Ship Drag Reduction by Air Bottom Ventilated Cavitation in Calm Water and in Waves

Abstract

The goal herein is ship drag reduction by air bottom cavitation in the moderate range of Froude number Fr (0.4 < Fr < 0.65) in both calm water and in waves. A ship hull with a bottom niche terminating in a cavity locker/seal (suppressing cavity tail oscillations and reducing the air escape from the cavity) was designed using nonlinear ideal fluid theory. The wave impact on the cavity shape and drag reduction was estimated with a novel analytical approach that takes into account the air compressibility in the cavity and air entrainment by the water. The model drag was measured in the Naval Surface Warfare Center linear tow tank at different drafts in calm water and in waves. The baseline configuration was with the niche closed by a flat cover. The attained total drag reduction at 0.45 < Fr < 0.63 was up to 25%, whereas the air supply power was under 4% of the gain in the required propulsion power. The air cavity was stable in waves (up to sea state 5 for a 90 meter ship) and the effectiveness of drag reduction by cavitation in seaway was greater than in calm water due to smaller wave-induced additional drag of the ship with air bottom cavity. Two identical models were built and tested also as a seatrain. However, the percentage drag reduction due to cavity ventilation in the seatrain configuration was less than for a single hull. The need for fine tuning the air supply distribution between the hulls was found.