Foiler Design

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

  1. Wardi
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    Wardi Senior Member

    Doug,
    Moths are currently running at B/L of 1:10, which is based on the minimum surface area for the length and this seems ideal, but I am sure you could get away with a beamier hull, provided it is light enough and you can get up to take off speed early enough.

    My own opinion is that a canoe stern would be ideal to accommodate attitude changes at takeoff, as you have proposed. The main thing is have as little rocker in the hull as possible, so that you maintain the high prismatic coefficient and achieve high speeds for take off. Conventional canoe shapes with lots of rocker can suck at speed causeing high drag at takeoff speeds.

    This is where the Moth with flat planing sections all the way aft and pintail stern excels.. the best of both worlds!
     
  2. Doug Lord

    Doug Lord Guest

    Cp /rocker

    Ian, thanks for your great and informative comments! I've seen designs for Moths that show the boat at max displace(with crew) sitting at static with both the bow and stern substantially immersed; I assume thats done to get a high Cp at a 10/1 beam to length with the least rocker possible. But it seems that could lead to slow manouverability at low speeds with the extra area fore and aft resisting the turn? I can see that with a foiler the other advantages probably outweigh this concern but I'm curious if it has been a problem?
     
  3. Wardi
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    Wardi Senior Member

    This is not really a problem, but probably only because the boat is so small. When tacking in displacement mode you can roll your weight aft, this sinks the fine stern, lifts the bow clear and they tack quite easily. A further technique is to heel well to windward after tacking which induces lee helm, pulling the bow away from the breeze onto the new tack, when you then sheet on, the rig pulls the boat and skipper upright, generating incredible acceleration.

    I doubt you could do this with a very large boat with a long hull. It may be less of a problem if it was extremely light!
     
  4. Doug Lord

    Doug Lord Guest

    wetted surface

    Ian-fascinating insights! In the case we've been discussing-a hull designed to takeoff as quickly as possible- would you think that a semi-circlular low wetted surface hull work in this application(high Cp-low rocker) compared to the relatively square sections you described? Seems to me that the thing is going to takeoff before it planes so I'm not seeing the benefit of the flat sections-what do you think?
     
  5. Berth
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    Berth New Member

    Hi Doug,

    We´ve performing an Axeman 6 around here in Germany that is pure semi-circular and we couldn´t find any disadvantages about other hull types in any conditions. Andy P. has done a good work with that boat. This hull does not plane other modern moth hulls do neither). More square sections provides more stability - but in a range you can ignore IMHO. Ian is right: most of design parameters are due to moth sailing technique and lightweight building. In theory you can change a lot - but for every advantage you have to pay a disadvantage.

    Yours Berthold
     
  6. Wardi
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    Wardi Senior Member

    Berthold is correct!
    Semi-circular, least wetted surface sections will give the lowest resistance. The main reason this is not used in Moths is the lack of stability, it would be quite OK for a larger hull where this is not an issue.
     
  7. Doug Lord

    Doug Lord Guest

    pre takeoff foiler design

    Thanks to both of you for shedding some light on this important aspect of foiler design that we really haven't touched on until now....
    =========
    Regarding rocker, it's interesting to look at the design of the main hull on the Rave-loads of rocker and a "normal" transom.
    =========

    When the Rave takes off it does so with little or no pitch up.
    On my F3 rc model using the same foil system(but different foils) it sits at static at about +2 degrees pitch up but sails on foils completely level-the nose comes down about 2° after takeoff.
    I'm trying to understand better the difference in pitch attitude at early takeoff between the Moth and the Rave/ F3 with the Moth sailing at a fairly high nose up attitude early.
    The Rave /F3 set up is +2.5 degrees on the main foils and 0° on the rudder foil.(both Rave/F3 and Moth have the angle of incidence referenced to the nominal static waterline-at least as best I can tell on the Moth). The F3 takes off in a 5-6mph wind and the Rave takes 8-10 but both tend to lift off straight up with no pitch up at all.
    From what I've gathered on John's boats the main foil with an asymetrical 63412 section is set at 0° and the rudder(I think) is a symetrical section set at 0°. Looking at Abbott and Doenhoff the lift coeficient of the 63412 at 0° is approximately the same as a symetrical foil at 2.5°.
    So why is there such a nose up attitude to the Moth? I understand why technically with the settings above but I'm wondering why from a design standpoint. If the main foil on the Moth could be adjusted to a +2 or +3 angle of incidence by moving the whole board/foil combo(no adjustments to the wand)with the rudder foil left alone then it seems as though the drag on the rudder foil might be reduced with the boat at a more nearly level attitude.As speed picks up the board/foil combination would be "shifted" to a lower angle of incidence.
    We've talked about this a bit before but not ,as best as I can remember, with reference to the angle of incidence of the rudder foil. Edit added: And referenced to the angle of attack of the rudder foil vs the main foil...
    Comments would be appreciated....
     
    Last edited by a moderator: May 15, 2005
  8. jimb
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    jimb Junior Member

    main foil angle of incidence

    The Moth's relatively tall rig means that the rudder foil spends a lot of time keeping the bow up to overcome the sail pressure rather than supporting the weight of the stern. It runs at negative incidence a lot of the time and this may have influenced the design concentrating the lift on the centreboard foil. The mast height is something like 3 times the distance from the centreboard to the rudder. Maybe Ian Ward's work with bow and stern foils would give a different relationship between main and stern foil angles, this distance is about 2/3 of the mast height.
     
  9. Doug Lord

    Doug Lord Guest

    mainfoil-rear foil

    I know that on the Rave and F3 the loading on the rear foil changes the more wind there is. The nominal design loading of the rear foil(at minimum takeoff windspeed) is 20% of the all up weight of the boat. But as the wind comes up gradually the rear foil becomes less loaded until finally it pulls down-all automatic with almost imperceptable change to pitch trim(and no alteration to the rear flap).
    On the Moth ,having only seen pictures and video I would imagine a similar thing happens and you can see a pitch change from light to heavy air. [In many pictures of the Moth it is clearly very nose up which means (assuming the settings mentioned before are correct) that the rear foil is operating at a positive angle of attack;ie,lifting.]
    Assuming that the loading on the rear foil of an Ilett Moth may be 20% of the weight or maybe more with a crew induced bow pitchup to facillitate early take off(minus the pitch loading of the rig) I'm wondering whether or not the rear foil needs to be run at the high angle of attack caused by this crew induced pitch up or if instead the foil/daggerboard combo could be given a greater(than 0°) angle of incidence (for takeoff up to ,say, 15kts where it would be shifted back to zero or nearly zero)so the crew would not have to induce such a high pitch up allowing the lightly loaded rear symetrical foil to operate at less angle with less drag?
     
    Last edited by a moderator: May 15, 2005
  10. Wardi
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    Wardi Senior Member

    To explain the need for high bow up attitude, I think you have to keep in mind that the Moth dinghy foiler has made a real breakthrough in low speed takeoff with hydrofoils. Windspeed of around 6-8 kts is the lower limit today.

    Both Rave and trifoiler have much more power and righting moment but need 10-12 kts of wind to take off and boatspeed at takeoff is around 10kts.

    To get enough lift from the foils on a Moth as such low windspeed and hull speed, they need to be at a large angle of attack initially. It also helps to utilise leeway to induce more lift over the foils as well, hence the benefit of windward heel.

    To initiate the angle of attack it simply lean aft a little and/or put some negative lift (suck) on the rudder foil to induce the bow up attitude. This is exactly how aircraft take off!!

    As this is done so easily, I do not see the benefit of having an adjustment to maintain level attitude at takeoff and re-adjust when up an running. Far better to use a mechanism which is simple and required no manual intervention.

    An alternative, if you want level running at take off is to have a fully articulated lifting foil as I have previously proposed.

    Larger foilers and models have far more relative power available and therefore have relatively larger foils at take off, so the need for such high incidence angles is not so apparent. The foils on Trifoiler however have very high incidence, but you don't notice this as they are fully articulated.
     
  11. Doug Lord

    Doug Lord Guest

    takeoff

    Ian, the Rave has 56sq. ft. SA per sq. ft.of foil area compared to the 78sq. ft. for the Moth ,a higher foil loading and a "sail loading" of 2.77 lb.'s per sq. ft. compared to the Moths 2.69 so it's logical that it takes off later. You can't (practically) change the Rave's pitch attitude at takeoff by crew weight since the crew sits all the way aft already!Or by rudder flap because the transom would drag too much. The nose up pitch attitude I've experimented with on the F3 definitely results in a quick takeoff and then an almost immediate drop in pitch attitude.
    I just can't help but think that if the angle of attack required for a quick take off was achieved by changing the angle of incidence of the main foil so the rudder foil was not lifting (much)that takeoff would be quicker and low end speed would be better because of the reduction in induced drag from the rudder hydrofoil; I'm going to do some experiments with this just to see what happens when I can.
    The gain is possibly even quicker takeoff and higher speed in lower wind; the disadvantage is, as you say, the greater mechanical complication and the need to shift as speed increases.
     
  12. Wardi
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    Wardi Senior Member

    So why not use a fully articulated main lifting foil?
     
  13. Doug Lord

    Doug Lord Guest

    theory

    Well ,if the theory is correct then a fully articulated foil would be one of the solutions . I'm concerned about how it(fully articulated foil) would work with a wand and whether or not it would be strong enough particularly in intentional jumping with the boat.
    But either that solution or a shifting board/ foil combo might be a good answer.
    But do you think the theory is correct in terms of an improvement in take off and "low speed" foiling by reducing unnecessary drag from the rudder foil?
     
  14. Wardi
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    Wardi Senior Member

    Sorry to dissapoint you, but one specific advantage of the fully articulated foil is that it will not be possible to "jump" the boat. My aim was to keep it in the water, low, fast and under control, not flying out.

    I have always used fully articulated foils. Stainless steel joints are strong enough if engineered right. I note that George Chapman also uses these on his Ceres cat foiler,
    http://homepages.rya-online.net/ejcchapman/ . Fixed foils like Airchair would be far easier to jump!

    With respect to take off and low speed foiling, I think we are breaking new ground!
    John Iletts method works really well, but I am sure it can be further improved. Here we need the help of some good hydrodynamicists.
     

  15. astevo
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    astevo Junior Member

    i know ive been out of the discussion for a while, but thought id jump back in.

    if we are looking at minimising the take off speed for moths we have to look at maximising lift, by either maximising camber of increasing angle of attack.

    the problem with the current boats is that in marginal conditions we get maximum lift by sinking the back and maximising angle of attack. Ideally we would just pivot the foils to maximum aoa and keep the boat level in order to minimise drag. with the nose up attitude the camber of the main foil is reduced due to the sensor, and the rear foil is flapped up. so we are really trying to get maximum lift from sections with the minimum effective camber. thats why the fully pivoted foils have such appeal. the only problem is the engineering. my calculaations require about a 18mm stainless shaft, which would be a real ******* to fit in the vertical foils. the i-14s use titanium shafts, but i feel this would be some issues with the toughness and fatigue of the more brittle material
     
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