C Class Banana Foil

Here is one of the best views I've seen to date of a "banana foil" used on C-class cats and ORMA 60 tri's so the foil doesn't violate the beam restriction in the rule and is still fully retractable while producing vertical lift when in place.
There's a story behind this that will appear on SA this coming week.......
C-Class%20awsome%202.JPG
Address:http://www.sailinganarchy.com/fringe/2006/images/C-Class awsome 2.JPG Changed:12:52 AM on Friday, May 26, 2006

 

Banana foil on C Cat?

Hi Doug, please explain the advantages of using a banana foil on a cat, and is there any comparative peformance data from 2 identical cats, one with the foils, and the other with standard daggerboards?
I understand the role on tris and that the lift of the lee float banana foil counteracts a considerable amount of heeling force acting on that float.
Somehow with a cat it seems different as it lacks the balancing contribution of a central hull.
Does the cat potentially fly both hulls clear of the water? :?:

 

Cat

Frosh, I've never used such a foil on a cat but I've read Steve Clarks explanation and it is basically the same rationalization as for the ORMA tri: it reduces the wetted surface of the hull by partially lifting it.If I remember correctly, Tom Speer says that for this foil to be effective it must lift the boat far enough clear that the wetted surface of the hull is reduced by four times the area of one side of the foil(the vertical lifting portion of the foil).
On this particular boat it appears to me that there is also a foil on the rudder but only projecting inboard-probably so as not to exceed the beam rule. I've written to a guy who crewed on this boat for an answer on that.
On the Orma's some of the boats have used rudder t-foils and others haven't. On both boats there is a potential problem that has led to serious crashes on the tris: these foils do not use an altitude control system so the faster the boat goes the more they lift;in certain conditions they can lift the ama clear of the water and then they become a surface piercing foil subject to ventilation . If the foil ventlates the ama can crash back in and a pitchpole can result. I wouldn't use this type of foil without a rudder t-foil since it can provide an extra margin of pitch control.
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Heres the link to Tom Speers comments on "foil assist"; see post #2:
Foil Assist
Address:http://www.boatdesign.net/forums/showthread.php?t=9732&highlight=Foil Assist
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Heres another link where Tom says"..at least twice the planform area.." vs " roughly four times..." in the previous quote-see post #105:
Little Americas Cup, Toronto (not Perth anymore), September 2007 - Sailing Anarchy Forums
Address:http://www.sailinganarchy.com/forums/index.php?showtopic=27245&st=100

 

It seems to me that fitting a banana foil to a cat is at this stage experimental, as no obvious (to me) physics, explains what might be achieved as a peformance enhancement.
With a tri, the lift of the foil exerts leverage counteracting the rig's heeling force, but I don't see this working with a cat. In the photo posted by Doug in his link, the C cat is sailing in a fairly good breeze and is heeling to an extreme angle.
The other possibility is lifting of the leeward hull sufficiently to reduce wetted hull surface to reduce overall drag forces on that hull. As Tspeer has previously posted, the reduction in wetted surface of the hull needs to be in excess of four times the area of one surface of the immersed banana foil to be a net benefit.
As the windward hull is likely to be flying in the C cat, and if the leeward hull is significantly lifted by a foil acting at one point, then almost all longitudinal stability will be lost, and a pitchpole could result.
The answer might be a large T-foil on the rudder blade, but due to the power and high CE of the 300 sq. ft. wing sail, I think that instability will still be a key issue, and at the speed they travel, a serious crash will probably destroy the rigid wing sail, which is relatively delicate.
No point going a little quicker than the opposition for a short time and then risk going into self-destruction mode before reaching the finish line.

 

C

Frosh, the Banana boards definitely work on the C Class cat according to Steve Clark; if I can find his analysis from the last Little America's Cup I'll post it. The reduction in wetted surface is real and exceeds the level mentioned by Mr. Speer in the right conditions. But like everything in sailboat design there is a price to be paid- in this case in very light air-and maybe in very heavy air.

 

The phyics goes something like this:

Lift + displacement = weight
Drag = skin friction + wave drag + induced drag

Skin friction = Cf * Swet * 1/2 * rho * V^2
Form drag = Kf * Cf * Swet * 1/2 * rho * V^2
Wave drag = Cw * Swet * 1/2 * rho * V^2
Cw ~ V^4 + interference effects, so Wave drag ~V^6 on average and ~V^2 at high speed.
Swet ~ displacement / Lwl, so the first three drag sources can be reduced by shifting the weight to lift from the foils, reducing the displacement.

Induced drag = Lift^2 / (pi * span^2 * 1/2 * rho * V^2)

All the drag components associated with supporting the craft with buoyancy increase rapidly with speed. But the drag associated with supporting the craft using lift from the foils decreases with speed. So at high speed, it's far preferable to fly than to float.

 

Flying a C class cat?

Tom, you have covered the maths of lifting foils and drag components which are reduced sufficiently to allow an overall speed increase.
However this is not what my question is about. The Int. Moth shows us the practical application of the above-mentioned maths, and most of the handling issues have been mastered by the best sailors within Moth ranks.
My real questions are (a) Is the aim of the banana foil on a high performance cat to fly the leeward hull, or both hulls almost clear or fully above the water surface?
(b) If the answer to the above question is not both hulls, then what stability problems are introduced by lifting the leeward hull almost, or fully clear of the water surface?
My gut feeling is that on C class cats longitudinal stability would reduce significantly as the leeward hull starts to lift clear as then neither hull is in the water. Also ventilation of the banana foil would possibly occur threatening lateral plane stability as well.
How do these issues impact then on the theoretical drag reductions in a real on the water racing situation?

 

The pictures tell the story!

Hi, I have just skimmed over the SA site and saw a spectacular video of the ORMA 60' Banque Populaire pitchpole on the 20th. May in fairly rough conditions. This tri is also fitted with banana foils on the floats, but unlike a cat it has a more bouyant centre hull which should have a significant longitudinal stabilizing influence (unless it is flying). The video in the link below is fairly small and it appears that the main hull is in the water (but it might not be). This is starting to look bad for the C class cat fitted with banana foils as I am fairly sure that instability of the cat hulls when being foil assisted is a much bigger problem than on a ORMA 60 tri.
http://www.multicup60.eu/videos/
Click on top right thumbnail.
The photos on this site are also excellent showing the hull bows 20' under and also completely inverted.

 

I suppose you'd have to ask the designers what their intent was!

I suppose the question then becomes, just how much of a percentage of the weight should the foil assume? And that depends on how far you want to push things. Since the bottom of a catamaran hull is fairly flat, there's a lot of wetted surface for the buoyancy provided. So if you leave some of the hull in contact with the water to provide stabililty - think of it as a hull-stabilized foiler instead of the other way around - then there's a significant amount of drag associated with it.

You're going to have very little trim capability or stabillity about an axis connecting the banana and stern foil. I suppose having two stern foils helps, but there's not much change in moment due to, say, the windward stern foil as the boat rolls about the banana-leeward stern foil axis.

To avoid unstable pitch-heave coupling, the forward foil has to have a greater heave-stiffness than the aft foil(s) and the aft foil(s) have to have a greater pitch stiffness than the forward foil. On a Moth, this is provided by the wand feedback to the flap. I don't see any such mechanism for the banana foil. An angled daggerboard changes its vertical lifting area with heave, and thereby gets some heave stiffness. But the vertical portion of a banana doesn't.

So I think it would be very dangerous to try to balance a C-class on the banana foil and stern foils without having any hull in the water for stability.

 

I wonder if a better design might be a straight lifting foil set at the diagonal in each catamaran hull, as seen on a previous generation of ORMA 60 tris, set further forward than the usual daggerboard position to gain pitch stability?
Could lead to serious weather helm. Would probably also need larger than usual and beefed up rudder blades with lifting foils also.
Any opinions on whether this might allow full flying of a leeward hull on a C class cat, and sufficient stability to avoid a catastrophic crash.

 

C

Frosh-more info:
Sailing Anarchy Home Page
Address:http://www.sailinganarchy.com/ Changed:11:59 PM on Monday, May 29, 2006
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Got an e-mail from my friend who's crew on this boat; says it just takes getting used to but that the foils work really well. Only use 2 at a time. The angle of incidence of the rear(rudder) foil is controllable and may have been what caused this incident: too much downforce on the rear foil allowing the forward foil to get too positive and voila! an unplanned jump-so very cool!
PS- they have excellent pitch stability.....
Frosh, the banana foil is ,in essence a diagonal foil that retracts; they couldn't use a diagonal foil on this boat because when it was retracted it would exceed the beam rule; same reason for the "L" shaped rudder foil.The banana foil allows lift to be developed and also allows the foil to be retracted w/o exceeding the rule limits.

 

Banana Board Dimensions

Here's a thread where Steve Clark provides dimensions for the Patient Lady banana boards...


Curved Banana Boards-Little America's Cup - Boat Design Forums
Address:http://www.boatdesign.net/forums/showthread.php?t=5027

 

Banana Boards C Class

Here is another shot of the Patient Lady board from the extraordinary "Foiling C Class " thread on Sailing Anarchy-see post 439 today:
Foiling C Class - Sailing Anarchy Forums
Address:http://www.sailinganarchy.com/forums/index.php?showtopic=59168&st=400&