The development of energy-efficient propulsion systems for autonomous sailboats requires innovative sail designs that balance aerodynamic performance and maritime operational reliability. This study presents a novel rigid wing sail system comprising a NACA 0020 main sail with an embedded NACA 0018 tail sail, specifically designed for uncrewed ocean navigation. Through systematic CFD analysis using ANSYS Fluent 2022R1, three configurations were compared: (1) the proposed hybrid wing–tail system, (2) a single main wing sail, and (3) traditional flap sails. The investigation focused on two key design parameters—tail sail area (25–40% of main sail area) and deflection angle (0–15°)—that were evaluated across angles of attack from 0° to 30° under typical marine wind conditions. The results reveal three critical findings: First, the hybrid system achieves a 29.5% higher peak lift coefficient than a single wing sail and an 11.6% improvement over slotted-flap sails. Second, increasing the tail sail area to 35% of the main sail optimizes both the lift coefficient (C_{L max} = 1.16) and the lift-to-drag ratio (L/D = 7.5 at 9° angles of attack). Third, as the tail deflection angle increases, the maximum lift–drag ratio shifts forward, and at small angles of attack, the maximum lift–drag ratio increases by 40%. The hybrid wing–tail sail design proposed in this study significantly enhances the aerodynamic performance of uncrewed sailing boats, providing new insights for the sustainable development of marine renewable energy technologies and autonomous vessels.

In response to growing demand for autonomous and energy-efficient offshore operations, unmanned sailboats have emerged as promising platforms for next-generation marine applications. As the primary control surface, the rudder plays a pivotal role in enabling precise maneuvering and maintaining course stability. This study proposes an asymmetric aft-twisted flap rudder integrating a symmetric streamlined main rudder with an asymmetric flap. The design aims to enhance lift generation at small deflection angles, thus improving the hydrodynamic performance and response characteristics of rudder systems. The flap size for the conventional symmetric rudder was first determined from computational fluid dynamics (CFD) simulation results. To further improve lift performance, a 45° curved transition section was introduced at the junction between the main rudder and the flap to enhance flow attachment and reduce viscous drag. Building on this configuration, the asymmetric twisted flap was incorporated into the improved rudder design. CFD results indicate that the lift coefficient increased by approximately 27%. Comparative CFD analyses with the conventional symmetric flap rudder and the streamlined rudder revealed distinct coupled flow characteristics under various combinations of rudder and flap angles. These findings offer valuable insights into the hydrodynamic optimization of control surfaces in autonomous marine systems.

Low-Reynolds-number flows are susceptible to premature boundary-layer separation and stall, which can substantially reduce the aerodynamic efficiency of small blades. In this study, a variable-section twisted blade, designated FX-01, is investigated using computational fluid dynamics with the correlation-based Transition SST model. The blade incorporates a smooth spanwise transition between two cambered airfoil sections, together with linearly varying chord and twist distributions. Its aerodynamic performance is compared with those of two full-span reference blades over a range of prescribed global blade angles and freestream velocities. The numerical method is assessed against published low-Reynolds-number experimental data for a finite-span NACA4412 model. At V = 8 m/s, FX-01 achieves a maximum lift-to-drag ratio of approximately 9.1 at a prescribed global blade angle of α_g = 15°, compared with approximately 8.85 and 6.42 for the NACA4412 and NACA4418 reference blades, respectively. At α_g = 15°, FX-01 reaches a maximum lift-to-drag ratio of approximately 13.06 at V = 28 m/s. This peak value is 5.0% and 25.1% higher than the respective maximum values of the NACA4412 and NACA4418 reference blades over the investigated velocity range. The flow-field visualizations indicate that FX-01 produces a smoother spanwise pressure distribution and a less concentrated low-speed region in the displayed wake under the investigated conditions. Within the limitations of the steady-RANS framework, the results indicate that the coordinated spanwise variation in airfoil section, chord, and twist is associated with favorable predicted aerodynamic efficiency for the stationary finite-span blade under the investigated low-Reynolds-number conditions.