Synergy of Resistance Reduction Effects for a Ship With Bottom Air Cavity

Abstract

Friction on a surface covered by an air cavity is much less than friction in water but there is a resistance penalty caused by the cavity tail oscillations. Nevertheless, there is a method for designing the ship bottom form for suppressing these oscillations. This study describes the design method and calm water towing tank tests for a ship with a bottom ventilated air cavity operating at Froude range 0.45<Fr<0.65, where both Fr and cavitation number influence the cavity shape. At this Fr range, wave resistance significantly contributes to the total ship resistance. Model experiments were conducted in the NSWCCD linear tow tank at three diverse drafts. The attained resistance reduction ratio was up to 25%, which is significantly greater than the calculated water friction resistance of the unwetted area of the air cavity. This is a result of the increased ship elevation over the water level due to cavity buoyancy. This contributes to the resistance reduction by decreasing the side wetted surface area and by reducing the submerged volume; thus, there is a synergy of resistance reduction effects. The power spent on air supply is under 2% of the propulsion power.