Windsurfing analysis breaks down propulsion mechanics of sailboards

Windsurfers often pump their sails back and forth to gain speed, but it can also cause slowing if done incorrectly. The complicated physics of the action has caught the eye of scientists.

Zhang et al. examined the propulsive dynamics of sail pumping on sailboards. The researchers established two quantities that measure the propulsive performance of pumping sails.

“The first one, defined as the propulsive efficacy, is the increase of the driving force coefficient compared to that of the non-pumping sail,” co-author Xin Zhang said. “The second quantity is the propulsive efficiency, defined as the ratio of the increase of the driving power and the power output of the sailor.”

Initially, the effects of sail pumping were compared to a more researched phenomenon: wing flapping. However, for wings, only the initial thrust of flapping contributes to forward motion, since lift operates in the perpendicular direction. Sail pumping channels both aerodynamic lift and drag into propulsion.

By altering the ground speed and vehicle direction, sail pumping enables the propulsive power to exceed the initial power input. This is impossible for wings in a single medium.

Aside from guiding the techniques of future windsurfers and their coaches, this research may prove valuable to fields that prioritize fuel conservation.

“In vehicles for planetary exploration, the energy carried by the vehicle is precious,” said Zhang. “It may be possible to use the sailing principle to provide auxiliary driving force to the vehicle and use the sail oscillation to enhance the driving performance.”

Source: “Propulsive efficiency and efficacy of a pumping sail,” by Peng Zhou, Siyang Zhong, Hanbo Jiang, Xin Zhang, and Raymond Chi-Hung So, Physics of Fluids (2021). The article can be accessed at https://doi.org/10.1063/5.0065561 .