Well known Yacht Designer / Naval Architect Dave Gerr approached me some time back as he was writing a keel and rudder design methods article for a well known Yacht Design School. He had noticed the article I had published for Sailing World that described a keel design that was used on a winning J36 on the Newport – Bermuda race.
I provided some details of how the design was intended to work and why I believed that it was an improvement over many keel designs of that era. Dave included the description of the keel in course work he was writing and also published a short article that compared several keel design types. At the time the keel I used on the J36 was radical because it defied the then current concept that the keel / hull joint was the source of a great deal of drag. The conclusion by many designers at the time was to dramatically reduce the root chord length (keel to hull location) as much as physically possible in order to reduce this keel to hull drag.
My design, seen in the downloadable article listed below, flew in the face of that concept by using an even longer hull to keel joint but done using a method perfected on aircraft wing to fuselage joints going back 80 years (1940’s). The most typical design could be seen on the exceptionally fast P51 Mustang fighters of WWI and still seen on modern Piper low wing aircraft today. The concept is to use a highly swept back (~45 degrees) short span segment that sharply breaks into a far lower sweep back angle on the order of 10 degrees. The highly swept portion of the root section of the keel or wing causes span wise flow to be induced away from the fuselage / hull joint until the sharp leading edge change is encountered. The sharp break in the leading edge induces a local vortex to form that effectively seals the upper portion of the keel and enhances steady flow on the remainder of the keel / wing span. The very slight leading edge sweep angle of the remaining leading edge resists span wise flow and reduces tip vortex drag and reduces keel drag overall. Leading edge sweep or at least quarter chord sweep angle of near zero degrees is required for maximum keel lift and reduced keel drag.
Here is the article showing the various keel types that includes the Vacanti Keel concept.