Over the past two decades, the numerical and experimental progresses made in the field of downwind sail aerodynamics have contributed to a new understanding of their behaviour and improved designs. Contemporary advances include the numerical and experimental evidence of the leading-edge vortex, as well as greater correlation between model and full-scale testing. Nevertheless, much remains to be understood on the aerodynamics of downwind sails and their flow structures. In this paper, a detailed review of the different flow features of downwind sails, including the effect of separation bubbles and leading-edge vortices will be discussed. New experimental measurements of the flow field around a highly cambered thin circular arc geometry, representative of a bi-dimensional section of a spinnaker, will also be presented here for the first time. These results allow to interpret some inconsistent data from past experiments and simulations, and to provide guidance for future model testing and sail design

Unsteady aerodynamics and hydrodynamics play a significant role in small boat sailing. Highly dy- namic motions of the hull, appendages, and sail can be controlled through deliberate actions of the sailor. Kinetic sailing techniques involve coordinated changes in steering, sail trim, and bodyweight position, which can improve performance relative to steady sailing. We study the aerodynamics of the “S-Turning,” or “S-Curving,” kinetic technique, used to increase VMG while sailing downwind. Drag forces, which primarily propel the boat, can be increased by dynamics of the unsteady flow. Full-scale, on-the-water tests are conducted in an Olympic Class Laser sailboat with a suite of on-board sensors: GPS, anemometer, inertial measurement units, and computer vision cameras. Laboratory tests use a representative 2D sail geometry in an XYθ-towing tank. Direct force measurement and particle image velocimetry are used to explore the fundamental vortex dynamics. We measure characteristic motions of the boat during an S-Turn and show how boat performance is affected. Laboratory experiments show sail driving force is enhanced by an increase in the apparent wind speed over the sail, and by large scale vortex shedding produced the lateral motion of the sail in each turn. It is reasoned that this technique has in-part evolved to capture the beneficial effects of vortex shedding.

This work presents a wind tunnel experimental study of the effect of curling on the spinnaker aerodynamic performance. Four spinnakers combining two different panellings and sail materials are tested at different wind speeds and wind angles in the Twisted Flow Wind Tunnel of the University of Auckland. Results show that the curling has a significant benefit on the propulsive force at an AWA ≥ 100◦ when this conclusion cannot be made at lower AWA where the best propulsive force is reached on the verge of curl- ing or before. Sail material and panelling have an effect on the sheet length where curling appears, stiffer material and cross cut panelling being the latest to curl. Finally, it is shown that the curling frequency increased linearly with the flow speed at AWA = 120◦.