ALS-induced propeller air ingestion and its effects on ship propulsive performance

Recently, the International Maritime Organization (IMO) has strengthened environmental regulations, making the reduction of carbon emissions and fuel consumption from ships a critical concern in the maritime industry. Therefore, the development of various energy saving devices (ESDs) to improve the efficiency of vessels is becoming a topic of growing importance. Among these, the Air Lubrication System (ALS) has gained considerable attention as an eco-friendly solution that reduces hull resistance by injecting air beneath the hull bottom. However, a significant challenge associated with ALS is the potential ingestion of the generated air into the propeller plane. This ingestion may adversely affect propulsive performance. To address this issue, the present study aims to evaluate the degradation of propeller performance caused by ALS-induced air ingestion through Computational Fluid Dynamics (CFD). To achieve this, the resistance and self-propulsion simulations of the K-supramax vessel equipped with ALS were developed using the Unsteady Reynolds-Averaged Navier-Stokes (URANS) method. As a result, the effective power and the delivered power decreased by up to 17.10% and 14.98% owing to the drag reduction achieved by the ALS. However, the quasi-propulsive efficiency decreased by 2.15% to 2.55% compared with the bare hull condition. This was caused by the injected air being drawn into the propeller plane during operation. Such ingestion was about 1.5 times more pronounced with the propeller in place, and the resulting reduction in the density experienced by the blades demanded a greater power input to sustain the same thrust.