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Keel and Rudder Design Methodology


I published a number of keel and rudder design articles over the course of about 3 decades in magazines ranging from SAIL, Sailing World, 48 North (Seattle Sailing Local Magazine) and most recently in Professional Boat Builder. I have provided a link below to the article that I last published in Professional Boat Builder because it is the most comprehensive article of all of those that I published. I have not seen any similar set of details provided by any other author to my knowledge. While there are excellent hydrodynamic articles by well known author C J Marchaj (Aero-Hydrodynamics of Sailing – Dodd-Mead) specific details of specific foil shapes and definition of the true value and function of the once popular Winged Keels are not provided in most books.

Our WINGS and FOIL programs are designed to allow the optimization of Keels and Rudders for low speed power ( Speed to Length Ratios under 2) and sail applications. True state of the art performance analysis of Keels and rudders would require the use of 3D Computational Fluid Dynamics software and a detailed knowledge of the modeling methods needed to use the this advanced analysis software. We have endeavored to offer programs and design methods (described in the download article) that are within the grasp and financial resources of the great majority of designers and hobby builders. We recommend the more advanced software in our companion blog article on “How to Make a Good Boat Faster”.

Please consider the following article as a technical introduction to Keel and Rudder design based on well known Aero-Hydrodynamic principles. Many keels and rudders have been designed with our software that have contributed to winning major offshore Sailing races, including the Newport- Bermuda race in which a Vacanti Keel modification to a J36 resulted in an overall victory over Farr and other well known boat designs.

Once last critical thing that must be understood in Keel and Rudder design is implementation accuracy. Any keel or rudder design can be rendered useless if the actual implementation or build of the design is not done with sufficient accuracy. A good example was a keel design I created for a Six Meter Sailing yacht named Steverino. The builder of the keel did a great job but at the last minute decided that the “look” of the keel was not agreeable to him. The builder then heavily modified the intended leading edge nose of the keel by grinding the off the forward 6 inches of the leading edge into a more visually appealing (to him) rounded shape. This modification destroyed the entire foil shape structure of the keel tip design and resulted in poor performance. Similarly if good CNC or other high accuracy mold methods are not used the expected performance of the keel or rudder may be destroyed.

Boat Stability and Keel Design

While most designers worry about athwartship stability normally provided by a keel, one client and user of our software reported that he was working on a boat that had the terrible tendency to bury its bow while running downwind. We looked carefully at the choice of foil shape used on the keel and determined that it had exceptionally poor stall characteristics that could result in very high drag. While most consider the keel to operate near zero angle of attack when running downwind, the boat is constantly being steered to keep the apparent wind at the required location relative to the spinnaker and mainsail. This particular boat required that all crew remain aft in the cockpit to keep the bow up and prevent submerging in waves.

After the keel was redesigned with an appropriate foil shape, specifically designed for the Reynolds number range of that boat, the boat downwind characteristics changed dramatically. Literally the boat was “tripping” over its own keel and was being forced nose down by the large moment arm of the keel drag located well below the waterline. Many have not accepted this hypothesis but none the less, accuracy of build and proper foil shape selection (use of FOIL and WINGS) resulted in a boat that won and was sailed safely off the wind.