Talk:Bruce foil

There are several statements in this article that need correction. First the missing reference:

Bruce's work on what became known as the 'Bruce foil' was published in AYRS 51 April 65 and AYRS 74 Oct 70. It was republished in 'Design for Fast Sailing' by Bruce and Morss, ed. Morwood in 1976.

Bruce's original design had a single foil on a tacking dinghy. It works whether to windward or leeward. So the statement that a tacking boat needs to carry two such foils is wrong.

The original foil was designed to be balanced in yaw and indeed was designed so it could be used to steer the boat. So the statement that it produces a yawing force that can impede the steering needs correction.

A major limitation of the original Bruce foil is that it is dynamically unstable. It is only balanced at exactly one angle of heel and any deviation from this angle leads to larger and larger forces that tend to capsize the boat. So the statement that 'In theory, the force from the foil and the sail will always be balanced, so capsize should be impossible' is wrong and the explanation of factors that can unbalance the foil is incomplete.

[ In practice, small boats using inclined foils can maintain heel stability using dynamic controls such as moving body weight in the same way as a normal dinghy. Curved inclined foils also overcome this stability issue. ]

The only mechanism described in the article is for the Bruce foil to generate force 'up and into the centre of pressure of the sails', which is the situation when the foil is to lee of the boat. So that description does not apply to the stated case of proas that always carry the foils to windward. In that configuration the force is generated down and away from the sails.

The statements of the main advantage and disadvantage are contradictory. The main advantage, as stated by Bruce, is indeed the ability to resist heeling and so permit the sails to generate greater drive so that the boat achieves greater speed. The main disadvantage is dynamic control problems such as a foil carried to windward 'popping out' in waves.

Dave Howorth 82.153.140.247 06:42, 2 April 2007 (UTC)


 * I knew I recognized that name--you sent me some AYRS material reguarding hull speed. I'll ping you at your e-mail address so we can discuss this here further.


 * The Bruce foil on either side I have (virtually) seen in practice, it's used on some models of trimaran; a foil at the front of each ama provides additional righting force (in addition to the bouyancy of the ama) and also acts to provide some lift when sailing downwind to prevent diving and reduce wetted area. See http://www.aegeanmarine.com/Concept.htm for information.  I'm not seeing how a single foil on a tacking craft would work, unless you're using a ballasted ama for stability on one tack, and a Bruce foil to provide lift to compensate for a lack of bouyancy on the other tack.  Apparently I need to draw some vector diagrams to get things straigtened out.


 * Right, just took a couple of minutes to percolate through my subconsious. Use a symmetric NACA 00xx foil, for example, mounted at zero angle of attack, and then the slip angle will produce lift to the windward side, and thus a corresponding up or down component as well.  scot 20:22, 2 April 2007 (UTC)


 * As for proas, you're right that the foil on a pacific proa is unstable in that it only works at one angle of heel--additional heeling will reduce the submerged area of the foil, reducing the downforce as heel increases. On an atlantic proa, or some hybrid with the foil mounted to the lee side, increasing angle of heel will submerge more foil and give more lift, which should be a more stable system--this is basically a shunting variant of the trimaran above.  Still gonna dunk you if you catch a sudden gust that heels you faster than it accelerates you though.  See http://proafile.com/view/weblog/comments/a_primer_on_proas/  Now there was one variant floating around the AYRS and/or the Proa File mailing list that involved hinged akas, which increased the foil angle as the heel angle changed.  That one relied on a surface skimming ama to keep the foil in the water; I don't recall how the force was transmitted from foil to the vaka/rig given that the hinged akas provided no rigidity; it may have relied on rigging from ama to masthead.  scot 20:13, 2 April 2007 (UTC)