Check your Keel
Do you have proper keel shape?
by Paul Bogataj
Are you fast upwind, but slower downwind? Perhaps it is the other way around. There are many factors that influence a Star boat's performance. One of these is the section shape of the keel. A keel optimized for upwind sailing is not much different than a keel optimized for downwind sailing. The proper keel shape balances upwind and downwind performance, but it is useful to know what the extremes look like.
When a boat is sailing upwind, the keel is producing a sideways force to windward to offset the force of the sails pulling to leeward. This equilibrium allows the boat to sail forward and results in the boat and keel moving through the water at a slight angle. This leeway angle is self‑governing in order to provide only the force necessary. If more force is required, the leeway angle will increase to satisfy the requirement.
The other factor that influences the amount of sideforce produced is speed. At a constant leeway angle, the amount of force generated is proportional to the speed of the boat squared. So, if the boat were going twice as fast, the force would be four times greater. Since changes in speed result in different amounts of sideforce, the leeway angle automatically varies to maintain its equilibrium with the sail’s force. Hence, if the boat is going faster, the leeway angle will decrease.
In order to design a keel section for upwind sailing, it is important to know how much lift the keel needs to produce. It is then possible to analyze potential sections using a computational fluid dynamic (CFD) method to determine an optimum shape for those conditions. The performance of the keel at upwind sailing conditions can be compared with its performance at downwind sailing conditions (basically zero leeway angle) to determine the most appropriate shape. The other condition of interest is maneuvering, when the boat is going slower than normal and trying to accelerate with the full amount of sail force. Examples of this occur when tacking and starting, and result in much higher leeway angles.
The CFD method computes the characteristics of the flow around, and in particular, very close to, a keel section. The water close to the keel's surface that is disturbed by friction with the keel is called the boundary layer. Some of this water moves along with the keel and acts to disturb additional water just off the keel's surface in a shearing manner. At a small distance (1/8" to 1/4") away from the keel's surface, the water is undisturbed by friction and passes at an undiminished velocity. But, the water affected inside the boundary layer requires energy and causes drag. By determining the details of the flow along the keel section and how the water is influenced by friction with the surface, the drag of the section is calculated with the CFD method.
Three sections are presented in Figure 1 that have varying fullness forward, causing differing sharpness to their leading edges. The performance predictions for these are shown in Figure 2. Drag was computed and is plotted at various lift levels for each shape.
Notice that it is possible to design a section (labeled “downwind keel”) that has very low drag near zero lift, which would be fast for downwind sailing. Unfortunately, this section has higher drag at the lift required for upwind sailing and significantly higher drag in highly loaded, slow‑speed maneuvering situations. It is also possible to design a section (labeled “upwind keel”) that has low drag at the high‑lift conditions, but it suffers severely at the low lift conditions of downwind sailing.
Through application of the CFD method, a section (labeled “compromise keel”) was designed that maintains low drag at upwind sailing lift conditions, and balances the compromises in performance downwind and when highly loaded. The downwind drag is acceptably low and the maneuvering drag is not unacceptably high, yielding a shape in between the two extremes. The printed shapes can be transferred to templates to check the shape of your own keel's leading edge. Finding it toward one of the extremes may explain your one‑sided performance upwind or downwind.
Paul Bogataj is an aeronautical engineer specializing in sailing applications. He applied CFD to design appendages for Team Dennis Conner for the 1992 and 1995 America's Cups and Young America for the 2000 America's Cup. He also has won North American sailing championships in two classes of dinghies. His background includes 11 years of experience on the aerodynamics staff at Boeing, but he currently consults independently, designing appendages for a variety of sailboats. He can be contacted at:
4223 ‑ 70th Avenue Court NW
Gig Harbor, WA 98335