Bruce foil
Encyclopedia
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. It was invented by Edmond Bruce
in the early 1960s, and first published in the Amateur Yacht Research Society publication in April 1965.
s, high above the waterline, and the sideways resistance, generated by the centerboard or other foil
below the waterline. The resulting torque
causes the hull to heel until the buoyancy of the hull provides sufficient torque to balance the heeling force. 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.
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.
Leeboard
A leeboard is a lifting foil used by a sailboat, much like a centerboard, but located on the leeward side of the boat. The leeward side is used so that the leeboard isn't lifted from the water when the boat heels, or leans under the force of the wind....
, consisting of a foil
Hydrofoil
A hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the...
typically mounted on an outrigger
Outrigger
An outrigger is a part of a boat's rigging which is rigid and extends beyond the side or gunwale of a boat.In an outrigger canoe and in sailboats such as the proa, an outrigger is a thin, long, solid, hull used to stabilise an inherently unstable main hull. The outrigger is positioned rigidly and...
and always set at an angle to provide both lateral and downwards force. It was invented by Edmond Bruce
Edmond Bruce
Edmond Bruce was an American radio pioneer best known for creating the rhombic antenna and Bruce array.Bruce was born in Saint Louis, Missouri, and raised in Cambridge, Massachusetts, Brooklyn, and Washington, D.C...
in the early 1960s, and first published in the Amateur Yacht Research Society publication in April 1965.
Theory
Nearly all methods that increase resistance to sideways movement also cause heeling, the leaning produced by the imbalance of the forces on the sailSail
A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...
s, high above the waterline, and the sideways resistance, generated by the centerboard or other foil
Foil (fluid mechanics)
A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift is substantially larger than the drag...
below the waterline. The resulting torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
causes the hull to heel until the buoyancy of the hull provides sufficient torque to balance the heeling force. 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
Inclined rig
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....
, and the other the Bruce foil.
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 NACANACA
- Organizations :* National Advisory Committee for Aeronautics, the forerunner of the U.S. federal agency NASA* National Association for Campus Activities, an organization for programmers of university and college activities...
00xx series, which relies on the leeward slip of the hull to provide the angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
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
Trimaran
A trimaran is a multihulled boat consisting of a main hull and two smaller outrigger hulls , attached to the main hull with lateral struts...
designs. In this case, the leeward foil works in conjunction with the leeward ama
Ama (sailing)
The term ama is a word in the Polynesian and Micronesian languages to describe the outrigger part of a canoe to provide stability. Today, among the various Polynesian countries, the word ama is often used together with the word vaka or waka or va'a , cognate words in various Polynesian languages...
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.