The response of a hydrofoil to wave orbital velocity fields

For study purposes, a simplified model of a hydrofoil craft is constructed with the assumption that it has:

• heave only motion;

• no surface proximity effect on the foil;

• no foil broaching.

It is then shown that a fully submerged hydrofoil, mounted at the bottom of rigid struts, can transmit large vertical force fluctuations to the hull, even in an idealized sinusoidal seaway because of the orbital velocity field in the water. But if the foil support struts are hinged, inclined aft and resiliently supported, so that the hydrofoil can swing about the strut's pivot in response to the changes in local water velocity, then the vertical accelerations transmitted to the hull are reduced. The more the strut is inclined to the vertical, the smaller are the accelerations transmitted to the hull. A hinged strut whose equilibrium angle (for 1 g) is 60° to the vertical can reduce the vertical accelerations by an order of magnitude. It also has two other practical advantages. The strut(s) and foil will ride up towards horizontal during the rare but inevitable impacts with large marine objects (such as whales or flotsam) and during groundings. And when they encounter a region of water moving rapidly downward (which can cause a conventional fully submerged hydrofoil to experience a violent hull impact on the water) they move in such a way as to maintain a roughly constant lift force on the hull, so that there is negligible hull motion in heave.