Development of ship drag reduction techniques is one of the urgent issues for ship builders to save costs, energy and environment. For the last several decades, microbubble techniques for ship drag reduction have been actively investigated and achieved 5-10 % fuel savings in practical applications. However further improvement is necessary to reduce the energy required to supply air bubbles, because it occupies 3 to 10% of the total energy consumption of a ship by using conventional bubble generators, which depends on the water depth of bubble generation (ship draft). In our research group, a power-saving device for air bubble generation using a hydrofoil was invented. The device could effectively introduce small air bubbles around the ship hull and succeeded in reducing the net power consumption up to 15% in the full-scale tests (Kumagai et al., Ocean Engineering, 2015). Recently, to enhance their ability of bubble generation, a novel hydrofoil, which has an air slot on the suction side, has invented. The negative pressure generated above the hydrofoil effectively drives atmospheric air into water. In this seminar, I' ll present the fundamental concept of the hydrofoil device based on a simple fluid dynamic theory, and show ongoing towing tank experiments which reveals the threshold of air entrainment, bubble size distribution, and air flow rate by the hydrofoil device. Finally, its impact on the costs (saving money) and global warming (saving our Earth) will be discussed.