Abstract
This paper presents two substantial innovations in keel design for racing sailboats - in particular the International America's Cup Class:
1. Multi-element foil sections
2. Planar winglets
It is shown that multi-element keel foils can improve upwind keel performance by approximately 10% over existing systems, while possibly yielding a slight downwind deficit.
When multi-element foils are integrated with planar winglets, it is shown that average upwind keel performance gains of 20% are attainable, in addition to a slight downwind advantage over existing systems.
The multi-element, planar-winglet keel concept has been dubbed the "Condor Keel" in honour of the planar winglet's originator: the bird.
The Context
A monohull sailboat keel is essentially a vertical wing with two major functions:
1. Provides hydrodynamic side forces to balance the aerodynamic (sail) side forces.
2. Structurally connects the ballast bulb to the hull.
When compared to the aerodynamic devices (sails and rig) of modern racing yachts - which are complex, highly tuned, and infinitely variable - the conventional IAAC keel (which generates similar forces) is a very simple device: a linear, symmetrical foil with a simple plain flap. The one successful innovation in recent years (since Australia II) has been the incorporation of non-planar (horizontal) winglets for induced drag reduction. This keel configuration has become the standard for contemporary yachts.
This paradox becomes understandable when one begins to study the problem of generating symmetrical (port/starboard) hydrodynamic forces from a fixed appendage (under the IACC rules, the keel strut cannot rotate), while supporting a massive ballast bulb and minimizing drag downwind. By conventional thinking, it remains the best solution.
Improvements are available, however, by invoking the unconventional.
To access this site requires a browser which supports frames.