THE RESPONSE OF A HYDROFOIL TO WAVE ORBITAL VELOCITY-FIELDS

For study purposes, a simplified model of a hydrofoil craft is constru cted with the assumption that it has: heave only motion; no surface pr oximity 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 id ealized sinusoidal seaway because of the orbital velocity field in the water. But if the foil support struts are hinged, inclined aft and re siliently supported, so that the hydrofoil can swing about the strut's pivot in response to the changes in local water velocity, then the ve rtical 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 degrees to the vertical can reduce the vertical acceleration s by an order of magnitude. It also has two other practical advantages . The strut(s) and foil will ride up towards horizontal during the rar e but inevitable impacts with large marine objects (such as whales or flotsam) and during groundings. And when they encounter a region of wa ter moving rapidly downward (which can cause a conventional fully subm erged 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 th e hull, so that there is negligible hull motion in heave. Copyright (C ) 1996 Elsevier Science Ltd