Foiler Design

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

  1. astevo
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    astevo Junior Member

    in aeroplanes you get large stability gain by having the rear stabiliser at a good angle of (negative) incidence. it is this angle which allows you to be forward of the centre of lift and it is this factor that makes aeroplanes with forward centre of gravity stable. in contrast if there is no incidence it make the plane a tandem have you ever heard of a stable flying flea . the flea is a plane which is notoriously unstable and one which modelers have always had problems with. .

    and this has major problems, it will not self correct in pitch and will go wherever it is pointed. they are also badly bahaved if asked to operate at a range of speeds. it is for this reason that i feel it is best to have the lift from one main foil. you also gain the benefits of greater efficience from having a single foil due to less tip loss. and in my case a better aspect ratio due the fixed cord section of nicley moulded prepreg 63-412 i have.

    when i said that you should have the centre of gravity in front of the centre of area, the downward pull of the tail is what i was alluding to.

    wardy and others have said that the wide arse boats will not be able to sink the stern enough to lift off. and so the forward sensor would not work without a canard.
    i question how much you really need to sink the back. if you have a sensor at the bow it will have the effect of both increading the section thickness and angle of attack when you are at the surface. with the current foils with no flaps neither of these two things happen and so the initial lift must come from an increase in angle of attack.
    anyway got a start in an hour and a half, better cease this rant.
  2. Doug Lord

    Doug Lord Guest

    Foiler loading

    My 16' monofoiler is designed with 80% of the load on the main foil and 20% of the load on the rudder T-foil. She hasn't flown yet so we'll see. But the Rave has flown ,of course, and is designed the same way: 80% on the two main foils and 20% on the rear foil. The area of the rear foil is exactly 50% of the two forward foils taken together. On the full size Rave the foils are all symetric(because they have to develop positive and negative lift?) and the main foils are set at + 2.5 degrees relative to the static waterlne with the rudder T-foil set at 0 degrees. My 16 footer uses an asymetrical (63412-I think-same as John Iletts-but bigger) main foil section set a 0 degrees and a symetrical rudder t-foil set at 0 degrees.The t-foil rudder area is 50% of the main foil area. The Rave generally requires no input to the rudder t foil. I've designed and built a 56"LOA X 71" radio controled model similar to the Rave(with Dr. Bradfields help) with the exact same foil settings and relative areas. It requires no input to the rudder t- foil whatsoever.
    From talking to Rohan and John it seems like the system they are using also requires very little input to the rear foil with the wand hooked up to a flap on the main foil.
    John, if you read this what relative foil settings are you using(angle of incidence) for the main and rudder foil,what section for the rudder and what size is the rudder t-foil compared to the main foil?
  3. Wardi
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    Wardi Senior Member

    It is of course possible to lift off with a fat stern by putting a positive angle of attack on the main foil. You can do this by simply raking it forward. This is what I have been doing with the scow foiler. Works fine!!

    I see no problem attaching a forward sensor to the rudder foil. This could work rather well.
  4. Wardi
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    Wardi Senior Member

    RVELL, Thanks very much for your contribution!

    I agree completely that this may work OK, and that you can take off by adjusting the lift of the main foil. I am already doing this with my scow foiler. It looks as though Astevo is building this configuration, so we will soon see.

    We should be careful to define our terminology!! In the arrangement I am proposing, my "canard" is actually my forward "sensor". It is not intended to take any lifting load, just set the angle of attack of the main foil....just as you are proposing in fact. It is just that rather than using a wand with linkages to control elevators on the main foil or rear foil (complexity), I am using the canard as a surface sensor and the feedback to the main foil uses the hull itself as a fixed connection (simplicity). I think in the end we are talking about the same thing, different methods!

    Accurate surface tracking:
    I think there is a major factor missing in our considerations so far!. It is related to large pitching moments of the sail/rig and the speed of response to pitch and height variations and ability to accurately track the surface.

    The current Moth foilers can ride quite high with deep foils and have a relatively large vertical margin to work with. As the boat pitches up they can ride quite high and most importantly if they pitch forward and down, there is perhaps 0.5 m of travel before the hull hits the water, and even if it does, the very fine bow allows it to bury another 0.3m with no big problems.

    Even so, in big waves downwind, the sensor says lift, but the wave has passed and the boat and foils leave the water. This is the problem Rohan has mentioned.

    What I am trying to achieve with the surface tracking canard, is a much more accurate surface tracking with instant and direct feedback to eliminate ride height variation and prevent the problem of launching off big waves.

    I think you are getting close to a good solution with your powered foiler, but you do not have the big pitch problems from sails to contend with.

    My current problem is that with a canard of fixed angle of attack, It lifts and runs well, but as the boat goes faster and the main foil lifts higher, the angle of incidence of the canard is reduced relative to the water and so when I get any forward pitching moment the canard provides very little lift and it simply goes under.

    I think I now have a solution with a simple constant lift canard which works at all angles of incidence, both submerged and on the under construction.

    Your arrangement may also help solve this problem...I am looking more closely again today!

    all the best, Ian
  5. Andy P
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    Andy P Junior Member

    My simple ( no moving parts )design inspired by Ian Ward/Miller has been modified due to these discussions, and now has a linkage to automatically adjust the canard(s)

    But I'm still going to try the bow rudder / canting main Y-foil configuration.

    The arrangement is a bow rudder, mounted fwd of the stem on a bowsprit.
    The rudder has a variable angle fully submerged T-foil at the bottom, and linked to the hinged surface tracking canard.
    The surface canard is on a trailing link, and when lifted by contact with the water surface, adjusts the angle of the lower T foil in order to provide instant extra lift.
    The pushrod through the rudder to operate the T foil will allow the foil to hinge into a neutral angle ( vs a negative angle / downforce ) in a big nose
    Perhaps a flexy wand with a small foil on the end may be needed to control the coarse movements of the surface foil. Maybe this will be the only control needed, and the surface tracker for emergency use only.

    The complete assembly is located on the rudderblade, allowing the unit to be retracted with the surface tracker above the waterline for displacement sailing. The linkages are also very short ( but a bit fiddly to make)

  6. Wardi
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    Wardi Senior Member

    Hello Andy,
    I think it is very good that you try all ideas and find out your own results.
    On the weekend I tried 8 different configurations during 4 hours sailing in 15-25kt winds. You certainly learn a lot in a short time...its fun too!!

    I think the bow rudder can work, but make sure you put the centre of effort well forward of the centreboard, otherwise you will have the tacking problems discovered by Phil Stevo.

    Canting the foil I am sure can work, but induces several handling problems. These are easily fixed on a sailboard by raking the rig, but this is not so easily solved on a dinghy with stayed rig. I have trialled canting boards very much, but have concluded for now that it is simpler to have a fixed board and heel the boat instead. Let's see if you can make it work! :)

    This is very similar to the sensor wand controlled submerged foil I have been using recently. The problem I have found with this arrangement is that our "wheelbase" on a Moth is very short and as the bow angles down, it significantly alters the angle of the canard strut...bow rudder in your the water surface. The result is that the relative effect of the sensor on the foil at the bottom of the rudder is reduced, resulting in too little lifting force. You end up with a slow porpoising of the bow, instead of level flight.

    One solution to this could be a forward facing sensor, which will probably need a surface tracking ski arrangement. With the sensor ahead of the rudder/strut, it provides relatively more lift as the bow is angled under construction!
  7. Andy P
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    Andy P Junior Member

    it's still a bit cold here.... and I have to get a boat before I can actually try this out.

    The idea of the fwd rudder is to get more horizontal separation - about 2.5 - 3m , with the board back a little more than normal. ( instead of the normal aft rudder ~ 2m )

    It's easier to make it canting first and then lock it up if it's no good.... but I feel it may have advantages .
    With the bow rudder on a bowsprit, the intention is also to explore canting the entire rudder assembly when sailing the boat heeled.

    The fwd strut/ski was tried here in Cowes some years Christopher Hook on small outboard boats.

    And now the latest flying car

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  8. Shane Hollis
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    Shane Hollis New Member

    Thoughts On Altitude Control


    I am a Moth Sailor in NZ, I have been following the boat design group discussion on foilers and I am grateful to all the participants for sharing their ideas...

    I have no experience with foilers so please excuse my ignorance.

    To summarize the situation as I see it...

    It appears that the problem with foiling Moths foilers is that the driving force is high above the water. As rig load increases off the wind, the hull is depressed in the water (not enough lift), as the load comes off i.e. a lull the boat rises up (too much lift) resulting in porpoising.

    The 'state of the art' solution as developed for Rohan's successful foiler Moth is to use a surface tracking sensing arm connected a foil flap via linkages. This solution is reactionary - a bit like reversing a trailer behind a car.

    Launching into flight is achieved by trimming the boat fore and aft ie. sinking the stern to increase foil angle of attack.


    Why not utilise the very element that is causing the problem in the 1st place to control the lift of the foil.

    When a gust hits, mast compression increases, when the wind drops mast compression reduces. Connect the mast base pin to a lever with an opposing spring (Perhaps elastic cord at the end of a lever arm with tension adjustment) connect the lever to a mechanism to control foil flap.

    Light rig loading mode:

    Light airs and maneuvering, decelerating through a lull and when insufficient wind to foil.

    Lever would be set to minimum load position and set flaps for low lift/low drag.

    High rig loading mode:

    When the downward force exceeds the lift offered by the untrimmed foil, crew weight cannot respond adequately to change A of A of foil, the increased downward force on the mast base will activate the trim tab on the main foil to increase lift, the more downward load the more lift.

    Beating to windward at full power - relatively low speed (8-10knots) with windward heel, requires maximum lift also for leeway reduction (see Rohan Veal's explanation)

    When hit by a gust in high wind off the wind sailing requires increased lift to counter momentary increase in downward force.

    The Positives:

    - No dragging sensor devices
    - Instantaneous response
    - Relatively simple engineering

    The Negatives:

    There would be slackening of the rigging in high rig loading mode, however only a few mm may be all that is required to get the travel and force required to alter the trim tab.

    ....Shane Hollis NZ
  9. Phil Stevo
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    Phil Stevo Junior Member

    It is not quite like that. There are a few interrelated problems:

    1. When you go faster the foils work better so they lift more. The boat does not weigh any more so more lift results in an upward accelleration, leading to the foils breaking the surface. To prevent this you need to reduce the lift coefficient by either a change in the angle of incidence of the foils or a reduction in the camber of the foil. This reduces the lift so that again: lift = weight and the boat does not take off. The John Illet wand does both of the these by lifting the flap which reduces camber and reduces incidence of the new decambered section.

    2. The sail force is about 3m above the drag force from the foils resulting in a bow down moment when the sail is driving. This has to be ballanced by locating the the crew aft of the centre of lift by a calculatable distance. In a wind lull the sail force is reduced but as the boat is still moving the drag force is not until it slows down, but the bow down moment is lost and because the crew did not move forward there is a resulting bow up action from the crew weight moment. This results in increased incidence on all submerged foils and hence the boat rises up. If the bow wand does not correct the height the boat will launch into the air and crash.

    3. The reverse of 2 happens in a gust. The sail force increases and unless the crew moves or the trimming devise reacts, then the bow will go down. If it goes down too far the foils will have negative incidence and the boat will dive big time. This can be minimised by moving the foils further appart, hence the transom boxes on the Illet boats, but somewhat limited by the length of a Moth. It also seems logical to have some trimming facility on the rear foil so at extreme dive situations it can actually pull down like a traditional T foil rudder.

    The way the boat reacts is also affected by waves and chop, depending how high you are and how close the foils are to the surface. Basically if the foils break the surface you crash one way or another.

    I have not completed my level control device yet and last Saturday in a puffy balmoral Southerly, I had numerous first hand experiences of all three events. This was with a couple of different settings, changing incidence angle between fore and aft foil and with lots of fore and aft body movements.

    Next week I will have my new Illet main foil with a self trimming flap so hope for a more reliable race.

    Phil Stevo
    Aus 9324
  10. Wardi
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    Wardi Senior Member

    It is akin to flying an airplane consistently 2 feet above a runway covered with 1 foot high bumps and potholes, inside a shed with a roof 2 feet above the top of the plane, in a gusty crosswind, with the engine mounted 9feet above the wing operating at full power...but intermittently, without landing.... :(

    Fly too high and you hit the roof and crash, fly too low and you crash anyway..The good thing is that if you crash you only get wet! :p

    Sounds like a script for the next James Bond action sequence! ;)
  11. National3434
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    National3434 Junior Member

    A better way of understanding the Miller configuration?

    I'd like to submit this for your collective thoughts. I think it is a collection of what you have all been saying, which I have been trying to understand!

    The Miller configuration uses a bow foil to track the water surface and a main foil to provide the majority of the lift. The angle of incidence of the main foil is controlled by the angle given by the distance between the two foils and the depth at which the main foil finds equilibrium. The faster you go, the greater the lift, the lower the incidence angle. So this should be a self trimming system and the main foil should never come to the surface (assuming smooth water). So this is a MONOFOILer which is controlled in pitch by reference to the surface tracking foil.
    (To call the surface tracking foil a canard is not wrong but is slightly confusing since it implies that it works in the same way as the main foil, which it does not.)

    Now enter the disturbances of real life on a boat.
    Waves mean that the water surface is no longer flat so the surface tracker has a harder job to stick to the surface (or an average surface). The surface tracker may become airborne, decieving the main foil into lifting the boat right out of the water. Or it may plough into a wave and not have sufficient lift to find the surface before the hull bouyancy takes over.
    Variations in wind speed (gusts) mean that even on flat water the vertical forces on the surface tracker are not constant at constant speed. The overturning moment of a gust in a rig can be significant and without reserves of lift the foil will burry leading to a descent of the main foil untill the hull bouyancy takes over (a crash). This does not happen on a windsurfer since the rig loads are taken by the pilot, not the boat.

    Wardi and the Stevos have found that a stern rudder foil works nicely with the suface tracker at the bow. It seems that this should have a couple of useful attributes. One is to add to damping in pitch (slowing the surface tracker's divergence from the surface) and the other is to apply a forward overturning moment to prevent the surface tracker rising off the surface (assuming the aft foil is set to lift).
    I think all this assumes that the main foil acts as a kind of pivot about which the bow and stern foil act and oppose one another. Is this actually the case?

    In order for the whole system to work better, the forces which enforce the surface tracking need to get greater, giving greater 'stiffness', meaning that the disturbances mentioned above will not cause a loss of tracking. So the 'reserve' lift of the bow foil at the surface needs to be high enough not to cause sinking when the boat is hit by a gust. And the lift of the aft foil needs to be sufficient to force the bow foil down onto the surface quickly should it become airborne. Also that if immersed into the back of a wave the bow foil does not lose lift but gains it untill it finds surface again.
    Am I right that this requires both larger foils bow and stern and careful selection of the bow foil section characteristics to give 'seamless' recovery of surface tracking after ploughing into a wave?

    Sorry for another tome!
  12. Olav
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    Olav naval architect


    that's exactly what I understand of the issue (so we're both right???)! :idea:

    I'm now thinking about having a fixed large canard (if you don't like the word, what do you think about calling it "waterski"?) with significant dihedral to allow the boat to slightly heel without digging the front foil's tips in. The main foil will be just a simple T. I want to start experimenting with a section operating at a high Cl (probably something like the NACA 5313 from one of Tom Speer's post in this thread) to ensure the boat will lift even in moderate wind conditions. Refining regarding optimum drag/lift ratio etc. can be done afterwards as I still do not know what wind velocity will be needed to go airbourne.

    Now the rudder: As far as I have understood the problem, an adjustable T-foil on the rudder blade seems to be good. It then can produce negative lift to help the boat to take off (greater AOA) and to avoid the canard digging in AND eventually positive lift as well to support the whole system during the flight. The flap or even the whole T-foil could be operated by rotating the tiller extension like on Rohan's boat.

    By the way: My Moth building campaign website is now online. Have a look at
  13. tspeer
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    tspeer Senior Member

    I think you've done a good job of summarizing things. The higher heave stiffness of the bow foil ensures that the boat rotates about the bow, and thus pitches down, as it rises. The stern foil does a good job of stabilizing the boat in pitch and providing the damping you mention.

    As you point out, once surface conditions get to be too rough and the boat speed too great for the bow foil to track the surface, it loses its effectiveness - especially for the case of a planing surface. So a surface-piercing foil may be the way to go. It doesn't suffer from losing contact with the water alltogether, and it can average out the effect of waves that are at too high a frequency for the boat to respond.
  14. Wardi
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    Wardi Senior Member

    I think this is a pretty good summary of the situation!.

    Miller has found in practice that the surface tracking canard sensor works really well in choppy waves, and big swells, because it tracks the general water surface very accurately. This is why I think this method can work even better than my original Bifoiler arrangement.

    I think you are imagining much bigger variations in general loading on the canard sensor than really occurs. When running I find the canard only touches the water surface perhaps 30% of the time and there is little loading on it or resistance from it. If it lifts too high, just roll your body forward a little or sheet on and it will hit the surface again, this is easily controlled.

    If you ease the sheet the pressure comes off the rig and the canard lifts clear of the surface. I have found that a small stabilizing foil on the rudder keeps the boat running level and compeletely manageable. This does not have to be very large and in fact is about the same size as the stabilizing Tee foil used on all modern skiff Moths these days. At this stage I have seen no need to make this adjustable, other than to keep set it at the correct angle in the first place. I do this by raking the rudder in its box.

    If on the other hand you get a gust which drives the bow down, the canard hits the water. The problem is that its angle of attack is quite low and decreases as the bow dives further. There is insufficient lift and it eventually goes under. To have a much "stiffer" response would be really good. The trick is getting a good canard design. It also will have to run under water efficiently, which the Miller canard does not. I have therefore designed and an currently constructing a self leveling canard which will give the same lift, irrespective of the attitude of the boat and will work both submerged and surface running.

    Any new ideas on a good canard design most appreciated..

  15. Wardi
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    Wardi Senior Member

    Here is a question for Tom Speer, as I think he can help us a lot to understand foil types and configurations we can use on Moth foilers.
    Assuming the boat plus skipper weighs 120Kg, The main foil is initially rectangular 140cm x 14cm (aspect ratio 10:1) and that this takes the entire lifting load.
    It is a simple symmetrical foil with thickness 14% of the chord length.

    1) What is the actual total resistance in Kg of such a foil at 8 kts, 16kts and 24kts..a graph would be nice :). What are the main components of this resistance, ie profile drag, induced drag and friction etc.
    2) What is the total resistance at the same speeds if I split it into 2 foils each half the width, equally sharing the load.
    3) How much improvement in total resistance can I achieve by making a thin foil say 8% thick, or using the special configurations you have previously mentioned.
    4) What happens to the total resistance if I increase the total width (area) of the single foil by 20% or decrease the total width (area) by 20%.

    The main reason for asking is to get some feel for those parameters which make the biggest difference to total resistance, so we can address these in the design first.

    What in your opinion would be the best single foil arrangement to give the very best all round performance. Ideally we are after lowest take off speed and highest top end speed...I think
    best regards, Ian
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