Stability curve of Dali foils

Discussion in 'Stability' started by Robert Biegler, Jul 19, 2023.

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Robert BieglerSenior Member

I am considering using Dali foils to give my sailing canoe more stability, and I am wondering what the righting moment curve looks like. The image below illustrates my speculations. It shows only the leeward foil. Assume the windward foil is not in the water.

I assume that, at 0 degrees heel, the inner part of the foil is angled so as to produce no lift in either direction, and that the outer part of the foil is the only thing countering leeway. So the resultant force from the foil, the black arrow, is perpendicular to the outer part of the foil.

At 10 degrees heel angle, there is more of the outer part of the foil in the water, reducing leeway, and the inner part produces some lift. The total force from the foil is greater, and closer to vertical. Righting moment has increased.

At 20 degrees heel angle, there is no more of the outer part of the foil in the water than at 10 degrees, and it is closer to horizontal, decreasing angle of attack induced by leeway (assuming no change in trim). However, leeway increases again, increasing the angle of attack. Also, running deeper below the surface should increase the foil's lift a little. Further, let's say that the inner part of the foil is back to producing no lift because of the increased leeway. Then righting moment from the foil would be less than at 10 degrees heel angle.

At 50 degrees heel angle, the lift from the outer part of the foil no longer depends on leeway at all, but only on trim and how much the outer part was toed in to begin with. It might suck the boat down. The inner part of the foil is the only thing resisting leeway, and so trips up the boat.

This is not an issue for IMOCAS because of their deep keels and, in the current generation, because the foils give enough lift that, at speed, enough of the foil is out of the water to allow for reserve stability.

Is my reasoning correct? And seeing that my canoe doesn't have a keel, would I need foils large enough to lift the boat so the tip of the foil never goes underwater at speed? And if don't want foils that large, to what extent could I limit the loss in righting moment by designing a hull that trims bow up as it heels?

I suspect that the Dali foils are not the best configuration for this purpose.

Last edited: Jul 22, 2023
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Alan CattelliotSenior Member

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Robert BieglerSenior Member

I thought the canting keel increases righting moment at moderate angles of heel, and the shape of IMOCA foils deviates from DSS to counteract the loss of lateral resistance from the canting keel. If I installed DSS, I would need a separate lateral plan. I would prefer the whole setup to be simpler than that. In any case, my canoe has an aluminium hull far too thin to be welded, so it is not easily modifiable. If I specifically wanted DSS, I would really need to replace the hull.

That would be technically possible if the hull were wood or composite, but even then, it would interfere with pulling the boat up the ramp, or up a beach.

The idea with the Dali foils was that I could use a single degree of freedom to both adjust angle of attack, and to get the foil out of the water for paddling or rowing, and to fold the foil away when I need the boat to be compact. Something like this:

I also want a frame that attaches to the gunwales and contains most of the forces from sail and foils, so that the hull doesn't need to deal much with forces for which it was not designed. Neither DSS nor a keel meet those requirements.

Last edited: Jul 25, 2023
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Alan CattelliotSenior Member

Yet I understand a little more what you're looking for. In my opinion, the foil isn't exactly the correct answer to the stability issue of your sailing canoe. Too dynamical. Used on Imocas, they are not generating all the righting moment needed to counteract the heeling moment of the sails. Only compensate. Regarding the speed of your canoe, and even if you're a strong man, you will need some awfully large and adjustable foils to get the effect that you want. Generation of sailors have worked on this tedious issue, and have come to efficient solutions, to be able to spare the cargo surface, as well as having a boat that one can sail. Looking at the images you've uploaded on this forum, i would say that you already know some good existing solutions : AMAs for the stability, like polynesian multihull, rotating daggerboard, like on traditionnal deutsch or netherland boats. I understand that you're looking for something more compact.

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revintageSenior Member

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For this type of boat, I myself have used a Bruce Foil with successful results.
JS

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revintageSenior Member

Hej Heinz-Jürgen,
Thats a another story, with the foil on the outrigger you can get the RM to gain the speed that make the foil work as intended. If we double the left one, we get Icaros ;-).

Cheers
Lars

Last edited: Jul 26, 2023
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Robert BieglerSenior Member

I like how you allow adjustment of the angle of attack of the foil. Could you say more about how much difference that makes? And how sensitive is this boat to the foil getting unstuck when going over waves with the foil to weather? The one description of practical experience I found said it worked fine on lakes, but on the North Sea, the foil lost its grip when going to windward in some seaway with the foil to weather. I am currently trying a hinged Bruce foiler, adapting a proposal by Hagedoorn. The hinge is supposed to let the foil drop down quickly when going over a wave, having less roll inertia than the whole boat. I have not yet had enough wind and wave to test that (and I will need to reinforce the mast before I do).

Explanations of my experience so far are at Hinged Bruce foiler - Amateur Yacht Research Society https://www.ayrs.org/phpbb/viewtopic.php?t=3876&p=4428#p4428

The simple foil shape that my limited skills allowed me to build has a lot of drag when to weather, and getting the boat ready to sail takes longer than I like, partly because I have to remove the foil and beam to get the boat to where I can keep it. This configuration also has the two drawbacks that the foil is in the water even when paddling or rowing, and it has a fixed draft. I have an idea for dealing with the second problem, but there would still be extra drag under human power, and if I want to go dinghy cruising in an area with winds as often very light as they are here, I will need efficient propulsion by human power. The Dali foils could easily be lifted out of the water for beaching or rowing, and fold away quicker for storage than what I have now.

For anyone appalled by the area of the foil, that's because I have experienced a small wetted area, high aspect ratio foil losing its grip at low speed, and this one still has a grip even when stalled, as intended. If high speed were the primary aim, I would be looking for a second-hand Trifoiler or reproduce its configuration.

Articulating amas on a single crossbeam would avoid putting twisting loads into the hull, and could hold a leeboard. That is the conservative solution. But I am willing to experiment a bit, for curiosity, and in the hope of finding something better, even if it is only better for some narrowly defined requirements that may not suit anyone else.

I think of the Dali foils I drew as Bruce foils with the beam in the water instead of in the air above. The Birdyfish uses foils with less lever arm than what I drew to fly. I am not convinced that righting moment at small heel angles would be the fatal flaw.

Last edited: Jul 26, 2023
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revintageSenior Member

Hello Robert, now remember I read your thread in the AYRS forum a while ago. I might have misunderstood your image in your second post, but if you intend to sail like Birdyfish or Persico 69F with both V-foils in the water I follow you. Then there will be less heel, depending on how undercompensated they are, but you still need higher boat speed to achieve lift and stabilization.

If you intend to have a foil on only one side you will need an arm to multiply the little lift you get from it at low speed. Bruce or Dali will probably not show a big difference. Both will give more lift the more heel and speed you have. I fully admit I have never tried this concept.

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The inclined foil has its limitations due to its location close to the water surface both when it is to windward or leeward . Therefore, I tried an L-shaped foil with much better results. By being able to rotate the foil around the inclined axis, I was able to optimize the result. Above all, it felt much safer even in waves.
JS

Last edited: Jul 27, 2023
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revintageSenior Member

Hej HJS,
I assume that the vertical foil had enough length to keep the horizontal foil well under the surface even with wind coming in from the outrigger side and high heel angle? Have you analyzed the lift vectors? Sorry, have no possibility to rotate the image, just tilt it with the wind and water horizontal.

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My solution creates a lifting or downward force in the same way as a Bruce foil. The resulting force is directly above the center of total weight. It always sailed completely without heeling.
JS

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revintageSenior Member

So you balance the angle of attack to keep heeling out, no problem to at sudden speed changes? But maybe they are never so sudden ;-) .

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