A Bruce foil is a variant of the leeboard, consisting of a foil typically mounted on an outrigger and always set at an angle to provide both lateral and downwards force. A leeboard is a lifting foil used by a sailboat much like a Centerboard, but located on the Leeward side of the boat A foil is a surface designed to maximize lift (force generated perpendicular to the fluid flow while minimizing drag (force generated in the direction of the fluid An outrigger is a part of a boat's Rigging which is rigid and extends beyond the side or Gunwale of a boat It was invented by Edmond Bruce in the early 1960s, and first published in the Amateur Yacht Research Society publication in April of 1965. Edmond Bruce (September 28 1899 - November 28 1973 was an American radio pioneer best known for creating the Rhombic antenna and Bruce array. The Amateur Yacht Research Society or AYRS is a Nonprofit organization headquartered in the UK and with an international membership
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Theory
Nearly all methods that increase resistance to sideways movement also cause heeling, the leaning produced by the imbalance of the forces on the sails, high above the waterline, and the sideways resistance, generated by the centerboard or other foil below the waterline. Heeling is the lean caused by the wind's force on the Sails of a sailing vessel A sail is any type of surface intended to generate Thrust by being placed in a Wind &mdashin essence a vertically-oriented Wing. centreboard is a retractable Keel which pivots out of a slot in the hull of a Sailboat, known as a centerboard trunk (US or case A foil is a surface designed to maximize lift (force generated perpendicular to the fluid flow while minimizing drag (force generated in the direction of the fluid The resulting torque causes the hull to heel until the buoyancy of the hull provides sufficient torque to balance the heeling force. A torque (τ in Physics, also called a moment (of force is a pseudo- vector that measures the tendency of a force to rotate an object about The limited buoyancy of the hull therefore limits the amount of force that the sail can effectively produce.
One solution to dealing with this limit is to bring the forces generated by the sail and the underwater foil into alignment, canceling as much of the torque as possible and thus reducing the amount of heeling. Two approaches to this have surfaced one being the inclined rig, and the other the Bruce foil. An inclined rig is a method of rigging a Sail to direct the force of the sails in such a way as to reduce Heeling.
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. Windward is the direction from which the wind is blowing at the time in question Angle of attack ( AOA, \alpha Greek letter alpha) is a term used in Aerodynamics to describe the Angle between the 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. Windward is the direction from which the wind is blowing at the time in question
Bruce foils can be mounted in pairs, one on each side, as used in some trimaran designs. A trimaran is a Multihulled Boat consisting of a main hull ( vaka) and two smaller Outrigger hulls ( amas) attached 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[1]. An ama is an Outrigger on a Proa or Trimaran, used to provide stability
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.
Advantages and disadvantages
The main advantages over a typical centerboard or leeboard is that the Bruce foil allows the sails to remain nearly upright and keep the full force of the wind available to drive movement without the crew needing to move their weight. centreboard is a retractable Keel which pivots out of a slot in the hull of a Sailboat, known as a centerboard trunk (US or case The main disadvantage is that the sails remain upright and keep the full force of the wind available, rather than depowering with increasing angle of heel as happens with traditional foils. The Bruce foil also produces increased drag as a result of the vertical component of force, as well as a yawing force due to the off-center placement of the foil, which may require alteration of the steering system to compensate.
See also
References
- A Primer on Proas at The Proa File discusses Bruce foils
- ^ Aegean Marine trimarans use retracting Bruce foils in the amas to provide additional stability. The Amateur Yacht Research Society or AYRS is a Nonprofit organization headquartered in the UK and with an international membership
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