A comparison of downwind sail coefficients from tests in different wind tunnels

Abstract

This paper contains results from five different tests on model sailing yacht rigs and sails. The tests were conducted by the author in four different wind tunnels over a fifteen year period between 1991 and 2007. The tests were conducted as part of development programmes for Whitbread 60 and America׳s Cup Class yachts and for particular racing teams. They were originally subject to commercial confidentiality so have not been published previously.Although the aim of the original tests was to compare sail designs and develop the performance of the individual yachts the aim of this study is somewhat different and uses the data to compare wind tunnels. The paper describes features of the wind tunnels that affect the results together with the test requirements for investigation of downwind sailing performance. A large number of individual results are presented from tests over a range of apparent wind angles and curves of maximum lift and drag coefficients from each tunnel are then compared.Although the original tests were not designed for benchmarking wind tunnels the lift coefficients from the different tests showed broad similarity within a 10% tolerance band and the drag coefficients within 20%. The difference between the tolerance bands being partly attributed to the dependence of induced drag on the square of lift. These together with similarities in the trends of the coefficients with apparent wind angle help validate the technique of wind tunnel testing of sailing yacht rigs. Conclusions have also been drawn from the results about the effect of lift on the drag of downwind sails and the overall accuracy of wind tunnel tests on rigs.