This would generate a degree of twist that could be adjusted with fore-and-aft tension in the sail, especially in a leg of mutton design with an elliptical boom (sprit). I haven't tried it yet as a model but it sounds like it would be fairly easy to do.Ī further assumption would be to have the angle between the sails surface and the wind direction constant with height. This would allow the section and profile to be related mathematically with some assumptions about differential air pressure across the thickness of the sail, which in turn could perhaps be constant to a first approximation. It seems a reasonable assumption anyway, if it is also reasonable to ignore the effect of vertical and diagonal tension. Which prompts me to ask if airfoil sections are really the place to go to? I know that the same theory applies, but the requirements are very different.Ī sail would intuitively take a shape that kept it at constant front-to-back tension, which should generate an airfoil section that would depend on the sail's (side) profile. There has been more emphasis, I suspect, on asymmetrical section development than for symmetrical airfoils. Lift is also (generally) one-way for a wing, not so for a keel. The keel of a boat starting in a goodly breeze (or going about hard) approximates to a plane trying to loop from standstill! In a light breeze the keel needs to perform like a glider wing but in a decent wind the same keel needs to perform like a mid-air refueling tanker. Speed range starts at zero and levels off at hull speed, planing speed or who knows what if we are speaking of foilers. The case of a boat keel (or sail for that matter) is quite different. In the case of an aircraft the weight and therefore the lift is relatively constant and the range of speed no more than an order of magnitude. The Bogataj link was thought-provoking, for such an apparently self-evident topic. In the linear lift range, the predictions from these programs are probably about as accurate as what you'd get from the wind tunnel, allowing you to go directly from the computer to full scale with a reasonable amount of risk. Today there are several inexpensive or free airfoil design programs that can be used to create a section tailored to your specific requirements. Most often the section is operating at lift coefficients that are way outside of the drag bucket. A symmetrical section only has thickness to use to determine its characteristics, limiting what can be done, and any drag bucket is necessarily centered about zero lift. The thinner a section gets, the more it acts like a flat plate. The earlier transition means that the big gains from laminar flow that are possible from modern airfoil shapes aren't going to be realized in hydrofoil sections. Two problems with improving on the NACA 4-digit sections are the fact that (for whatever reason) transition occurs earlier at the same Reynolds number in water than in air, and many classes specify fairly thin sections. More recently, Paul Bogataj has designed some keel shapes for classes that specify thin keels. Coordinates are available from the UIUC airfoil database. See his "Airfoil Design and Data", published by Springer-Verlag. Įppler designed sections specifically for use as yacht keels, the E836, E837, E838. Vieweb & Sohn, 1981.Ī couple of sections that might be of interest are FX L V-152 K25 and FX 71 L-150/20. X, "Stuttgarter Profilkatalog I", Friedr. Wortmann designed some sections for sailplane rudders that might be suitable. There have been a number of more modern sections designed. These characteristics have made them good all-round sections. Their front-loaded pressure distributions didn't promote a lot of laminar flow, but the long gentle adverse pressure gradients promoted short laminar separation bubbles that were quite robust when laminar flow was present. Click to expand.The NACA 4-digit thickness distributions were derived from sections that had proven to be good performers at low- to mid- Reynolds numbers.
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