Outward facing J/foils on a cat

I've seen some threads devoted to this question, but I'd like to raise it again. What is the problem with outward facing foils on a cat? In particular, in the case of an A-class cat, for which both foils are generally in the water, what would be the difference between boats set up with inward and outward facing foils?

 

Just as a wild guess id say an established class like the A class would not allow their maximum beam measurements to be exceeded. An outward facing foil tip would have the same affect as a greater beam on a conventional cat.
Ill await the actual hydrodynamic arguments with interest.

 

If you have outward facing Uptip foils, you loose the whole point of the things. Uptip foils are designed to stabilize flying height. If you cant them the other way, they destabilize flying height and any excursion from the desired height would tend to get worse.

On an inward uptip foil, if the hull flies too high, the boat's leeway will increase because the surface area of the vertical portion is less, hence the side force/sq foot is greater, hence the AoA must be greater, therefore more leeway. When the leeway increases, the AoA of the uptip area gets reduced, reducing lift and returning the boat to the proper height.

 

I understand this, and it makes sense if only the leeward foil is deployed. But what if the foils cannot be retracted and the windward foil is also immersed? In this case are both foils actively working the opposite to what you would want in terms of righting moment? Ignoring altitude control (for the moment) wouldn't outward facing J foils be more effective in terms of righting moment?

 

Sure, but it doesn't matter if you are pitchpoling. I think they would have to be aft on a catamaran running on 4 foils.

Unless you can control the angle of attack on a lifting foil, everything else is pointless (or at least impossibly complicated). That means dealing with pitch and heave stability control first. Once you can control AoA (meaning it is steady in time), you can design something to produce the forces you want and evaluate the drag of the design. Since pitch and heave stability is essential 100% of the time, you want to look for very low-drag ways to accomplish this.

I also wonder how you launch and land if the foils have an effective RM greater than the hulls. Wouldn't the max RM requirement be expected to occur near takeoff?

 

Are you talking about uptip foils(invented by TNZ for 34) or "L" foils or "J" foils?
According to Greg Ketterman using a "J" foil(his term for his foils as best I remember-) with the tip pointing out is draggy(see following post) compared to the same foil with the tip pointed inwards-mainly because when pointed outwards there is high and low pressure on the same side of the lee foil. He backed up his theory by testing the "J" foils on two Hobie Trifoiler's and the boat with the foils pointing inboard was faster.
UptiP foils have intrinsic altitude control that varies by design. The more "up" the tip the greater the intrinsic heave stability. The lower(and more like an "L") the less drag there is and also less heave stability there is. One of the classic comparisons of the two types of uptip foil occurred in the last Little America's Cup between Groupama(more "Up" tip and heave stability) and Hydros(less "Up" tip, less heave stability, more speed) but Hydros reduced heave stability required a tremendous increase in crew workload which proved their undoing against Groupama's more heave stable uptip foils.
Uptip foils need to be used one at a time because of the way they work with leeway coupling. If the windward foil was down it would be in the same situation as Ketterman's outward pointing lee foil-high and low pressure on the same side of the foil. If two main foils are down and both lifting, then RM is drastically reduced. If a wand system is used on the main foils then the windward foil could go to zero lift and then create downforce increasing RM.
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See post #49 here for an explanation of how uptip foils work by Tom Speer: http://www.boatdesign.net/forums/hy...r-design-foil-assist-full-flying-40894-4.html
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This is important from the standpoint of using the right name for the foils invented by TNZ in 34. There are all kinds of names being used by many different people for these foils. The newest I've heard are "Tick" foils! But the inventors named the foils a long time ago:
Link to "Foils...." Part 1 and Part 2: http://www.cupinfo.com/en/featuresindex.php

Pete Melvin quote from the article,Part 1:

When we were working on the rule, we knew you wanted to get as much lift as possible when you were going fast downwind,” Melvin says. "For instance, in the 2010 America’s Cup, sailed on giant multihulls, the maximum amount of lift we thought we could get was about 50% of the weight of the boat. At that time, we were still relying on the hull to provide pitch control, so what’s come out of this is the boats all now have elevators (the horizontal foils on the rudders).

“At Team New Zealand, we developed a new type of foil that allows you to keep your height above the water more or less steady. No one had been able to do that before, at least not on a course-racing boat that was not going downwind. We developed that mostly on our SL33 test boats -- they came with the stock constant curvature “C” foils and with those kinds of foils, you can generate 50% boat weight lift before they get unstable. But we noticed that when we could get one boat up fully foiling for a few seconds it would really accelerate away from the other boat – and that got the wheels turning. How, with such a huge potential benefit, can we achieve stable flight downwind? So our design team came up with the “up-tip” type of boards. We refined those on the 33s and our 72 is designed to do that and fortunately it worked right of the box.”
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Right to Left-1)Hobie Trifoiler "J" foil, 2) & 3)-Morrelli and Melvin illustration of an UptiP foil, 4)-DL illustration, 5) UptiP foil illustration by Dario Valenza:

 

Ketterman

At one time I thought I had discovered the holy grail with outward pointing foils and I received the following from Greg Ketterman-this is the whole post:
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This is an e-mail I got from Greg Ketterman in response to the question:"Is there any special reason the trifoiler foils point inward?". It occurred to me that the Trifoiler case was similar to the reverse curve foils and since Greg has been very helpful to me I asked him. It should be noted that Greg actually tried outboard foils and sent pictures of them. I have his permission to post this here. Greg is also a member of the boatdesign.net community. I urge anyone with an interest in hydrofoil design and development to read the enclosed paper-page 14 and Figure 14a&b on page 32 are directly related to this discussion. For those interested in model hydrofoils there are details of his model throughout this paper including sail area, foil area and improvements warranted by testing. I'm grateful to Greg for taking the time to put this together and for his help:

(emphasis by DL)

Hi Doug,

Forgive me if I am too condescending but I am amazed at how many people do not get this. ***** argues that it would provide more righting moment. Imagine how ineffective a curved daggerboard would be if it curved out. I would have expected you to understand.
The boat in the attached photo did not perform well at all.

My senior project report explains it here; https://picasaweb.google.com/112349...ReportTriFoiler#slideshow/5075757835053693858

A simpler way to look at it is the aspect ratio of the horizontal foil adds to the aspect ratio of the vertical foil when the foil points in. Aspect ratio is everything with hydrofoil sailboats.

All the best;
Greg Ketterman

VP Engineering Hobie Cat Co.
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Illustrations/picture: Left to Right-1)&2) from Kettermans paper, 3) Hobie Trifoiler with outward pointing foils:

 

Good point, Doug, but I am deeply suspicious of this explanation.

If the foils consist of a vertical piece and a slanted piece and have ...

1. Same side force.
2. Same lift force.
3. Same vertical center of lateral resistance.

... then the primary issue is the effective span of the foils. So lets say that the lift is twice the lateral force. Draw a line perp to the total force vector and look at the projected span of the foils. It will be less for the outboard design than the inboard design. If the total length of the foil is the same, the lesser projected span puts the outboard unit at a disadvantage. The way the pressure washes out along the span can be designed as one pleases. (Building it is another matter). So it isn't the fact that the pressure reverses (it is the rate of change spanwise, not it's absolute value that matters), it is that the effective span of the outboard foil is less if the foils are mirror images. So an outboard foil would need a different shape to get the same span. It would be trading a bit of extra overall length for a bit less bending moment at the hull.

<<edit>> had to run mom around town for two hours. Crossposted with Doug's second post>>.

 

Thanks for the explanations. In summary, would it be correct to say that if we ignore heave stability control, the outward facing foils should be better from a RM perspective, but may provide less lift and add drag?

In terms of heave stability control, if there was a flap at the base of the vertical section of an UpTip foil that deflected in response to Leeway (Leeward foil only), could this be tuned to provide elevation control?

 

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As you mentioned(I think) earlier, some in the A Class are using "4 in the water" foils where both main foils are lifting OR where the windward main foil is moved to a no lift position upwind(zero degrees angle of attack).
There is a big difference in design: if both are designed to lift all the time, RM is reduced but foil area(of each foil) can be smaller. If designed for the lee foil to lift around 80% or so of the weight , then the foils have to be larger in area(span is limited in the A Class rule).
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PS- See the render and description of Steve Clarks new C Class cat with very unusual foils in the 2015 Little America's Cup thread in "Multihulls".


Quant 23 monofoiler:

 

I may not have explained the flap position properly:

 

No, unfortunately. There are a few problems here.

The flap is nearly perpendicular to the surface of the foil you are trying to influence. Flaps work by changing the circulation field about the foil. The perturbation of the circulation caused by the flap's movement does't care about the geometry of the rest of the foil and doesn't follow the bend around. So the outboard part of the foils that is around the bend doesn't see any circulation change because it is about 90 degrees out of plane.

The second problem is that the perturbation field doesn't extend spanwise very far. It decays rapidly in the spanwise direction. The only part of the foil that will experience a significant change of pressures is right where the flap is.

The third problem is that you fundamentally can't do what you are trying to do with a passive device. You could attach the flap to a wand and extract power from the linkage, but you can't do this passively by letting something relax. It requires an energy source. Not a very big one, but something.

Instead, lets say you had two flaps. One on the vertical strut near the desired waterline, and one at the outboard edge. You could connect them so that leeway would force the lee side outboard flap down. As the boat rose, the flap on the strut would loose effectiveness and the lifting flap would decrease lift. On the windward side, the strut flap would be clear of the water, and the outboard flap deenergized. This is just a different type of wand. I think you would need some toe-in in addition to leeway to make it work, so that is where the drag penalty comes from.

 

Thanks again for the detailed explanation. I like the two flap idea. It seems to me that there should be other ways to exploit the leeway other than the way it is used for the UpTip foils.

While I'm proposing crazy ideas, something else I've been wondering. Could fences on the lifting sections of the foil be used to reduce the lift with increasing leeway?

By the way, I didn't really understand your earlier question: "I also wonder how you launch and land if the foils have an effective RM greater than the hulls. Wouldn't the max RM requirement be expected to occur near takeoff?"

 

https://www.youtube.com/watch?v=ghMO7g4M8vI

See from 13:30.

 

Not a catamaran but I tried this out on a trimaran foiler a couple of decades or more ago; worked fine but I listened to criticism from the Kettermans (meaning the leeward foil would be lifting up, yes, but also lifting to leeward as well) and so changed to a more conventional infacing setup. The foils had varying AoA twisting from 3 degrees at exit from float to zero at the tips. There was also a main hull dagger.