Quantifying Hydrodynamic Trade-Offs in Regeneration: Four-Quadrant Mapping for Sailing-Yacht Propulsors

This study investigates the hydrodynamic feasibility of integrating regenerative hydro-turbines into a 12 m sailing yacht, as part of a zero-emission propulsion system. A custom two-bladed propeller was designed using systematic series methods, lifting-surface analysis, and validated Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) simulations. The propeller achieved ~54% open-water efficiency at the 6 kn, 6 kW design point, meeting thrust requirements while also reducing drag in freewheeling mode. Two hydro-turbines added only 9-10% resistance across 3-7 kn. In generating mode, each turbine produced 200-300 W under a thrust load of 5-6 kn, with a manageable speed loss of approximately 0.3 kn. This performance enables continuous charging during passages. Despite the increasing use of hydro-generators in practice, yacht-scale hydrodynamic data coupling four-quadrant propeller behavior with hull resistance remain scarce. Prior studies have optimized propellers or turbines in isolation, but few have quantified the net energy-drag trade-off in realistic sailing conditions. The current study fills this gap by producing yacht-scale four-quadrant KT-KQ-J maps with turbine operation, integrating them into validated hull resistance models, and deriving non-dimensional “net-energy-versus-drag” charts. The results provide actionable guidance on when regeneration yields net benefit, offering designers and operators a framework for practical, performance-optimized deployment of regenerative systems in sustainable yacht design.