CFD-Based Design of an Asymmetric Twisted Flap Rudder for Lift Enhancement at Small Deflection Angles

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.