As a ship moves through water it encounters three types of drag: wave drag, pressure drag and frictional drag. Wave resistance is mainly a problem at high speed, and can be minimised with a carefully designed hull. Streamlining can also almost eliminate pressure drag – the backwards pull generated by the pressure difference between the bow and stern as the water through which a ship passes divides and then recombines. The greatest component of drag, and the main problem to ship designers, is frictional drag. This comes from the interaction between the hull and the water around it. Its effect, says Kodama, means that a ship pulls a large body of water along with it as it moves.
Japan is working on the Super-Eco-ship project aims to reduce a ship’s greenhouse gas emissions by a quarter while increasing its cargo capacity by 20 per cent, through a whole series of propulsion, control and design changes. so far Sea trials have shown a net drop in drag of only 3 per cent and reduction of less than 10 per cent in scale models.
Air pockets sandwiched between the boat and the water should make a highly effective lubricant. Tests on models show it is possible to create stable cavities that cut drag by a factor of 5. Yet the project aims to reduce hull contact with water by a whopping 80 per cent, and to sustain these air cavities at all speeds and in all sea conditions.
Experiments currently show microbubbles were the least efficient, saving just a few per cent. The air film was better, and the air cavities performed the best.