Bruce foil

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A Bruce foil is a variant of the leeboard, an arrangement to provide a sailboat with resistance to sideways movement so that the sails will produce nearly forward movement under a variety of apparent wind directions. It was invented by Edmond Bruce in the early 1960s.

Nearly all methods that increase resistance to sideways movement also cause heeling, the leaning produced by the leverage of the pressure on the sails above the line of sideways resistance. A Bruce foil eliminates or reduces this by being mounted on an outrigger and angled to direct the force up and into the centre of pressure of the sails. Bruce foils are often mounted in pairs, one on each side. Bruce foils are well suited to proas, since when mounted on the ama (outrigger) they are always carried to windward. By angling the sail and foil so that the forces of each are colinear, the rig will be balanced and no heeling moment will be developed.

The main advantages are that the sails remain upright and keep the full force of the wind available to drive movement without the crew needing to move their weight, and that with only a downwind Bruce foil in the water the hull is also lifted slightly so reducing drag. The main disadvantages are that the sails remain upright and keep the full force of the wind available to increase stresses, and that there is increased drag from the extra resistance needed on the foil because of its vertical component of force (and maybe a slight component of yawing leverage that might interfere with steering).

See also Amateur Yacht Research Society.

[edit] External link

• Amateur Yacht Research Society http://www.ayrs.org/