CFD-Based Assessment of Stern Appendage Optimization and Propeller Condition on the Propulsion Efficiency of an Existing Passenger Yacht

Improving the propulsion efficiency of existing vessels is an effective approach to reducing fuel consumption and greenhouse gas emissions without major structural modifications. This study presents a high-fidelity computational fluid dynamics (CFD) investigation of a 40 m passenger yacht aimed at identifying practical retrofit measures that can be implemented during routine dry-docking. A Reynolds-averaged Navier–Stokes (RANS) model with a Volume-of-Fluid free-surface formulation and overset-mesh propeller modelling was developed and validated against the measured self-propulsion operating condition of the vessel. Three complementary retrofit strategies were investigated: hydrodynamic optimisation of stern appendages, propeller surface roughness, and propeller pitch variation. The appendage optimisation reduced the total hydrodynamic resistance by approximately 7%, allowing the equilibrium engine speed to decrease from 1740 to 1698 rpm (2.4%) while maintaining the service speed of 12.1 kn. The corresponding delivered power decreased from 311.8 to 305.4 kW per propeller, demonstrating an improvement in propulsion efficiency under equilibrium self-propulsion conditions. Increasing equivalent sand-grain roughness progressively degraded propeller performance, reducing the generated thrust by up to 10%, increasing the required torque, and lowering propeller efficiency, thereby highlighting the importance of regular propeller maintenance. Additional self-propulsion simulations showed that blade-pitch variations of ±2° significantly modified the equilibrium operating point through changes in engine speed, propeller torque and delivered power. The results demonstrate that relatively simple retrofit measures involving appendage refinement, maintenance of propeller surface condition and accurate control of propeller geometry can provide measurable improvements in propulsion efficiency without modifications to the hull form or propulsion machinery. The proposed methodology provides a practical engineering framework for prioritising retrofit actions for existing passenger vessels.