This paper describes the experimental study on the drag reduction of an inland waterway ship scale model by a system of air cavities. The cavities were generated underneath the flat bottom of the ship model. As the cavity length grows with the velocity squared, a system with a variable number of cavities was suggested in order to ensure a high efficiency of the drag reduction at different velocities. Underwater cameras were used to visualize the cavities during the test. The video recordings provide the data on the contours of the cavities and their dynamics in the bottom plane. It was observed that there is an effect of the flow around the ship model on the cavities. The cavities at the forward side of the bottom are affected, most likely, by the pressure and velocity nonuniformity generated by the bow. This effect is expressed in the extended cavity length and thickness. The local flow characteristics around the ship might significantly influence the cavity parameters. The drag measurements show the efficiency of the drag reduction by the suggested system for a representative ship model in calm water and in head waves. In addition, the stability of the of air cavities system is discussed.@en

One of the most promising viscous drag reduction techniques for ships is so-called air lubrication. Particularly, a stable air layer created on the bottom reduces the wetted area and therefore reduces the resistance of a ship due to the negligibly small viscous drag of air compared to water. The air cavity and air chamber concepts are considered as the most effective and practical ways to form a stable air layer on the bottom capable to reduce the total drag by 10-20% for cargo ships ([1], [2]). These concepts are shown in Figure 1. The air cavity concept is based on injecting air behind a small obstruction that separates the flow which is called a cavitator. The cavitator is extended in the span-wise direction and typically has a rectangular or triangular cross section with a sharp edge. In the developed stage the length of an air cavity is limited to the length of a half the gravity-wave length and it should be restricted by skegs/keels on the sides. The air chamber is created by injecting air into a recess formed in the bottom. Provided the recess has sufficient depth, the free surface is not limited by the wave length and can have a multi-wave profile. The shape of the free-surface at different flow conditions and air injection rates is of interest.@en