Objectives This paper explores the effects of air flow rate and sailing angle on the air layer retention and energy efficiency of the bottom groove of a ship, focusing on a large scale model of a bulk carrier.

Methods An air layer drag reduction self-propelled model system and hull cavity scheme are designed, and drag reduction experiments are conducted under open water conditions. The jet drag reduction effect on the model in a positive floating attitude of ship is examined, as well as the impact of a certain trim angle on the speed and shaft power of the model.

Results  The results indicate that, when the main engine speed is constant, air injection can significantly improve the speed of model; after stopping the jet, the air layer within the air cavity of the bottom groove can be maintained for a long time, with better drag reduction efficiency when the ship is in an upright state and the trimming is within 0.25 degrees. When the trimming angle is too large, the gas will overflow from both sides of the model head and the air layer will not effectively cover the bottom of the ship, decreasing the efficiency of drag reduction.

Conclusions Several meaningful conclusions are obtained from the above experiments, providing useful references for the engineering application of air layer drag reduction technology on full-formed ships.