Bruce foil

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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.

1 Implementation
2 Advantages and disadvantages
3 See also
4 References

[edit] Implementation

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 far out from the centerline of the hull, 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, so that the foils will work on either tack. 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.

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 throught 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.

Bruce foils are well suited to shunting proas, since when mounted on the ama (outrigger) they are always carried to windward. This obviates the need to carry two foils, as are required by tacking craft.

[edit] 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. With proper angling of the outrigger, only a downwind Bruce foil in the water and the hull is also lifted slightly so reducing drag. 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-centre placement of the foil, which may impede steering.