Bruce foil

The Bruce foil is a type of leeboard variant designed to generate both lateral and vertical force when set at an angle. Invented by Edmond Bruce in the early 1960s, it was first published in the Amateur Yacht Research Society journal in April 1965. The foil is typically mounted on an outrigger and operates similarly to a traditional centerboard but with distinct advantages. The primary function of the Bruce foil is to reduce torque caused by the imbalance between the upward force of sails and the downward resistance from underwater foils. By aligning the forces generated by the sail and the foil, it minimizes heeling, allowing the hull to remain upright without requiring crew movement to depower the sails. Implementation involves positioning the foil far from the hull's centerline and angling it so that its force counteracts the sail's torque. Symmetric designs, such as those based on the NACA 00xx series, rely on leeward slip for lift generation. When mounted on the lee side, the foil produces upward lift, while on the windward side, it generates downward force. In some trimaran designs, foils are used in pairs, with the leeward foil working alongside the leeward ama and the windward foil retracting to reduce drag. While the Bruce foil allows sails to remain upright and fully powered, it has drawbacks. These include increased drag due to its vertical force component, a tendency to cause yawing because of its off-center placement, and potential instability if the windward foil lifts out of the ...