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Asymetrical Sails

In the continuing quest to build faster sails, an old school of testing has been reborn with some fascinating results. Driven by the International Americas Cup Class (IACC) and the Volvo Ocean Race, wind tunnel testing has proved to be indispensable in the research, design and testing of the new breed of asymmetrical sail shapes.

During the past America’s Cups, asymmetrical spinnakers increased their effective range from 11 knots True in San Diego to 14 and sometimes to 18 True in Auckland, thereby replacing many symmetric sails used in the past. In the quest for better performance, the Volvo Ocean racers use asymmetric sails exclusively in their downwind inventory.

Over the last half-century spinnaker sail shapes have been developed through the countless succession of build, test, recut, and try again. In the last few years a design revolution has been developing with the design and use of asymmetrical shapes. However, without the luxury of time to develop the full size sail shapes, sailors and sail designers have struggled to come up with specific 3-D shapes that lead to faster sails. With the use of twisted flow wind tunnel studies and full-scale 2-boat testing, asymmetrical sail shapes have improved tremendously. The wind tunnels used in Auckland by North Sails are said to be twisted flow. This is a reflection of the real life situation.

As wind flows over the surface of the water the air closest to the water moves slower than the air just above it because of the friction between the water and the air. Therefore, the wind flows faster as you move up and away from the water. Since boats move through the water at a constant velocity, the slower moving air closest to the water crosses the boat at a tighter angle than the faster moving air above. This effect is called the wind gradient and can vary from 6 to 16 degrees from deck to masthead on most boats. The amount of twist seen by the sails will change due to several effects…. variations in true wind angle, boat speed, wind speed, air & water temperature. In wind tunnel testing an average gradient is used to model the wind gradient effect.

 Practical experience and the wind tunnel testing reveals that at AWA less than 90 degrees, asymmetric sails will out-perform symmetrical sails. At the AWA range between 100+ and deeper it was thought that Gennakers would have less of an advantage but wind tunnel testing is proving that newly shaped Gennakers have an advantage which only decreases as the AWA approaches dead down wind where symmetrical shapes still have a strong hold. Wind tunnel testing is helping sail designers unravel how to build Gennakers that will out perform spinnakers through the entire wind angle range.

The development and acceptance of asymmetrical sails in the last 10 years have seen their use in AC, Volvo, Sport boats, CSA, IMS, PHRF, etc. If this growth continues Gennakers may replace the symmetrical sail sooner than we expect.

Andrew Dove is Area Manager for North Sails.

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