Hydrofoils operating at shallow depths ( e.g. keels and rudders) will be affected by the interactions between the lifting surfaces and the free surface. This problem is approached computationally and experimentally by examining the simple case of a surface-piercing hydrofoil operating at steady forward speeds and small angles of attack. It is shown that there are two contributions to the lift and drag on the foil, one due to the vorticity shed into the wake and another due to the port/starboard asymmetry of the radiated wave field.

The mathematical problem is specified as a linearized boundary-value problem to be solved numerically. The two contributions to the lift and drag on the foil are determined independently from far-field momentum integrations. Calculations of the lift and drag on surface-piercing foils with variations in aspect ratio and heel angle are compared to experimental results. Comparisons of the predicted and measured radiated wave fields generated by the foils are also presented. It is found that the efficiency of the foil is reduced by the presence of the free surface and that the use of simple reflection-plane models can lead to significant errors.

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