Numerical Study on the Influence of Air Bubbles around the Ship Hull on the Propulsive Efficiency

Injecting air at the ship bottom to create a mixed layer of seawater and air bubbles between the hull surface and seawater is well known as an efficient energy saving technique to reduce the frictional drag of ships. Various studies have been conducted to promote the efficiency of frictional drag reduction by bubbles. In contrast to the extensive studies on frictional drag reduction involving bubbles, very little attention has been given to the influence of bubbles on the self-propulsion factor. However, the self-propulsion factor is one of the major ship performance parameters. The purpose of this study was to investigate the influence mechanism between a hull covered by air bubbles and propeller open water characteristics. A numerical simulation approach that can be applied to a self-propelled ship with air bubbles around the hull was used for this purpose. It was found that air bubbles around the hull could increase the inflow velocity to the propeller. It was also found that bubbles around the stern could affect the viscous pressure resistance and that the effect could be strongly affected by the propeller. Furthermore, the results showed that air bubbles could reduce the hull resistance and increase the inflow velocity to the propeller, which in turn could reduce the thrust loading coefficient and improve the propeller efficiency. In conclusion, the results indicated that air bubbles around the hull could influence the self-propulsion factors (the wake fraction and thrust reduction fraction) and propeller operating conditions. This suggests that these factors should be considered when improving the energy efficiency of air lubrication ships.