Unsteady Sail Dynamics due to Bodyweight motions
Authors: Schutt Riley R. , Williamson C. H. K. 
Published: June 2021

In small sailboats, the bodyweight of the sailor is proportionately large enough to induce significant unsteady dynamics of the boat and sail. Sailors use a variety of techniques to create sail dynamics which can provide an increment in thrust, increasing the boatspeed. In this study, we experimentally investigate the unsteady aerodynamics associated with two such techniques, “upwind leech flicking" and “downwind S-turns". We employ a two-part approach.

First, on-the-water experiments are carried out using a Laser class sailboat sailed by Olympic and world championship level sailors. Data collected from an on-board GPS, IMU, anemometer, and camera array is used to generate characteristic motions of the boat and sail relative to the apparent wind.

Second, laboratory experiments using the characteristic motion of the sail are run in a computer-controlled 3 degree-of-freedom (X, Y, and θ) towing tank. We use water as the working fluid. Rather than directly experiment with three-dimensional sail shapes, we represent the primary effects of the sail dynamics using rapidly prototyped two-dimensional flexible sail geometries. Shapes are based on extruded draft stripes from the upper third of the sail. The laboratory experiments approximately match the key non-dimensional parameters of the on-the-water sailing conditions, including the reduced frequency and heave-to-chord ratio. Particle Image Velocimetry and force measurements are used to analyze the flow field and thrust generated by the model sail during the dynamic motions. On-the-water testing shows that the characteristic sail motion in leech flicking is a combination of periodic heave caused by the actions of the sailor and a passive twisting of the sail due to rig flexibility. The heaving sail motions are due to rotation (roll) of the rig around the longitudinal axis of the hull. This is at an angle to the apparent wind, resulting in heave that has components both perpendicular and parallel to the oncoming wind flow. This is distinct from classical aerodynamic studies with heave purely perpendicular to the incoming flow.

In laboratory experiments, the characteristic flicking motion is applied to a NACA 0012 airfoil and a 2D sail, both angled at 15 deg to the flow. Lift increases and drag decreases, leading to an overall increase in resultant driving force of the boat. The beneficial effect of this dynamic motion becomes greater as the apparent wind angle increases. In the case of leech flicking, the experiments show that the formation of vortex pairs is fundamental to the augmented thrust due to heaving.

The presence of S-turns, whereby the sailor changes the boats direction simultaneous with rolling the boat, generally in the downwind direction, is also associated with vortex formation and pairing, which will be described at the conference. During downwind S-turns, large amplitude heaving motions are paired with substantial rotations of the sail caused by both adjustments of the main sheet and changes in heading.

Increased velocity made good downwind is measured from the on-the-water experiments, and is associated with an increase of thrust during characteristic dynamics of the airfoil or sail shape in the laboratory.