Nearly all methods that reduce 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.
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.
External link