Implementation
A Bruce foil reduces torque by moving the foil far out from the centerline of the hull, and angling it so that a perpendicular line through the center of the underwater portion of the foil will intersect the center of pressure of the sails. Like a traditional centerboard, the Bruce foil is generally a symmetric design, such as the NACA 00xx series, which relies on the leeward slip of the hull to provide the angle of attack needed to generate lift. When mounted on the lee side, the Bruce foil generates an upwards lift in addition to lateral force, and when mounted on the windward side, it generates a downwards force.
Bruce foils can be mounted in pairs, one on each side, as used in some trimaran designs. In this case, the leeward foil works in conjunction with the leeward ama to provide lift, and the windward foil can be retracted to reduce drag.
In theory, the force from the foil and the sail will always be balanced, so capsize should be impossible, but in practice that is not the case. Since the foil's force is relative to the hull's movement through the water, and the sail's force is relative to the wind moving past the sail, sudden changes in wind speed or direction can unbalance the foil and cause heeling. In addition, a foil on the windward side presents an unstable situation, since any lifting of the foil out of the water, by mechanisms such as heeling or wave action, will result in reduced downwards force, potentially leading to capsize. A lee foil provides more stability, as loss of lift will force the foil back into the water.
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