Unsteady Aerodynamics of Sailing Maneuvers and Kinetic Techniques

Abstract

Small sailboat kinetics are significantly affected by bodyweight motions of the sailor. Highly dynamic motions of the boat lead to unsteady aerodynamics around the sail. These unsteady flows are studied using a two-part approach. First, on-the-water tests in a modified Laser sailboat capture the motion of the boat and sail. Subsequently, these characteristic motions are the basis for towing tank experiments exploring fundamental 2D flows around representative sail sections. The “Sail-Flicking”, “S-Turn”, and “Roll-Tacking” kinetic techniques are explored in depth. Each technique is used by racing sailors to improve the performance during specific portions of a race and are most effective in select wind conditions. Sail Flicking consists of repeated small amplitude sail pulses which increase straight-line speed in upwind and cross-wind sailing. In the 2D case, a vortex pair is shed during each individual “flick”. The benefit in driving force is maximized at apparent wind angles near 45°. Sail Flicking effectiveness scales with the non-dimensional parameters reduced frequency and heave-to-chord ratio, making the technique most effective in light wind. The S-Turning technique uses coordinated turns, rolls, and sail adjustments while sailing downwind. It is named after the boat's “S” shaped path through the water. On-the-water tests show a measurable increase in downwind Velocity Made Good, VMG, while laboratory experiments indicate constructive interaction between the sail and vortices shed during each motion period. Sailboats tack back and forth to make progress into the wind. During a Roll-Tack, sailors allow the boat to reach a high heel angle during the turn. An aggressive bodyweight shift then rolls the boat upright, sweeping the sail through the air and increasing driving force. On-the-water tests show that Roll-Tacking in light wind increases windward VMG relative to straight-line sailing and relative to tacks which do not roll the boat. 2D laboratory experiments representing a slice of the sail show the formation of a strong vortex pair associated with a six fold increase in driving force.