shape taken with the bottom two minima. The idea was not to make existing keels illegal, but to bring order to chaos and give a target to anyone wanting to modify legally. (If you use the John Lashley templates, the bottom will come out too thin if you fair it down from the bottom-most template station; you must flair it ever slightly to make the 3 inch minimum at the bottom.) See the 1999 Amendments to the Constitution .

If your keel is fat, consider whether you prefer to have weight in your keel or a thinner keel, as you will probably have to shave lead.
Weigh your boat accurately with all equipment out except class and Coast Guard mandated equipment. Calculate the volume of lead keel to be removed to fit your new templates, multiply by the density of lead, subtract from boat weight. If the new light weight is under the class minimum boat weight you may want to reconsider, as you will have to add penalty weights.
Regardless of thickness, make sure there is good, strong epoxy-glass on the joint between the keel stub and the keel, or this will crack. The trailing edge where the stub joins the hull might also be beefed up with eglass to avoid cracking.
Elsewhere on the keel, avoid glass as this can possibly blister. (Aliens‘s old keel had developed big, pointy, smelly blisters.) We used straight epoxy in layers alternating graphite/no graphite for color to aid in fairing, and some microlight.
Shape is important but not exactly specified for the class as all boats have slightly different shapes. This is hand-and-arm work, not computer-lathe work; the tolerances are inexact. A fair shape is extremely important. If it is not fair it will not work well, thick or thin. You can tell if it is fair by getting it wet and looking at the reflections for distortion.
The NACA sections were developed over 70 years or so in a repetitive cycle of refinement, theory/design/try-out/modify-theory/modify-design, and so on, but for AIRFOILS NOT HYDROFOILS. The Reynolds number is different for water. Whether this means the NACA airfoils are not the final answer for keels is certainly debatable. However, some people who know about theory say, “take an existing fast keel and clone it.” In any case, the NACA sections will not fit an O30 keel — as George Olson discovered post-facto, though he had an NACA section in mind, apparently. It is possible to use NACA for the front part but the trailing part must be thinner. Some thin keel shapes were made by shaving the fat side of a disintegrating, asymmetrical keel to match the existing thin side of that keel. So the shape is not NACA but is in some sense a factory shape … from one side.

Happy sailing —

John Rahn