Numerical investigation of propulsive performances of a ship with a gate rudder system

Abstract

Shipping contributes more than 3% of global carbon dioxide emissions. Several energy-saving methods have been implemented on ships to improve propulsion efficiency, reducing fuel consumption and carbon dioxide emissions to attain IMO regulations. Recently, a new concept of a unique twin rudder system known as the "Gate Rudder System" has been introduced, which acts as a novel energy-saving and maneuvering device. A conventional rudder is a source of additional resistance, whereas a gate rudder system reduces resistance, generates extra forward thrust, improves propulsive and maneouvering performance and, hence, reduces the power requirements. In the sea trial of a container ship with a gate-rudder system, it has been shown that the system can save up to 14% on fuel compared to a conventional rudder system, and it can increase up to 20%–30% in service condition. The power savings achieved by the ship with the gate rudder system exceeded projections based on model tests and preliminary CFD analyses. 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 model scales of 3 m and 6 m in length, as well as a full-scale ship of 69 m in length. RANSE-based CFD analyses were carried out for the selected ship to investigate propulsive performances and potential power savings at full load and sea trial load conditions with both conventional and gate rudder arrangements. For multi-phase flows, the volume of fluid (VOF) method was used to account for free surface effects, and the Shear Stress Transport (SST) k-omega turbulence model was used to simulate the turbulent flow. 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 gate rudder system have been achieved in both loading conditions compared to the ship with conventional rudder. The study also thoroughly investigates the scale effects on gate rudder performances, and it is found that the gate rudder of the larger scales generates more thrust compared to the smaller one, whereas conventional rudder contributes higher resistance in the larger scales.