Significant frictional drag reduction of ships and boats can be achieved by forming extended air cavities underneath hulls. However, the air cavity benefit may degrade or disappear in off-design loadings and speed regimes, as well as in rough seas. To make this technology more viable, means for controlling the air-cavity properties are needed. One of promising compact devices that can alter the air-cavity shape and size is a hydrofoil. In this study, a hydrofoil-equipped model-scale boat with a bottom recess for holding air was tested under adverse loading conditions in a water channel. States with significant bow-up and bow-down trims, as well as a few heeled conditions, were produced by appropriately ballasting the hull. At these states, the air-cavity dimensions were substantially smaller than the hull recess. A hydrofoil mounted on side struts under the boat was shifted along the hull, and its effect on the air cavity length was recorded. The hydrofoil positions, corresponding to the longest cavities, were determined. Variations of the cavity length at different hydrofoil locations, sample photographs of the cavity boundaries, and investigated experimental conditions are reported in the paper. In addition, computational fluid dynamics simulations were initiated with the purpose to facilitate design of actuators for air-cavity flow control. A reasonably good agreement was found between experimental observations and numerical results. The present findings can help guide developments of compact hydrodynamic actuators for manipulating properties of air cavities under ship hulls.

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