Foiler Design

Discussion in 'Sailboats' started by tspeer, Nov 12, 2003.

  1. astevo
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    astevo Junior Member

    just a quick update.
    Dad launched his foiler today.
    it had a forward rudder with canard and a main foil very deep with 5degree dihedral and about 5degres of sweep. lifting area about 100 cm^2.

    first observations were that with the forward rudder it was imposible to tack. i put this down to the fact that it will tend to act as a pivot point, around which the boat weather vanes once it is head to wind or stoped. once the steeing mechanism was removed (bow foils still in place) we put on a conventional moth rudder with a t foil which pulls down at about 2degrees to steer with. basicaly with a normal rudder when stoped the drag is at the back of the boat and you can swing around the stern to get out of irons.

    with the bow foil locked to max attitude it was clear that you needed to sit well back in order for the foil to lift to the suface. once there the main foil lifts out violenty and the boat launches itself then crashes. to remedy this you have to put a lot of weight on the canard as it meets the surface.

    next test it to make the rear foil at a positive angle of attack. so that is doesnt lead to the boat going skywards due to the rudder pulling the transom down.

    there were a few helm problems with the boat too. when displacing the moth hull provides a significant amount of lateral resistence of course once out of the water it dosent and so the centre of lateral area moves back and you get massive lee helm. yet another reason to have the center board raked forward a long way.

    id advise anyone planning to build boats with bow rudders to not go down that path.
    Wardy was on hand to witness the frst steps and has some photos which might get posted in the near future.
  2. tspeer
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    tspeer Senior Member

    There are actually two conditions for static stability:

    - When the boat pitches up, it must produce a negative (bow-down) pitching moment.
    - As the boat rises in heave, it must also produce a negative pitching moment.

    The first condition implies that the aft foil must be more lightly loaded (carries less weight per unit area) - possibly even negatively loaded - than the front foil. Then when the both experience the same change in angle of attack, the aft foil produces proportionately more lift, creating the negative pitching moment.

    This condition also makes the craft pitch up when the speed increases, making for positive speed stabilty because it will tend to slow down as it rises. Although a hydrofoil doesn't have much altitude to play with, and other factors start to come into play.

    The second condition implies that the forward foil must have greater heave stiffness (change in lift per change in vertical displacement) than the aft foil. This condition is needed to prevent unstable pitch-heave coupling. The unstable coupling occurs when the boat wants to pitch up as it rises, causing an increase in lift, and a more rapid rise, leading to more pitch-up, etc.

    The two conditions are readily met by a fully submerged T foil as a rudder and a surface piercing forward foil. They are also met by a planing canard and fully submerged main foil.

    Besides stability an important factor is the ability to control the trim. A flap on the stern foil, or a variable incidence stern foil is a good way to achieve this. Stability has to do with what happens when small changes are made, starting from a steady, balanced state. Trim has to do with achieving the balance in the first place, or altering the conditions where the forces and moments become balanced. A craft that is out of trim can behave as though it were unstable.
  3. Andy P
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    Andy P Junior Member

    I am also trying the canard rudder and foil, with canting main Y-foil on the daggerboard.

    This is all recycled from my earlier foiler, with basically similar idea. The main difference is that instead of rear foils on each wing tip, the new design has the main lift foil at the normal daggerboard position.
    The old boat had surface piercing V foil on rudder, with a flat bottom foil fitted to the bow. This was hinged so that it could not generate downforce in a nosedive. ( however it broke off whilst reversing trying to get out of irons,)
    The V foil could too easily be forced under water, and so a nosedive resulting.
    The new design with the main lift foil much further forward should make trimming easier.

    pics showing main Y with daggercase and canting bearing
    canard rudder with mockup lift foil.

    Attached Files:

  4. astevo
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    astevo Junior Member

    Andy P.
    I'm interesed in what has made you decide on the canting main foil and the large anhedral? other than that it was what you have?

    from what i found yesterday it seemed that a rear lifting foil is required to prevent liftoff as the canard leaves the surface if you are not far forward enough.

    since yesterday ive come up with a few different ideas. some of which might make there way onto my boat. at this point i will have adjustable height and adjustable angle canard. variable angle main foil and fixed small tfoil rudder (at about +3degrees at static waterline).
    possibly using a submerged bow foil with sensor wand to keep it within about 300 to 400mm of the surface.
    other things under consideration are possibly moving the main foil back to increase the seperation then steering with both foils so that the centre of lateral resistence does not move back with the main foil. itd be a ******* to design, build and operate though.

    any lateral area in the bow foil will seriously comromise manouverability at low speed. though it could be worth considering to keep the centre of lateral resistence forward once lifted.
  5. RVELL
    Joined: Feb 2004
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    RVELL Junior Member

    Front foil configuration

    Hi Wardi,

    Here are some ideas in response to your questions.

    At 450 cm^2 your front foil does sound proportional, however if it is operating on or near the surface the lift coefficient could be perhaps 33% of the lift coefficient of a submerged foil. In that case, maybe the foil size could be reduced.

    For example, let’s look at the ladder foil idea. Your 450 cm^2 foil bouncing on the surface and could be replace with two smaller foils each providing half the required lift. The surface skimming foil would be 450 / 2 = 225 cm^2. Another foil mounted 30cm lower on the same strut need only be 75 cm^2. This is because, under these assumptions, the submerged foil provides 3 times the lift of a surface skimming foil ( 1 / .33 = 3).

    Quote: “I think such a stabilizer would significantly improve your powered foiler also!”

    You are correct. Manual control, except as a game of skill, requires pilot input too intense for long flights.

    Quote: “Your solution looks to provide the control I need, but has the disadvantage that the upper foil is high drag when submerged in displacement mode. Do you have any suggestions to overcome this?”

    I can throw out a few ideas, but careful design and skillful construction will be the keys to creating something that works.

    The US Navy’s Pegasus used a front foil that swings forward. A drawing is shown below. Of course the brakes go on while the foil swings up.

    Another idea comes from my friends Steve Ball and Sid Shutt. Steve’s boat is shown below to illustrate lift-up mechanism on the bow. It is in the form of a parallelogram on each side of the bow. The incidence of the foil remains constant as it is raised. They use it to get a running start while pedaling vigorously. When take-off speed is obtained the bow is popped up.

    How about a hollow front strut fixed to the hull. Through the strut goes a rod attached to the foil. When in the displacement mode, the rod is rotated 90 degrees presenting the tip of the foil to the water’s flow. This would present a very small profile. The angle of attack should be held close to zero to avoid the high induced drag associated with low aspect ratios.

    Last idea: Assume a “T” shaped front foil. Create an indentation or “nest” in the underside of the hull. Retract the foil into the nest. This nest could be transverse, i.e.,
    Chine to chine, or fore and aft. The later would require 90-degree rotation.

    Just some ideas, I hope they help.

    Attached Files:

    Last edited: Feb 14, 2004
  6. Wardi
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    Wardi Senior Member

    Stevo Bow Foiler

    Here are some photos of Phil Stevo's latest foiler with a bow rudder...
    Some very good engineering here and important lessons to be learnt.

    The bow rudder works when sailing along, but makes it nearly impossible to tack. It prevents the head of the boat falling away from head to wind. The only practical way to use a bow rudder would seem to also have a stern rudder coupled, which would probably defeat the purpose!

    Attached Files:

  7. Wardi
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    Wardi Senior Member

    Scow foiler

    I have been working for some time now using a non-winged Scow as the platform for trials. Appart from being simple, easy to rig and very stable, it is also a pleasure to sail..even without foils!

    I have a canting centrecase, up to 40 degrees either side of vertical. Have built and trialed an inverted Y foil, see the photo of it under sadly lost to the depths...
    When canted to leeward I get a lot of lee helm and so much lift from to leeward that the boat is easily pushed in on top of me...windward capsize tendency. Canting the other way produces the opposite effect.
    It was concluded that the canting produces more problems than it is worth.

    The main foil has two door hinges and can be set at variable dihedral angle while on the water. Last week I tested the main foil at two angles of dihedral, straight Tee and two angles of anhedral with interesting results.

    Dihedral provides very good initial stability and rolls the boat in to windward. Initially this is great, but it does not stabilize and you simply keep rolling in to windward. Anhedral is initially very stable and provides no windward heeling moment. The problem is that when you heel more than about 10 degrees, it takes over entirely and tries to flip the boat over in the direction of heel. It is completely unstable and a disaster to foil with. I have developed a complete respect for Miller in being able to foil on his inverted Y foils on a sailboard. There is no doubt that the ability to balance in all directions while standing up and also to move the rig while standing up is the only way this could be achieved.

    It is concluded that a straight Tee or slight dihedral, like an aircraft wing is the best compromise.

    All of these results are quite fortunate, as it means the best arrangement seems to be a simple Tee foil in a standard centrecase.

    My Scow foiler uses a simple bow canard. You can see the photos for details. The canard can also work with a sensor lever for running deep. I have had a lot of success with this, but when running on the surface it has far less lift than when submerged, maybe only 25% and not nearly enough to counter the pitching moment of the rig. I think a submerged bow canard is the only practical solution, but will continue to trial a larger surface area planing canard, as it offers the hope of improved surface tracking in strong winds, big waves.

    Attached Files:

  8. Phil Stevo
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    Phil Stevo Junior Member

    2 days of experiments

    Here is the story partly told by Andrew (ASTEVO) and WardI, and his photos. The boat is my standard moth as sailed with fin and rudder up till this weekend. The mast and fincase are in the normal position.

    The design was to test the bow planing foil as per Miller and bow steering, to decide if a new hull should have the mast and main foil pushed furher aft.

    AS Ian said the inital test was with a bow rudder and small foil located 300mm below the bow which was intended to surfase plane. The main foil is on a deep fin. It has sweep and dihedral. The section is a conventional airplane section like a E205 maybe thinner. (Probably too much camber and too thick forward.) With the boat level the main foil is set with about 3 deg incidence. I also have some sections from John Illet for some new wings.

    With this configuration the CoG needs to be in front of the main foil, otherwise the bow foil comes clear of the water and the the main foil climbs too steep to the surface. But I found that the bow foil has insufficient power to lift the bow unless I move aft initially. The result is a sudden climb requireing a move forward to trim.. or a launch out into air with subsequent crash.

    Bow steering was a disaster. Stiffness in the linkage as tollerable and probably fixable but the main problem was getting into irons. In 45 minutes I manage only ablout 3 good tacks. Mostly it involved minutes of reversing. It appears that when the vertical foils are stalled the CLR goes way forward and the boat just weathervanes. It might work with a sloop rig but not a cat rig.

    After a while I went ashore and disconnected the bow rudder linkage and installed my std moth rudder. This made steering easy and allowed much more time for foil experimentation. We had some weird windward and lee helm which we could not work out - turned out to be the free swinging bow rudder jambing just off center.

    The boat would not lift the hull out while going to windward in maybe 12 kts, but reaching it lifted well. In the Balmoral chop both Andrew and I had turns and gradually got into trimming with body movements. With no bow foil it is obvious that there is a problem with excess height as speed build up, lots of take off and lots of crashes, but no damage.

    Wardi was watching and suggested we retrim the stern rudder T foil for slight +ve angle of attack.

    Day 2: Northbridge, smoother water, patchy winds up to 12kts.
    Launched with three foils, retrimmed stern rudder with packing under top gudgeon. But bow rudder was stlll jambing, making tacks and steering bad.
    Boat lifted more easilly and was easier to trim with body movements. After a while I nosedived and capsized and while down removed bow rudder completely. This seemed to make a lot of difference, les drag, easy steering.
    Slight diversion ashore to straighten some bent alloy, but I completed the race with only one more swim. Had some extended runs with hull clear of the water and only an occasional launching and crash. NOt sure about speed. I was long way behind after long swim and shore diversion, Chris Dey sailed with me for a little while after he finished but no real comparison available.

    Bow rudder is out for cat rigs.

    Bow planing foil control might work but needs to be bigger than 150 x 400. But if you need and aft rudder this means three apendages, which seems excessive.

    I want to separate the main and secondary foil for better pitch control. If I can not use the bow rudder and foil with aft placed rig and main foil, then I look like favouring the rear rudder and seconary foil like John Illet's set up.

    Basically in two days I can appreciate why John and Gath have gone the way they have.

    Next step is to build a larger T foil for the rudder. and assemble the Illet foils with surface sensing control.

    Will report when tested.

    Thankls to Wardi for local help and the WA guys for inspiration and leading the way.
  9. astevo
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    astevo Junior Member

    from what I have seen on the water, and read on this board. it seems the simple configuration will not always work. the rudder needs to be at the back in order to tack. and some lifting area behind the centerboard is also required as a safety. basicaly i have come to the idea (that wardy had proposed) that the a submerged foil with sensor at the bow, instead of the surface foil will be more efficient.
    when sailing Phil stevos boat (thats me in the photos) there was clearly a problem with the pitching moment of the rig forcing the bow foil down. miller does not have this problem with the sailboard (you can't put much torque through a rubber joint) and so the loads on the bow foil do not vary as much as in a dinghy. it is my feeling that the change in mode between submerged and planning will always present a problem.

    all of the problems with the canard system have lead us to move more and more in the direction of the illets foils. indeed the more i think about the most sucessful system out there, the more it makes sense. only things to do would be to increase the seperation between the 2 foils. so maybee its time to move the rigs in dinghys forwad again ?
  10. Andy P
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    Andy P Junior Member

    Some nice photos and reports.

    My original foils had 40 degree angle to horizontal, which is apparently the best angle when using split foils, for best compromise on lift and sideforce, and also helps with the differential lift due to leeway.
    With large anhedral angle, and up to 40 dgree of canting, then one wing handles just lift, and the other just sideforce.
    So it was easy to only join one wing, leaving the other in place. May have to redo both though, but will try it first.
    I would have thought that the off centre lift would be like catamaran sailing, ie just pivoting about the lift foil llike a cat hull to leeward, and allowing much more effective righting moment / sail power. I suppose the problem is controlling the lift.

    Is the lee helm because the hull's side force is missing when flying?

    The idea of the forward ( of the hull) mounted canard is so that the rudder can be partially retracted, leaving the normal blade part in the water, but moving the surface lift foil to above the waterline in non-foiling conditions. And also increasing the lateral separation of lifting surfaces.

    The plan was to have this canard system requiring no adjustment when sailing, ( like a normal moth + T foil aft rudder ) and unlike the Ilett foils.

    Perhaps instead of planing canard, the sensor controlled immersed T foil would do the same thing , except below the surface.

    To help the displacement mode turning problems, a short low aspect aft rudder that clears the surface when flying?

    I still want to try the canard arrangement ( and find out for myself that it doesn't work - maybe)
  11. Wardi
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    Wardi Senior Member

    This is the same logic I was using and is how the Miller sailboard works. The two main problems I found were:
    a) The foil at 40 degrees is way to leeward and produces massive lee helm when you lift out of the water. Miller can balance this by raking the rig aft and cats have the rig move over to leeward and long directionally tracking hulls, both of which counter this effect.

    b) The lift from so far to leeward is actually too much and the boat tilts in to windward, you actually find it best to sit in the middle of the boat to get going. With my canting case, I found the ideal angle about 15-20 degrees to leeward to balance lift and side forces, so the anhedral angle should be closer to a straight Tee if you are goung to continue with these trails.
  12. Wardi
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    Wardi Senior Member

    Dinghy foiler height control and longitudinal stability

    The biggest question with Dinghy Foiler design right now is how to manage both height control and longitudinal stability with the most efficient foil setup.
    While John Ilett has developed an excellent surface sensor controlled bifoiler, it is thought that this could be further improved by placing all of the lifting load onto a single more efficient lifting foil.(Just as aicraft now use a single large wing instead of a biplane arrangement.) The question then is how to best manage height control and most importantly, longitudinal stability.

    The idea of using a bow canard for sensing and stability, is that it involves only one small foil taking little load at the bow, which is always lifting, as opposed to stern foils, which would need to sometimes suck to lift the bow.

    I have trialled this arrangement but found several limitations. Firstly, to keep the canard lightly loaded you must be at or near the balance point of the main foil. This means that if you get any extra drag in the rig due to a gust, or ease the mainsheet etc, the boat will tip aft, bow sensor leaves the water and you launch into space...and ineviatbly crash, unless you can power on or lean forward fast enough. I have found that the best solution is a small stabilizing foil on the rudder, just as all skiff Moths have today. This works incredibly well! You can fly quite OK in flat water and steady breezes this way, with not lift control at all, simply by moving your weight for an aft. The problem is that it is quite sensitive and you cannot "power up" to ket maximum speed, without risking loss of control.

    I tried to rely on the aft stabilizer to prevent diving, but it is not effective enough alone. The problem then is to find the best solution to balance the forward pitch and provide good height control. It may be possible to use a bow sensor controlling the rudder lifting foil, but this would require both positive and negative control and would not produce lift off in boats which could not sink the transom.

    My feeling is that the bow canard is the best solution as it provides both pitch stability and height control. and is always lifting. It is akin to sitting on a see-saw and balancing in the middle. By providing a stabiliser at one end and lift support at the other, it is being supported in two places, making it quite stable. In fact if the aft stabilizer always had a slight lift angle compared with the main foil, the boat would always remain stable and not launch off into space!.

    The question then is what is the best canard arrangement?.
    I have found that the bow lifting foil while under water gives more than enough lift. As it nears the surface lift decreases noticeably. When it clears the surface, lift is even firther reduced and it feels like it has only 10-20% of the lift generated under water. This may not be a problem at high speeds and if you can keep pitching moments to a minimum as is done with sailboards, but if you get a big air bubble on the upper surface and it goes under, it provides little lift and plenty of drag. It is only when the foil goes right under and sheds the air bubbles that it can lift again.
    My conclusion is that the best arrangement would be a small sensor controlled bow canard permanently immersed. Which is what I am now trialling.
    I am sure this would be a good sloution for RVELL as well.

    The sensor canard arrangement is really mechanically quite simple. The control line goes down the strut, out the sensor arm and then down to to front of the foil. Initially it is let loose and deactivated. This means the sensor arm floats horizontal and the canard runs neutral and free with no loads and minimum drag.
    As you pull the control, initially it activates the sensor and the foil lifts the bow. It will rise until the sensor arm leaves the surface and provides level control.
    By pulling further on the control line, you are adjusting height control and can raise the boat higher for larger waves.
    So far, I think this is the very best arrangement. It also has the positive effect that the canard provides a steering moment to counter the rig heeling to windward or leeward, so the boat tracks straight at all angles of heel. It becomes very easy to sail.

    While I agree with Phil that two foils looks best, this does not seem to have worried the mini transat, multihull and canting keel enthusiasts... so why should we be too concerned.

    Any comments or ideas for further improvement greatly appreciated!
  13. rohan

    rohan Guest

    rig position

    andrew, why do you think the rig should go forward? if anything the centerboard case should be further forward i reakon.

    from my experiences, mast rake makes no difference to speed or control. only with the mast raked forward it makes it a little easier to get under the boom. it would have to be a big movement of the rig to make any difference i think.

    good body position and movement makes all the difference. in a controlled situation, by just moving body weight back 20cms could mean the difference to flying and not flying.

    i am finding that i am using my manual rudder trim adjustment less and less now too. all i will do is wind it up to stop the bow from pitching to much in high wind/big waves a few times upwind and downwind. in flat water, you don't have to trim anything but your sail and body weight.

  14. astevo
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    astevo Junior Member

    there are only two things which i would consider a problem with your set up rohan.

    basically i think the seperation should be increased to improve longitudinal stability. with your system (not your boat) the easiest way is to move the rig and centerboard forward and leave the rudder where it is. im looking at this as a potential new boat for next season, obviously with existing boats the large rudder gantry is the way to go.

    the other thing i think which might be worth considering is having the rudder foil as a symetrical section. essentilly the rear foil must be pulling down at some time for you to be sitting as far forward as you do when sailing upwind. the section you have now is not ideally suited to negative angles of attack, though it is not far wrong. the section shape of the 63-412 is very symetrical for the forward 70% bt the cusp at the back curves down violently. obviously your trailing edge flap reduces this effect. but i would favour having a symetrical section with no flaps but instead pivoting as a whole.

    im interested in the method which john has decided to use to regulate height.
    why has he chosen to only adjust the trailing edge flap not the angle of attack of the entire foil?

    also do you know if the illets have ever tried to use the sensor as a control of the rudder?

  15. rohan

    rohan Guest

    we are aware that we would like to have the main lifting foil further forward to increase distance between the two foils and john is testing this on his own boat that he is building at the moment.

    my rudder is angled at 0 degrees or +1 degree normally, so therefore when i sit back, i am only inducing more angle of attack on the foils to create more lift initially to get going, that shift my weight forward to trim out and go for speed.

    the main reason for not pivoting the entire wing on the rudder is because at high speed, it is way too sensitive and i am sure if you make a small movement on this it will pitch you or down quickly. it is also more sophistocated, more strength is required to construct and hence more margin for error.

    however saying that, i would like to try it one day and see how it reacts.

    we have not tried using a sensor of some kind to adjust the rudder as yet. as mentioned before, i am hardly using it these days anyway, and really only make one big adjustment before i start the leg depending on the conditions.
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