Foiler Design

This is essentially Bradfield's wand system, used on EIFO, the Rave, and his new offshore boat. It doesn't have to be all on or off - proportional control is far better. The Rave has bungies that vary the download on the wand and allow the pilot to bias the flaps and change the gain in the system.

When you consider a flapped foil going through waves, the ideal control system would maintain a constant lift on the foil, despite the orbital motion of the wave varying the foil's angle of attack. So you have to consider three different conditions: zero flap deflection at the design angle of attack, positive (down) flap deflection at negative angle of attack, and negative flap deflection at positive angle of attack. The range of flap deflections and angles of attack at which you can maintain the design lift coefficient while still staying within the low-drag bucket of the foil determines the roughest conditions you can efficietntly sail in.

As for takeoff and height keeping, you may want to give the pilot the ability to have the flap in the lowest drag setting as the boat sails in displacement mode, then switch to height control for takeoff.

A flap on the stern foil is important as a trim device. Moving weight aft affects the boat's stability as well as its trim. Being able to control the lift on the aft foil alows you to trim the boat without destabilizing it, and it lets you reduce the lift on the front foil by changing angle of attack so the flap can operate about its neutral deflection instead of being saturated one way or the other. Of course, the stern foil could be tilted instead of having a flap, but trim control is important for performance and preserving control authority.

 

I have been following the thread and only understanding some of what is being discussed.

In simple terms, what are super- and sub- cavitating foils?

 

The rave has a suface trailing type wand whereas the Dyna foils sensor may be below the water surface for half of the time.
I can see that a variable/gradual system of adjustment is best for the Tee foil-flap design. I had only suggested it as a way to keep the design simpler (without hull mounted sensor arms etc). It's just one of those compromises I guess, full speed or production version. Should it be carbon fibre or polypropylene?

Really, the question for now is Tee foiler or Y foiler (Ians) or other?

 

Super and Sub-for SP

Sp- Tom or one of the other guys would be able to give you a more detailed explanation of sub cavitating and supercavitating foils. But a basic answer goes like this: foils tend to have a speed limit(say above 35knots approximately) caused by cavitation. A foil designed to minimise or eliminate cavitation thus allowing higher speeds is a supercavitating foil and is shaped like a wedge pointed end first. A hydrofoil operating at lower speeds is generaly shaped like a wing and is sub cavitating.
It appears, if I understand it correctly, that Miller with his surfboard an Ian with his new Moth have found that a wedge shaped supercavitating foil operates well right at the surface in a canard/sensor application.
Ian and John, do you think that the new concept is far enough along to include in this effort or do you think that since both of you have great experience with a mainfoil on the daggerboard and rudder t-foil that that should be the focus? In terms of other people adapting to foiling which concept do you believe is easiest to handle?
Which concept do you think is least likely to result in an inadvertant crash?
Given Chris Dey's(sp?) proposal on the Aussie Moth site for a single configuration thruout a regatta which system do you think could be adapted most quickly to a viable allround(competitive in lite and heavy air) raceboat?

 

First of all, just because a foil has a wedge-shaped section doesn't make it supercavitating. A supercavitating foil has to be going fast enough that a low pressure cavity forms on the suction side that is filled with water vapor and the speed also has to be great enough that the cavity extends behind the foil so when it collapses it doesn't collapse on the foil (which causes erosion of the surface and other problems). It's highly unlikely that a racing sailing craft will reach those kinds of speed.

A sharp-edged section operating near the surface is more likely to be ventilated with air than supercavitating. The result is much the same in that the suction side is dry. But the flow mechanism id different. Whether the cavity is air or water vapor, the drag of the separated flow is high.

It's not clear to me that if you want a planing surface it couldn't be a subcavitating section as easily as a wedge shaped section. In either case, the foil is basically planing on the surface, and the bottom of the foil could have the same shape. But the subcavitating section would have lower drag prior to its emergence at the surface.

The trailing edge would be thinner and weaker, however, and this is where much of the load would be when planing. So the foil may need to have the trailing edge beefed up some, but I don't understand why it would have to go all the way to a wedge.

 

As for retraction, a slender hull like the Moths currently have is quite efficient over a broad range of speeds. So there could be a broad overlap in wind speeds between efficient displacement sailing and flying. Greg Ketterman told me that it's better to err on the small side with foils and have a takeoff speed on the high side. The work I've done with the cruising foiler have borne this out. It's quite possible to be able to fly but have more drag than if the hull was in the water.

So I think the risk of going out with the wrong configuration is acceptable. There will be days when you clearly don't have enough wind to fly and days when you clearly do. In between are days when the wind comes up or drops, leaving you wishing you'd either gone with the flying foils or left them on shore. But I think the boat can have a wide enough crossover region, where either configuration gives comparable performance, that the simplicity of interchageable foils is worth the risk of making the wrong choice.

If you look at the cruiser foils in the jpg I posted, you can see a couple of different retraction schemes. The bow and stern foils rotate about a transverse axis to retract. The lateral foils have hinges at the ends of the struts that allow the two struts + foil + hull to act as a four-bar-linkage. The lifting element esentially goes vertical as it comes out of the water.

I looked at arranging it so that the lateral foils would form leeboards when retracted to the vertical position. But what I found was this basically shifted the lifting foil inboard and when combined with the dihedral didn't give enough righting moment.

But for the moth, this wouldn't be such a problem and you could go with surface piercing foils similar to Burville's Pivac design, but still have them form vertical elements with most of the surface out of the water for displacement sailing.

 

Tom and Doug, you are both very quickly on the ball here!
You have raised many issues I hope I can clarify:
1) Rich Miller uses a supercavitating section becaues he is always in foiling mode on the sailboard and has found it sheds water more easily than an aerofoil section, it seems to plane a little better without burying under load. It is clearly not used for anything to do with cavitation. It is just an appropriate section shape he has found works well. Also sounds good!

2) I use an aerofoil because I want to run in displacement mode with least resistance. I also have the foil pivoting freely so there is no load on it at all.

Agree entirely with Tom's comments on this, with the exception of the need to strengthen the back edge of the foil. I see no problems here, as there is in practice very little load on the forward surface running canard. Ideally it should just skip on the water surface and much of the time it is completely clear of the water.

3) Doug has asked if "The new concept is far enough developed, or if the bifoiler arrangement using the daggerboard and rudder t-foil should be the focus? In terms of other people adapting to foiling which concept do you believe is easiest to handle?"

While my Moth efforts with the Miller concept are very new, Rich has been doing this for several years, standing on sailboard and going perhaps double the speed of the Moth foiler. I think the answer, is Yes it is too early, but I think it is very promising as it it is far simpler than the bifoiler, with few or no moving parts, cheaper, less resistance in displacement mode, and potentially more stable and easier to handle in strong winds and less likely to crash. It just requires some brave souls to make one and give it a go...or wait for me to get around to it.

4) The Moth class has a current dilemma with foils being used only in stronger breezes and removed in light air. This is destabilizing for the class and results in specialist foils being used for different wind strengths. I happen to agree with Chris, that we should use only one set of foils in all conditions and for all races in a regatta. This should in the end force us to use develop and use smaller, more efficient foils, perhaps with a higher takeoff speed but with less drag in light winds.
Smart configurations will find a way to use the area for lateral resistance in light winds and lifting in stronger breezes. I believe my new Unifoier configuration does this, but time will tell!. This will also result in much faster top end speeds.

5) For anyone contemplating using foils, then the bifoiler arrangement of John and Garth is a very good start. If you are brave or just cost conscious, build one of my Unifoiler designs. A Tee foil centreboard will suffice to get you going in the beginning, although I believe Rich is correct to go to the inverted Y for best performance and control.

 

Name

Sorry, last reply was from Ian Ward.
I seem to have some trouble with my registration.

 

If you're going to keep the foils immersed in all conditions, then there may be one other approach and that is to use a single foil for lift and keep part of the hull in contact with the water for balance.

With a foil lifting forward and the stern riding on the water the pitch-heave coupling is unstable. The boat wants to pivot around the stern because there's a large change in displacement with a comparatively small change in vertical movement of the stern - the heave stiffness of the stern is high.

As the forward foil lifts the bow out of the water, the support from the stern foil decreases, and the boat pitches up, keeping the stern in the water. This increases the angle of attack on the foil, increasing the lift and causing the boat to rise more and rotate more about the stern.

Eventually the foil stalls, or the foil broaches and ventilates. Either way, it's likely to flop back down and then start the whole process over again. Or worse.

But with the right feedback gain to the foil, it should be possible to stabilize the boat with most of the hull out of the water, planing along on the stern.

So the candidate configurations are:

- T foil daggerboard, trimmed and stabilized in pitch and roll by the crew's weight

- T foil daggerboard and T foil rudder, trimmed and stabilized in roll by the crew weight, stabilized in pitch by the foils, and trimmed in pitch (and heave) by the crew (either by weight or by aft foil control)/

- Twin surface piercing foils plus T foil rudder, stable in pitch, roll and heave, but additionally trimmed by the crew weight in roll and possibly aft foil control in pitch.

- Forward planing surface and T foil daggerboard. Stable in pitch and heave. Stabilized and trimmed in roll by the crew weight. Trimmed in heave by crew weight placement - weight aft will make the foil run deeper until the planing surface is lifted off the water.

- T foil daggerboard, planing stern. Actively stabilized in pitch and heave. Trimmed and stabilized in roll by crew

 

During my trails with the main foil and canard arrangement, it regularly ran along under control with the canard on the surface and the stern of the hull in the water. It felt nice and under control with little slapping of waves.

The main problem was that this modewas very slow compared to lifting the whole hull clear of the water.

I see that the 60ft trimarans in France use the technique you mention, but they have enormous power to spare.

DO you think this is really a viable option for smaller craft?

Ian Ward

 

I mainly brought it up to round out the tradeoffs. I don't think the single-foil/tail-sitting configuration is the fastest way to fly. But it might be the fastest when you're not flying. The crew might be able to raise the stern out of the water by moving weight forward, going to the unicycle mode, and get more performance at the expense of having to manually stabilize the boat in pitch.

I think there are several basic questions that have to be answered in designing a foiler:
- What is the total, trimmed drag?
- How is the craft trimmed in heave?
- How is the craft stabilized in heave?
- How is the craft trimmed in roll?
- How is the craft stabilized in roll?
- How is the craft trimmed in pitch?
- How is the craft stabilized in pitch?
- How is the craft steered?

This may seem a bit pedantic, but stability and trim are two different things and have to be explicitly considered. The two are often confused and people often use the term "stability" when they really mean "trim". By trim, I mean the equalization of forces and moments at the desired operating point. By stability I mean the change in forces and moments as the craft and whether or not the craft will return to equilbrium when disturbed from the trimmed condition.

I think we should make up a table showing how each configuration addresses these questions, plus perhaps the non-flying drag when sailing in displacement mode, and see which configuration best meets the desired combination of performance and crew workload.

 

Fly!

Most of you know that the big 60 foot tri's use curved lifting foils placed in the amas; they are curved so they can be retractable in lite air.
I wrote Nigel Irens a few weeks ago to ask why those foils were not being used on his new 75' design for Ellen McArthur . He responded that he considered them to be very unstable in pitch and that the sponsors would not permit it. His preference was to utilize something like the configuration of his Scat design for Dr.Bradfield but that much more testing had to be done first. The 60 footers using these foils used them for "foil assist" theoretically which meant that they were full submerged foils taking a high percentage but not all of the displacement of the boat when the main hull flew. Partial immersion of the ama was relied upon for pitch control not dragging the main hull.
In reality what happened was that since the foils had no altitude control they would continue to lift until the ama was free of the water. Now all of a sudden they were surface piercing foils ready and willing to gulp some air and CRASH a big tri. Which happened on dozens of occaisions-some catastrophic. Geant or Gitana is the only 60 footer with a rudder t-foil but that doesn't stop the ventillation of the main foils in waves or because of excessive altitude.
Back to the small boat: this thing has got to fly-dragging any part of the hull doesn't seem like a reasonable solution. The boat should probably use a mainfoil on the daggerboard and a rudder t-foil because thats what we have the most control and handling information on now.
I think the hull could be designed to be just wide enough that the mainfoil would retract flush with the bottom-this won't be easy to accomplish but done carefully might work well.The daggerboard that holds the main foil would be 60-75%(or more) hollow allowing a smaller chord daggerboard to slide up and down within it for use in light air.
Altitude control would be automatic with a wand though I'm looking at the application of "radar on a chip" for this purpose--but thats another story.
We have to decide what the intent of this boat is; the character of it.Is it to be the fastest, the prettiest, or what?
My sugestion is that it is designed to appeal to a wide wange of sailors that have never foiled before; that it be designed to fly early in the lightest air because flying is fun--in other words that not only will this boat make a seamless transition from displacement sailing to foiling it will make a transformation of the way foilers are veiwed by most people. It should be beautifull ,fast and have wide appeal--maybe it doesn't have to fit an existing development class. I'm torn on this because from a proof of concept test winning a worlds would be pretty cool-- then we could do a mass market version--whatever. But if we use what we know works well and refine it something really terrific could result...

 

Doug,
You have raised many more wide ranging and interesting questions. I think that crew ballasted small craft can get away quite different foil configurations to bigger craft.

We need to develop a good all round solution first and prove it within a competitive racing forum. That is where the Moth class comes in. Following that, I am quite sure many other designers and builders will copy the concept and introduce a range of mass produced pop-out craft to make some money.

I agree that it is important to decide the nature of the craft and its potential sailors before coming up with a final design. But as this can only be done around a successful configuration, we need to concentrate on this part first, which is by no means settled yet.

The ultimate aim for me is a very simple dinghy foiler, super light at about 25Kg all up on the water, not much bigger than a sailboard, easy to car top, which is comfortable, reasonably stable for kids, women and beginners, fast in light displacement conditions, a decent sail area say 8.0 sqm. etc. I have a good idea of the parameters, but no clear picture of the final design yet
...it would be good to have some thoughts on this.

 

Registration

C'mon, Mr Guest why don't you register. It will make your case or views more credible.

 

Registration

John,
Previous entry was from Ian Ward. Sorry for the anonymity. I also prefer to know who is commenting.

I have registered 3 times now and sometimes I am recognised, others not. This time I forgot to sign off in the text.

all the best, Ian