Trifoiler-style foils on IMOCAs?

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Robert Biegler
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Trifoiler-style foils on IMOCAs?

Postby Robert Biegler » Mon Sep 10, 2018 7:44 pm

Is there anything to prevent IMOCAs from using Trifoiler-style foils on either side of the hull, perhaps attached to the outriggers that provide a broad base for the standing rigging? One constraint is that there should be only one degree of freedom to how the foils move, but that condition would be met. Or would such foils be inferior to the Dali foils for any reason?

John Perry
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Joined: Tue Nov 08, 2016 6:39 pm

Re: Trifoiler-style foils on IMOCAs?

Postby John Perry » Tue Nov 06, 2018 2:56 pm

By Trifoiler style foils do you mean fully immersed inverted Tee foils that have flaps linked (as with the Moth main foil) to a 'wand' that senses height above water surface? If that is what you mean, then I am not sure it would be allowed under the current IMOCA class rules (2018).

Rule C.7.1(e) says: Trim tabs as well as deformable surfaces are prohibited for hull appendages, except for fixed keels, which can have a trim tab

Would the flap on a trifoiler foil be a trim tab? - if so it would not be allowed. And the alternative option of making the whole Tee foil rotate to change foil angle of attack would probably fall foul of rule C.7.1(h)

Would a foil control wand be an 'appendage'? The rules allow no more than 5 hull appendages so a trifoiler stye arrangement would use up 4 of those, then you do need a rudder or two and a keel.

The rules require foils to be retractable, an extra complication with Trifoiler style foils.

But the above is based on only a quick look at the rules, I have not studied them in detail.

As for wether Trifoiler style foils would be better than the current crop of shallow Vee foils I would have thoght that they would be more complicated and more prone to damage. If you look at the Utube videos of these boats sailing in heavy weather you can imagine that is a big factor to consider. Have foil control wands ever been used under severe ocean conditions? But possibly they could be more efficient in terms of L/D since the lifting surface could be more deeply immersed. The foils on IMOCA60 and also on trimaran floats seem to be operating so close to the surface that I suspect that much of the lift for much of the time comes from the underside (HP side) of the foil, which limits efficiency.

Robert Biegler
Established member
Posts: 31
Joined: Sun Jan 08, 2017 7:54 pm

Re: Trifoiler-style foils on IMOCAs?

Postby Robert Biegler » Thu Nov 08, 2018 9:26 pm

John Perry wrote:By Trifoiler style foils do you mean fully immersed inverted Tee foils that have flaps linked (as with the Moth main foil)

No. The Trifoiler uses L-foils linked to sensor surfaces well ahead of the foil. See https://www.youtube.com/watch?v=zXSgZCDVWOM

John Perry wrote:The rules require foils to be retractable, an extra complication with Trifoiler style foils.

Could be done by rotating around the same horizontal(ish) axis as used for adjusting angle of attack.

John Perry wrote:Would a foil control wand be an 'appendage'? The rules allow no more than 5 hull appendages so a trifoiler stye arrangement would use up 4 of those, then you do need a rudder or two and a keel.

The control surfaces could be rigidly connected to the foils. I would count that as one appendage. The IMOCA class association may count it differently.

John Perry wrote:As for wether Trifoiler style foils would be better than the current crop of shallow Vee foils I would have thoght that they would be more complicated and more prone to damage.

For Moth-style foils, I don't know why there would be a difference. The lever arm to the farthest point looks pretty similar to the Dali foils in use now. The trifoiler style foils would be able to give a bit by rotating aft, and so may be less vulnerable, unless the sudden pull down is a problem.


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