planing pods/skis/amas

Discussion in 'Multihulls' started by sigurd, Dec 12, 2004.

  1. sigurd
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    sigurd Pompuous Pangolin

    I think some special speedsailing boats use skis that are free to pitch, and some like the yellow pages use high AR pods that are locked in pitch.
    In which circumstances would you use which alternative?

    There has been some discussion here about tris with vakas optimized for displacement speeds and amas that are optimised for planing. I guess that if only one high AR ama is used for each side of the tri, some means of pitch stability must be implemented, foils?
    Or maybe use a fore and an aft ama on each side?
    Otherwise I think the ama would be so long that it would have a lot of skin friction.

    The yves parlier boat uses stepped hulls but these are compro/opti-mised for both displacement and planing.
     

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  2. Doug Lord

    Doug Lord Guest

    how fast is fast?

    Foils on multihulls especially those with a trimaran configuration allow the amas' to be very small since they don't have to fly the main hull but more importantly they can be used to develop the righting moment for the boat, like the Rave, Hobie trifoiler and the 40'SCAT.
    Planing ama's seem to have a lot to offer above a certain speed but it's hard to beat the virtually unlimited power to carry sail of the independent altitude control systems on Bradfields or Kettermans boats. The bigger the boat using these systems the more structural problems to engineer it right.
    For ultimate speed planing hulls seem to be the way to go but "hybrids" like Parlier's boat may have the worst of both worlds since the stepped hull is draggy between 10 and 20 knots more or less. Because of the stepped hulls foils are required to control the pitching moment of the boat which limits top speed. On seaplanes with stepped hulls the elevator on the plane controls pitching-something has to because stepped hulls are unstable in pitch....
     
  3. sigurd
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    sigurd Pompuous Pangolin

    huh?

    Can you elaborate why stepped hulls are unstable in pitch?

    I thought a stepped hull is where there are two planing surfaces, one fore and one aft, like the parlier cat? It seems to me it could be as stable as an unstepped one, since you can have one contact surface far forward, and one all the way aft.

    What I also am curious about is whether there is any merit to free-pitching, planing units, (skis, pods, short hulls whatever), for instance used as "front legs" in a tri.
     
  4. Doug Lord

    Doug Lord Guest

    stepped hulls, "free pitching planing units"

    As to stepped hulls being unstable in pitch one article I read was in Seahorse magazine earlier this year where a member of Parliers team used that fact to explain why the boat had rudder t-foils fitted(if I find the issue later I'll post it). Another good article on powerboat applications of stepped hulls is in Issue # 88 of Professional Boatbuilder which talks about a unique stepped hull solution to enhance controllability by using an aft "stabilizer". (idea by John Plum)
    The idea of "free pitching" planing units might have some value depending on the specific application; what did you have in mind? More importantly, exactly what are you trying to achieve with a "free pitching " planing unit?
    The Gougeon brothers designed a trimaran(Adrenaline) that allowed the ama to pivot in pitch and they said it worked well. The problem I see on a "free pitch" planing system is how do you keep the rest of the boat under control in pitch? If you use foils you've probably limited your top end speed and you've already affected the lower end by using planing hulls in the first place.
    It all depends on what you're trying to achieve....
    ===============================
    Here are a couple of excerpts from the article in Professional Boatbuilder by Eugene P. Clement:
    "This interestng result shows an inevitable shortcoming of a stepped hull in which the needed lift aft is provided by a fixed afterbody rather than by an adjustable stabilizer. The designer in the former case must locate the afterbody in a compromise position, suitable only for some conditions of weight,CG location,
    and speed. For most running conditions, however,the afterbody will inevitbly be in the "wrong" position, giving a running trim angle that results in less-than-optimum performance,and possibly porpoising."
    ---
    " In short, the tests showed that a suitable stern stabilizer allowing operator control of the trim angle will not only offer minimum drag at different loading conditions, but will also overcome the porpoising problem so frequently encountered by stepped boats."
    ===============
    The interesting thing about Mr. Plums stabilizer is that it was a planing section that was raised up and down(and remained level as the boat banked); the t-foil may work better on a hull like Parliers that has a large force that could tend to lift the transoms....
     
    Last edited by a moderator: Dec 14, 2004
  5. sigurd
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    sigurd Pompuous Pangolin

    Let's say I were to design a small ocean going triscaph - three short, wide planing hulls like Seaspider or Yellow Pages Endeavour. I could make them free in pitch or I could lock them relative to the frame. on flat water the pitch of the locked one would be relatively constant independent of speed. Not sure how the freely pitching hulls would/could behave though.
    From powerboat experience I think that it would be desirable to have a low AoA to get planing - after that some increase (how much) in AoA gives a lower surface area and thus less resistance. I do not know whether such a trait could be built into a freely pitching hull - my powerboating experience tells me that the opposite generally happens.

    Then there is the issue of waves. a locked triscaph ama would change its center of lift and "pitch relative to the water" while a pivoting one could be more constant in AoA and thus more efficient?
     
  6. tspeer
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    tspeer Senior Member

    This is a popular belief, but I think a mistaken one. Dave Keiper started out with small amas, and he capsized the boat three times - each time he was moving slowly and got hit by a gust. If you look at his book, over time the amas got bigger, and bigger.

    There are four regimes to consider: light wind sailing hullborne, flying in moderate winds, sailing hullborne with the foils down in heavy conditions, lying to a parachute anchor in survival conditions. An ocean-going foiler needs to be able to cover all of these conditions. The foils allow you to optimize the amas more for stability in heavy conditions without being as concerned about performance in medium conditions. They allow the main hull to be optimized more for light wind conditions. And the hydrofoils can be deployed or retracted to allow the hull to "change gears" just like changing sails does for the rig.

    But it's not a good idea to drastically reduce the size of the amas.
     
  7. Doug Lord

    Doug Lord Guest

    reducing amas

    I agree that on ocean going foilers serious consideration has to be given to how much ama size can be reduced but I think the size can be reduced from the size required to efficiently fly the main hull on a 60 footer(for instance)-but it definitely has to be looked at closely.
    On smaller multi foilers I think the ama's can be reduced more drastically but they still have to be able to take a gust when the boat is off foils w/o an immediate capsize. I think Dr. Sam has got it about right on the Rave.
    One of the most important factors here is overall beam which with a Rave type foiler can be substantially more than a conventional tri. The extra beam allows a smaller ama to have an effect on stability and gust responce off the foils comparable to a larger ama closer into the main hull.
    On the rc foilers I've designed I found that an ama with a total buoyancy about 85% of the boats displacement works good in the gusty and shifty conditions prevalent on small lakes/ponds. Overall beam of the F3 is 72" with an LOA of 56".
    So I think it's fair to say that a multi foiler can have smaller amas than tri's designed TO FLY THE MAIN HULL but their size has to be looked at carefully especially for ocean use.
     
  8. Phil Locker
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    Phil Locker Junior Member

    stepped hulls

    The above article, plus the development of Yves Parlier's boat, and the growing popularity of T-foil rudders in the I14 class have all been rolling around in my head, leading me to wonder if a stepped hull with T-foil rudder might work on a small skiffy dinghy. The step would likely have to be aft of the daggerboard to prevent excessive ventilation.

    I'm thinking in terms of a single hander about 11' long, while not aiming to fit in the box that defines a moth. I'd think this would lend itself to having your weight further forward (I14's are just about trapezing off the transom now) and a minimum wetted surface while planing. One question that comes to mind is - how small can the step be and still be effective? For the unfortunate times you're in displacement mode you'd want as long a waterline length as possible (such a short boat) but if built very light you will be drawing very little water, and hence the vertical height of the step will be limited by that alone. Just thinking out loud a bit here...

    A bit of a doodle is attached.
     

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  9. Matt Lingley
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    Matt Lingley Junior Member

    Wow that looks cool! Can see how in very light winds it might even work well as you could sit right forward and lift the back section right out of the water to reduce wetted surface. Would the step really make much difference to waterline lenghth though? I would have thought that the biggest problem it would have would be very similar to having really bad transom drag at displacement speeds.
    I guess being a wide boat it will have less of a hump to get over between displacement and planeing than Parlier's cat.
     
  10. Doug Lord

    Doug Lord Guest

    steps and speed

    Phil, very interesting. One of the things Mr. Plum proved in his early experiments referenced in the article above was that the step could be very small so your idea may be worth looking further into. I have experimented on models with the idea of pumping/pulling air into a openable slot at about the same position you show the step with the idea of ventilating the back end and reducing drag. No clear results but I know of experiments like this that have been tried on full size boats with good results.
    It seems to me, though, that in the speed range
    of around 10mph to 30+mph fully flying foils would be almost untouchable. Moths already fly in 7-8mph of wind and that speed(takeoff windspeed) is headed lower. Rohan Veal has been clocked at over 21mph on foils(11' Moth) and that speed is headed up.
    Foils seem to allow a hull design that is optimized for lite air and I'm sure some future foiler incarnations will feature fully retractable main foils.
    ============================
    Phil, speaking of foils , have you built any hydrofoils yet? Interested?
     
    Last edited by a moderator: Dec 19, 2004
  11. gggGuest

    gggGuest Guest

    Steps and Moths

    the Moth box is fairly big, unless you are going for big rigs or tunnel hulls its quite a challenge not to fit it!

    As I remember there was at least one Moth back in the 60-s witha stepped hull, but only a small one, about 6 inches off transom or something...
     
  12. Doug Lord

    Doug Lord Guest

    steps....

    Phil, I found this in the article above:"The testing also demonstrated the important point that a small step(which is desirable for reduced drag at low speed) will give satisfactory separation of flow from the afterbody bottom when used in conjunction with a hullform whose width decreases appreciably between midlength and transom. This latter feature, by facilitating separation of the flow from the sides of the hull, results in ventilation of the afterbody bottom with even a very shallow step."
    One of the succesfull steps they tested was 1/16"( 1.5mm) on a 10' model!
    Again, the article is in issue number 88 of Professional Boatbuilder(article by Eugene P. Clement) and this info was on page 84.
    I've been doing a lot of research on steps for a project and this one article has been a treasure trove-Plum was one of the unheralded early step pioneers in power boats..
     
    Last edited by a moderator: Dec 19, 2004
  13. Phil Locker
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    Phil Locker Junior Member

    Figure all up weight of boat + sailor at something well under 300 pounds. I'd have to refine the model and see where it sits on its lines, but with it properly trimmed fore/aft so that the sheerline is parallel to the waterline, I'd like to see the hull submerged from the knuckle at the bow right through to the transom. Too high of a step (and too much rocker) and this won't happen.

    You're much further than me in researching this. But as I said, that article got me thinking. I wonder if too small a step will make the boat fussy on fore/aft trim when planing. You wouldn't want the boat dragging its transom. But then I haven't been thinking about this so much as ventilating the back end as simply having a planing pad amidships that takes advantage of the lift off the t-foil rudder to reduce wetted area of the hull without the need for trapezing too far aft (as the 14s are doing).

    .

    No doubt. But fully foiling isn't the sandbox I want to play in.

    I wouldn't want to be restricted to the Moth rig limits. This is a blank sheet of paper approach just for fun. If I were to actually build this, I'd be flying a small a-sail off a short fixed sprit so I could play with the odd assortment of boats that come out to our Monday night "skiff" races.

    Sorry all, I didn't mean to hijack this thread.
    Phil
     
  14. Phil Locker
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    Phil Locker Junior Member

    I've done a few sets of the lifting foils themselves for I-14 T-foil rudders... both fixed (where the rudder itself itself is raked fore/aft to adjust pitch, as in Ovington's approach) and to be fitted with axles (a la Bieker).

    Haven't done a complete T-foil rudder yet, although one is in the queue.

    The small hyrofoils are a bit tricky due to scale. Requires a bit of planning to support them well after flipping to machine the 2nd side.

    If interested in pursuing, contact me offline.
    Cheers
    Phil
     

  15. sigurd
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    sigurd Pompuous Pangolin

    Usually stepped (one step) hulls are ridden on both the stern and on the bow right? or did I misunderstand and the stern is sometimes lifted clear off the water?

    Where do I start when deciding what AoA the planing surfaces should have?
     
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