Aerodynamic Analysis of the Bi-Radial MKII Sail for the Olympic Sailing Class Dinghy ILCA 7

Abstract

The most widely used model for sail force coefficients is probably Hazen’s model. The model, which is presented in [18], offers great flexibility and can be used for a wide range of different sail plans. However, even if it has been extensively used since Hazen presented the model in 1980 and is still used today 4 decades later, it has shown not to be fully satisfying for downwind analysis of sails. To properly analyze the Olympic sailing class dinghy ILCA 7 sailing downwind, the aerodynamic sail coefficients of the new bi-radial MKII sail first has to be retrieved from experimental wind tunnel tests. The sail coefficients can then be used in a Velocity Prediction Program (VPP), where they are balanced against the corresponding hydrodynamic coefficients, obtained from experimental towing-tank tests of a full-scale ILCA dinghy in 2014 at SSPA, a Swedish Maritime Consulting Company, situated at the main campus of Chalmers University of Technology in Gothenburg Sweden. With the output from the VPP, in the form of a polar plot, the optimal way to sail the ILCA 7 dinghy can be established, which is of great value for the Swedish elite ILCA 7 dinghy sailor, who will have the honor of representing Sweden at the Summer Olympics 2024. A 1:7 scale model of the new bi-radial MKII sail, released in 2016, was manufactured by North Sails and tested for downwind sailing in flat water conditions, in the low turbulence subsonic wind tunnel, at Chalmers University of Technology. The sail model was tested for two different apparent wind speeds that represented light and strong wind conditions. The Reynolds numbers for these two conditions were 1.8×105 and 3.0×105, respectively. Even though the flow at these Reynolds numbers presumably is turbulent, a grid, that induced turbulence with an intensity of 2% was used and the sail model was therefore with certainty tested in turbulent flow. For the light wind condition, the sail model was tested for five different headings, three different heel angles (upright condition included), seven different sheet angles and for each configuration of these, three different settings with the kicker were tested. For the strong wind condition, only the heading corresponding to when the ILCA 7 dinghy is sailed dead downwind in a planing mode was of interest; for that heading, the sail model was tested for the upright sailing condition, i.e. with a heel angle of 0°, for four different sheet angles and three different settings with the kicker. For each configuration of these variables, for both wind conditions, the other two trim controls, the outhaul and the cunningham were continuously trimmed. Altogether 327 different configurations of the above mentioned variables were tested. This Master’s Thesis describes how the aerodynamic sail coefficients were retrieved from experimental tests in Chalmers low-turbulence subsonic wind tunnel and how the measurements were corrected for blockage effects, after performing a numerical analysis with Reynolds Averaged Navier-Stokes (RANS) simulations, using the commercial CFD-code STAR-CCM+. The final results is a full set of sail coefficients needed for optimizing downwind sailing of the ILCA 7 dinghy, using a VPP.