Foiler Design

Windward on Foils

Regfade,
What you suggest is exactly how the foiler moths have developed as the technique for sailing effectively to windward. (Probably developed by Rohan Veal but I'm not certain). The Boat is equipped with T foils on daggerboard and rudder. But heeling the foilbourne boat to windward the foils are put into a sideslip where the foil lift is balanced by the sink of the sideslip in the windward direction. Not only do they foil but they point too, even 'crawling' to windward.
Find pictures of foiler moths going to windward and you will see what I mean.
Because the side forces (lift forces) on the daggerboard and rudder are small the tendency to ventilate is not great. But, from the video clips I have seen ventilation of the foils seems to occur just before the foil breaks surface (and the boat goes down the mine!).
Interestingly it is technique rather than design that has solved the windward problem for foiler Moths.
Tom Edom

 

Rohan Veal

It was Rohan Veal and it's a really important contribution! In a way it goes against the grain in terms of what many people thought: some thought early on that the bifoiled monofoiler should be flown as low as possible to keep the CE of the rig low to reduce heeling moment.
Instead Rohan with John Ilett's foils showed that by flying high and heeling the boat to weather upwind vmg was improved. Besides better upwind ability and more RM flying high keeps the boat completely clear of waves...
I'm sure other advances in technique are to come as others get more and more experience..

 

History & Righting moment

I would like to provide some perspective to the current subject of foiling history, before we get a little off track!

Firstly, it appears that most sailing hydrofoils of the past have used three or four point foil systems of some kind to support the boat.
Irrespective of the hull format being mono, cat or tri, the foil arrangements have generally provided significant lateral stability. In effect they have been "multihull" configurations. This includes Bakers monohull foiler and the Burville/Pivac Moth trifoil arrangement.

Centreline foiled human powered craft have been around for many years, the most recent being the Foilkayak. I presume that some had stability problems and perhaps it therefore looked like they could not be balanced laterally with a sail attached. Certainly there do not seem to have been any developments from this arrangement toward sail powered craft. I do believe however that there are many good ideas that can be borrowed from these developments for sailing craft, especially height sensing canards.

Centreline foiled "Airchair" foils have been used for many years for water skiing and more recently are being applied to surfboards and sailboards, but so far not to dinghies. My own thought is that control of pitch and height for this arrangement requires at least 3 degrees of freedom and that this is almost impossible to do sitting down, hence this method can only be used when the rider is standing up to provide accurate balance and control.

Bradfield built a sailboard centreline foil arrangement which was sold in the late 70's I think, for Windsurfers. While it got the sailboards up and foiling, it was difficult to control and not that fast.

I suspect that no one tried or considered building a dinghy with centreline foils alone, simply because it looked impractical in terms of stability, control and "obviously would not work"!!.

Rich Miller, has built and patented a centreline foiled sailboard using a forward canard for accurate surface tracking. He also used the windward heel of the hull and foil to provide righting moment to reduce ankle loads and in addition solved the problem of ventilation of the vertical strut by unloading it by tilting the board/foil. In this regard, I think he was the first to use the windward heel of a centreline foil to provide stability.

John Perry has developed a small trimaran hull with centreline foils and sailboard rig. I am unsure about the details of his arrangement as there are no photos, but he certainly appears to have got up on occasions. Whilst the hull is not a dinghy as such, the foil arrangement does appear to be using centreline foils.

I am aware that Andy Patterson in the UK has also trialled foils on a Moth, but I think he also had wing mounted foils for stability.

My own claim is to be the first to design build and sail a "dinghy" with centreline foils. This "bifoiler" arrangement is the same as that currently being used by John Ilet and others on Moths and 14'skiffs. The history and ideas behind this development are documented in the latest Seahorse magazine articles.

John Illet should certainly be accredited as the first person to master the development, design and construction of a robust, properly functioning centreline foiling dinghy. His efforts should not be underestimated as many others are still trying to emulate his success and to date his design has not been bettered, despite several people trying, including myself.

There seem to be some contrary claims regarding the David Lugg/Alan Smith designed 14'skiff foil development. As John Illet and David both reside in Perth, perhaps they can provide more detail of the design history.

Rohan Veal has been extremely active with sailing, tuning and developing handling techniques using the dinghy foiler design made possible by John Illet.

Stability from windard heel:
Centreline foils offer several advantages, including simplicity, retaining dinghy style handling qualities and increased stability from heeling to windward.

Heeling the boat to windward to gain stability is a simple functional advantage of the centreline foil arrangement. This was a clear design advantage right from the start of my development of the bifoiler Moth and is clearly documented in the latest two articles in Seahorse magazine. It has also been used by Miller. I do not think it is right to say that Rohan has invented or even discovered this, but he certainly uses it to good effect.

One item for discussion raised by Doug Lord is that Rohan seems to be the first person to sail very high and heeled at the same time. This certainly appears to be true on occasions, but it would be good to know if it is a real benefit. My own thought is that ideally the hull should be just clear of the water at all times and no more. I do not see excessive ride height being any advantage, as the benefits of windward heel will apply, irrespective of the ride height.

One reason for riding high, is that it is very difficult to control both pitch and height accurately, and so flying high is more a consequence of innacurate control. It also looks spectacular in the action photos! Once you lift high, there is less board in the water, so it can ventilate more easily, windward heel also reduces this ventilation effect, so the two effects go hand in hand, hence Rohan's technique. Windward heel also allows you to utilise the leeway to give more lift and get you up on foils earlier.

I am therefore questioning whether there is a real advantage to fly high and heeled to windward. I think it should be enough to sail heeled to windward and just above the waves. If the height and pitch control were accurate enough, this would also mean shorter boards could be used with consequent less resistance less sensitivity to windward heel angle. It is the method of accurately controlling height and pitch which I am concentrating on currently, as better control offers many advantages.

I would be interested in any comments on this.

 

Heeled Foiling

Ian , sure would like to get Rohans input on this.
It seems to me it's the same argument for having a long keel vs a shorter one except upside down. In the Moth case it seems that you would want a long distance between the hydrofoil and boat so as to maximize the windward move of the CG with heel. If thats true then to sail with the least resistance and max RM would mean sailing pretty high-at least upwind; what do you think?

 

Hello Doug,
In principle you are quite correct!
What I notice in practice is that it does not take much windward heel to get a big effect. You are then fighting another problem altogether...the boat wants to tip in to windward on top of you as you go past the point of "balance". Then you need to trim sails and body to ensure you stay upright. Under these conditions you are most efficient and very fast, but it is not all that easy to stay there, I guess it is a sort of metastable equlibrium.

In fact, for me it feels like you are balancing on a knife edge, so you then feel that you then want far less depth of foil so that the pendulum arm becomes shorter. Narrower foils would then be beneficial, as well as some dihedral on the main lifting foil.

I would certainly also be interested to hear from Rohan and John about this.

 

You're not likely to achieve your end of reducing the lift on the strut through section design alone. Nearly all section shapes produce the same amount of lift per degree of angle of attack. Just compare the lift curve slopes in Theory of Wing Sections or http://naca.larc.nasa.gov/reports/1945/naca-report-824/naca-report-824.pdf. The lift curve gets biased up or down, depending on the camber, but the slope is the same.

You might reduce the lift somewhat by using a blunt trailing edge. But some blunt trailing edges can still be surprisingly effective - at least in theory. Take the P20012 section, for example (http://www.basiliscus.com/ProaSections/ProaIndex.html)


These XFOIL results say the small separation at the trailing edge still makes for an effective lift-producer. Although this hasn't been confirmed through testing. So maybe that's not such a great way to go, either.

You could reduce the lift curve slope on the strut by inviting separation and stall, but that is more likely to cause ventilation and will definitely increase the drag. That pretty much rules out all the ways of reducing the lift with section design alone.

Another way to reduce the lift curve slope is to sweep the strut.

Perhaps the best way is to simply make give the foil a much greater vertical area than the strut, say, by giving it a larger chord. This won't change the pressure distribution on the strut for the same angle of attack (leeway), but since the total lateral force is independent of the strut and foil design, the more force provided by the more you offload onto the fully submerged foil the less will be carried by the strut because the leeway angle will be reduced.

 

Larger forward foil? à la CBTF

Snipped from a post by Lorsail on the subject of forward rudders
"
Canting Ballast Twin Foil boats such as the two 86'maxZ86's, Genuine Risk, the Schock 40, and soon on the 115'Maiden Hong Hong plus numerous cruising or semi cruising designs have proven the viability(and speed) of a forward rudder combined with an aft rudder. The forward rudder is about 25% of the LWL aft of the bow and the aft rudder a little more than 75% aft of the bow."

Would this not also be an option for a moth foilrer as it would enable both a tighter turning and also more area on the front foil? I get the impression that the lenght of the boat itself is a limiting factor.

Just a thought

 

Foiler

Theoretically, your idea could work because the forward foil could be further forward but I imagine the practicality of making such a highly loaded foil pivot easily would be tough-as would be the linkage and set up.From what I understand ,though, the Moth on foils is quite manouverable and probably wouldn't benefit too much from the twin rudder set up. Some of the gain possible with collective is already achieved by sailing the Moth at an angle of heel to windward allowing the hydrofoil to "lift" a bit to windward.
The Moths currently use a "gantry" or hull extension that's not in the water to get the rudder foil aft far enough and they angle the daggerboard forward to get enough separation between the two hydrofoils. That's because most of the bifoiler Moths now are conversions of existing hulls. A newer design the same length could work with the mast moved forward , the daggerboard moved forward and the rudder on the transom.But if the rule allows more separation using it will help the boat in pitch.

 

Moth foiling

I am one of those people Wardi refered to as still trying to emulate the design and developments of John Illet. In the past 10 months I have built many foil sections in various configurations, then tested, modified, retested, broken and even lost them.

I am now at a stage where I almost have control but not yet any speed gain over a standard narrow moth. I might be able to add something here of value from someone who is actually trying something on the water every weekend.

Control is all about height. That is the only worry. I built my boat with the mast and fin further forward to gain separation but I am not sure it is really necessary. The boat is only unstable in pitch when one of the foils comes out of the water, ususally the front one. The boat is also really stable in roll, because the foils are so much wider than the hull. Yaw is a problem only if you get too high and there is not enough rudder lect in the water.

Height control is everything. The wand does most of the work, and moving body weight fore and aft is definately needed in transition from floating to flying. But as you need to operate over such a wide speed range you need something extra. That is where the Illet adjustable rudder foil flap comes in. I have a diferent system where I tilt the main foil fore and aft to change its angle of attack but it amounts to the same thing, both change the relative angle of attack between the two foils, just like an aeroplane trimming forward or down elevator to reduce drag and gain speed.

Getting all the right ratios of wand length, softness, angles, relative to flap angles, size and how much tension in the bungee, is all a black science and can only be developed from understanding and testing. Only those who buy a complete Illet kit can skip any part of this stage.

If you want to use a different configuration you have a long path of development ahead.

One of my first attemps was with a bow rudder and canard foil. Control wasminimal and the boat was impossible to tack because the high drag from a stalled bow rudder meant the boat just weather vaned. Now I do not see the point of huge separation becasue there is no intrinsic pitch instability problem.

 

Windward Heel

Phil, what is your take on heeling the boat to weather to increase stability and improve vmg upwind? Do you think the more separation between the main foil and the hull the better?

 

Phil, Any thoughts on why you are not yet getting better performance than displacement hulls yet? What about on reaches?

 

i had my boat going really quick and under control on reaches last saturday and again last tuesday. upwind i have had troubles though upwind it was comparableto the a well sailed narrow skiff on tuesday with nother days practice. To improve upwind performance i hope to be foil at lower speed and be able to point higher with some bigger foils.

in answer to the leaning to windward quesion, you definatlye need to lean upwind, why this is so im not really sure. i guess this allows the boat to crab to windward a bit. once in the air the boats need a lot more righting moment, so you really have to hike your arse off, the lean to windward really helps here, as you get extra power from increased leverage. I dont really think the weight of the hull has any significant effect, it is only 30kg and the cg only moves 300 to 500 mm to windward.

in terms for separation of the foils, my boat is a std moth with a rudder frame. phils' is longer separation with no rudder frame

 

I think total weight must have a lot to do with foil performance. I am 80kg and my boat is probably a bit over 30 so I am a probably 10-15kg at least heavier than Rohan. On such a small wing area this is a big increase in loading and hence the foil must fly at a higher AOA to lift more weight, and that means more drag.

I have vary rarely got airbourne upwind, but I do get an increase inperformance when the boat lifts even onlt 75mm or so above its lines. It definately goes better upwind if healed to windward. The best relative performance upwind has been in about 10kts with a rather small flapless foil. It did lift the boat a bit, but was hard to control when it got windy. I have since lost it when it tore off in a crash.

Last week for the first time I felt I had positive control from the wand and flap. Still some trimming of rudder foil AOA and flap angle, but maybe we are getting there. Found it very hard to get in the groove upwind. Hull almost out and healed back well but no real height. Good control downwind though, and apparently good sped but not close enough to other boats to know.

As Wardi says when you get going upwind healed back to windward there seems to be a reduction in need to hike so hard. Not sure why, but I suspect that you need to depower and flatten the rig so much to get up and going, than once up and healed back there is not enough power in the rig to hold you out of the water. I can not do it reliably enough yet to power up the rig again once out.

 

History

Wardi

Firstly I agree re your comments on "flying high" just clear of the waves is better than higher. It is a matter of looking at the moments and forces in the transfers plane to understand this.

Re history
There is a world of differences between surface piercing foils and full submerged centre line foils. Full submerge foils must be configured to be near dynamically stable in pitch AND require either automatic height control (as per John Illet) or man in the loop control (as per David Lugg's Int 14)

The design and stability study that lead to David's success was carried out in the last quarter of 2000 and David first "flew" undercontrol and stable on the 9th of August 2000. David took Bullet to the 2001 Australian Nationals and the Australian Sailing and Seahorse articles were published in the first half of 2002

So our claim is to have been the first to sucessfully use fully submerged in line foils on a mono hull, the facts may prove otherwise and I would be pleased to see a factual chronology

alans

 

foiling at height

Alans, if you go to http://www.monofoiler.com and look under "Other Foiled Sailboats" you'll see a pix of Ian's (and the worlds first) bi-foil monofoiler sailing in October 1999. You and David are without a doubt the first to sail a two person spinnaker equiped monofoiler on just two foils...
I think you are probably wrong about "flying high". Rohan Veal has proved that it works and is valuable foiling upwind.
If you look at the concept then the idea is that the more separation you have between the foil and the bottom of the boat the more distance to weather you will move the CG at any given angle. You can, of course, achieve any angle flying "low" but that means a lot more wetted surface compared to flying high( when looking at the deepest fin possible to maximize windward moving of the CG). So when you're trying to squeeze the last bit of power out of a foiler it seems that designing a deep fin with the idea of flying high to move the CG to weather and reduce wetted surface is a good one. The downside is the extra board area on some Moths can't be retracted-but I'm sure future boats will have a board you can retract (two board "settings")to some degree when you don't need the extra power...