Numerical assessment of the scale effects on the propulsive performance of a ship with gate rudder system

Several energy-saving methods have been implemented on ships to improve propulsion efficiency, reducing fuel consumption and carbon dioxide emissions. The Gate Rudder System, as a novel energy-saving and maneuvering device, reduces fuel consumption by up to 14% compared to the conventional flap rudder system, as confirmed by the sea trial test of a container ship with a gate rudder system. The power savings achieved by the ship with the Gate Rudder system exceeded projections based on model tests. With the motivation of evaluating this discrepancy, this research aims to investigate the propulsive performance of a cargo ship with a gate rudder system as well as the scale effects on its performance, considering two different models of 3 m and 6 m in length, as well as a full-scale ship of 69 m in length. RANSE-based CFD analyses with the Shear Stress Transport (SST) k-ω turbulence model were performed to investigate the ship's propulsive performance and power savings at full load and sea trial conditions with both conventional and gate rudder arrangements. For multi-phase flows, the volume of fluid method (VOF) was used to account for free surface effects, and the model was unconstrained by heave and pitch. Grid convergence, verification, and validation studies were carried out to ensure the accuracy of the numerical studies. Improved propulsive performance and more than 12% power savings for the ship with the gate rudder system have been achieved at both loading conditions compared to the conventional rudder for the selected cargo ship. The study also observes scale effects, which reveal that the gate rudder of the larger model generates more thrust compared to the smaller one.