Apparent-Wind Aerodynamics of High-Performance Sailing: Speed Ratio, VMG, and Comparison with VAWT Kinematics

This study develops a compact apparent-wind framework for high-performance sailing craft and uses vertical-axis wind-turbine (VAWT) kinematics as a bounded comparison. By establishing closed-form equations, the model connects key variables: sailing angle, lift-to-drag ratio, resistance, speed ratio, apparent-wind angle, and velocity made good (VMG). The model is formulated as an upper-bound framework limited to established, approximately steady foil-borne operation; it is not intended to replace a complete velocity-prediction program or dynamic model of takeoff, maneuvers, touchdown, and recovery. A constant loss factor is used only as a transparent sensitivity parameter, while a velocity-dependent formulation is identified as the appropriate iterative extension for class-specific prediction. Predictions are compared with representative published ranges for iQFOiL, SailGP F50, and racing iceboats. Differences between ideal and observed performance are interpreted through aerodynamic and hydrodynamic drag, foiling-transition nonlinearities, cavitation, ventilation, structural loading, and control limitations. The VAWT–sailing analogy is restricted to relative-flow kinematics and lifting-surface control because the systems have different optimization objectives and flow physics. Finally, an exploratory energy-harvesting analysis distinguishes the unloaded zero-driving-force speed limit from a generator-loaded operating point governed by a coupled force–energy balance. The resulting framework provides clear, practical relationships while identifying the additional class-specific and dynamic physics required for predictive performance modeling.