Numerical study of shallow-draft air-cavity hull with two-step bottom in deep and shallow water

Air-cavity boats utilize air injection to the bottom hull surface to reduce water drag. While being attractive due to potentially large energy savings, this technology has not yet found broad applications due to challenges in confidently predicting air-cavity flows and a lack of standard development practices for air-cavity vessels. In this work, computational fluid dynamics modeling is undertaken to demonstrate the air-cavity system implementation on a simple shallow-draft hull. After conducting verification and validation study with data available for a displacement barge-type air-cavity boat, its hull is numerically modified with intention to operate at higher speeds up to the planing regime. The modifications include replacement of a sloping beach in the bottom recess with additional step and reduction of skeg volume. Computational simulations are carried out for the modified hull at several speeds and center of gravity positions. Numerically obtained resistance, trim, heave, and air-cavity shapes are reported and discussed. A favorable loading condition is identified. In addition, simulations were conducted for shallow-water scenarios. In steady-state regimes, significant performance degradation is found near the critical speed, while performance gains are demonstrated in the supercritical regime. In one case of extremely shallow water, the hull exhibited vertical-plane instability resulting in cyclic motions.