Proa design

Discussion in 'Multihulls' started by Tcubed, Sep 13, 2008.

  1. Tcubed
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    Tcubed Boat Designer

    Right now i have the ama wbeam of 0.8 M and lwl of 15 M with semi circular underwater sections from end to end. This gives a slenderness ratio of 18.75.

    As it comes out of the water it will be less and less affected by buoyancy forces so will tend to pitch less and less. By contrast, as the LH gets immersed under the transferred weight off the ama, it becomes more affected by buoyancy forces. However as it is almost empty of stuff and the weight of the ama acts upon the mast step it should have an excellent pitch response.

    Yes i just got michlet a month ago but haven't yet had time to learn how to use it. Any hints on how to transfer a shape from rhino or maxsurf into Michlet? I obviously must learn to be fluent with this program.
     
  2. sigurd
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    sigurd Pompuous Pangolin

    About that flexing. Howcome is it that regatta cats are requesting stiff beams? Probably it is faster. I can't really see how flexy beams will reduce the design load requirement a lot - if the difference in pitch for the hulls are big, the torque will still be transferred between the hulls. You also have a potential of getting non parallell hulls in yaw. Also I am not sure you can build a flexy beam as strong as a rigid one?
    Also, when the ama is in the air, I think it will go resonant or at least add a phase delayed pitch input to the main hull, since you apparently haven't thought out a method of damping the structure when unsupported by water?
     
  3. Tcubed
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    Tcubed Boat Designer

    Some excellent points.

    No i do not think resonance will be a problem as it would require resonant frequency that is close to values such as 1, 1.5, 2 etc. A final mass distribution calculation can determine whether or not there might be reason for concern. If it seems too close to a integer or integer fraction the weights (cargo) can be shifted slightly.

    You are right of course that fluid dynamic damping reduces to almost nil when in the air. But at that point the only pitching input would be from whatever stiffness is in the crossbeams, which i hope to be able to make very flexible in that direction only. There is also a small amount of damping from the inverted Y shroud arrangement. I see that it could possibly be a problem when sailing over long waves ie slow cycles, and the small amount of input could create pitch resonance in the ama. In practice though i don't expect to ever be consistently flying the ama, but rather be at 90 % say ideally so there is a buffer zone and of course a bit of damping.

    The yaw thing is not difficult to prevent as the cross beams , even if very flexible up and down will not stretch or shorten. The much greater engineering challenge is how to make the beams very flexible vertically but very stiff horizontally. So the hulls don't move for and aft relative to each other.

    I have not had opportunity to speak to other designers about the preponderance of rigid multihulls but a simple observation is that with the usual staying arangements it is essential to have a minimum rigidity to counter rig forces. In a tri or cat the rig will try to lift the stern of the windward hull, so if they were made 'floppy' the rig would collapse forward pitchpoling the windward hull while the leeward hull keeps its normal trim. In the case of the proa the shroud points to the middle of the ama and so the rigs trimming moment is transferred directly to the only hull that needs to resist this moment , the leeward one.

    Yes you can build a flexible beam just as strong as a rigid one . Those or two different things ; Young's Modulus of elasticity and breaking force.
    And if you do not force the hulls to move just the same despite not being in the same piece of water it seems obvious to me that there will be much less forces transmitted along the beams.
     
  4. sigurd
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    sigurd Pompuous Pangolin

    "Yes you can build a flexible beam just as strong as a rigid one."

    Yes sure, but it would be heavier, right? A stiff beam can make use of very tall shear webs, creating great strength with little flange material.
    If you want the same advantage, you would have to find some very stretchy flange material, or have unsupported parts of the tension flange take "shortcuts" (increasing opportunity for it to buckle when compressed), or deform the shear web, which make the stiffness of the beam lesser the more it is bent, opposite to what one would want (the flanges come closer together). None of those are cunductive to using as little as possible of any high strength/weight material, as far as i know.
    carbon tow is the ***** for strong things, its not that expensive either. but unfortunately for you it is very stiff!

    The most obvious way is to use a thin, large chord beam - it needs more material for the same strength as a tall one.
    If you have fore aft wander, cross two ropes in the middle of the trampoline or lay the net diagonally.

    "And if you do not force the hulls to move just the same despite not being in the same piece of water it seems obvious to me that there will be much less forces transmitted along the beams."
    So what?
    You still have to design it for the most load, which, i imagine, will not be less just because the hulls are skewed when it happens. When is that?
    It's analog to the bridle on the mast. (It should be either bridle, OR mast, prefer the former.) The hung ama or wide staing base looks superb until you take into account the rig being broken from the wrong direction. Then you have a pissepoor staying base - worse than a mono, but your righting moment is big as a multi. And the bridle now does nothing to help your akas either.

    The yaw thing. Did you try in your model to bend one of the beams and not the other? If I am not mistaken it is perfectly possible to ged yaw disalignment without shortening the beams. Unless your bridle can prevent this.

    I'm confuddled with the ama movement, it will be very interesting to see how the boat behaves! When you find out that it sails better on the other tack, you can just make the Y bridle a V bridle!
     
  5. sigurd
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    sigurd Pompuous Pangolin

    I am curious how you are going to make a flexible beam. What do you think about something like this?
     

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  6. ThomD
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    ThomD Senior Member

    What about the Gougeon approach to a beam, suspension? The have used it several times with success, F-40 Adrenaline Tri and the C-cats trimaran. Hard to know whether it was really and truly the future since it kept getting banned.
     
  7. keith66
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    keith66 Senior Member

    Saw a article in recent Yachting world magazine on L, Hydroptere the foil bourne tri, she utilizes suspension struts derived from Airbus landing gear on her main foils and this is reckoned to give her the flexibility not to break?
    Apparently it is the one aspect of the boat that the crew won't talk about!
     
  8. Tcubed
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    Tcubed Boat Designer

    Thanks for those tantalising tidbits, now where i might i find out more?..

    My idea is something vaguely similar to Sigurd's drawing except the 'holes' are rectangular, not round.

    Essentially, picture two planks, one above the other, spanning from main hull to windward hull. They are 'flat', such that their smallest dimension is vertical, and the second smallest dimension is longitudinal.
    They are separated by approx the same as they are wide.

    Then they are connected together in such a way that the planks' separation from one another cannot change, by struts every two widths or so. Imagine the struts are all individually pointing for and aft.

    This would be one crossbeam. The whole thing is duplicated for the other one.

    It seems to me that this setup would permit a considerable amount of bending and twisting yet have a decent breaking point.

    Also it is very easy to build and presents a very small frontal area.
     
  9. sigurd
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    sigurd Pompuous Pangolin

    In my drawing, the black lines are flanges.
    The red lines are hoops, they are connected to each other and to the flanges.
    The green stuff is solid stuff, cored fiber laminate or whatever.
    The idea is that the beam will be able to utilise a large separation between the flanges, for strength, but since the hoops can flex, it is also flexible.
    Now the key to it is that, when it is flexed enough, the hoops are squeezed into full contact with the green stuff. Ergo, at the ends of the flex the beam is stiff - it behaves as a tall, prebent I-beam, and the hoops are supported all the way so they don't get broken by overstrain.
    One would have to think some more on it or try it to see if it works as intended.

    With your idea each flange must be reasonably stiff as not to kink at the strut points, or buckle inbetween. Also you cannot utilise a lot of height (I think) so you need more material for the same strength.
     
  10. Tcubed
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    Tcubed Boat Designer

    I must say it is a pleasure to have intelligent input.

    Now that you explain the diagram you posted i understand what you were meaning.

    It has me thinking on methods of improving my conceptualization but without involving too much high tech materials and least of all complications or excessively elaborate construction methods.

    If one were to use tension limiters instead of compression limiters, maybe it would be easier to engineer it in a reasonably low tech way.
     
  11. Timothy
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    Timothy Senior Member

    Sorry to be off subject but Sigurd I couldn't help thinking that your idea would work well for flex limited battens.The battens would be free to assume thier intended shape in light winds and maintain that shape as the wind gains strength rather than let the sail become fuller as happens with conventional battens.
     
  12. James Brett
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    James Brett Marine Designer

    :) Hey All,

    Just to let you know of my 5 metre proa design.

    I am using a junk rig and ogive foil section rudders. The forward rudder is locked in a straight line to act as a centerboard while the aft rudder steers. The forward rudder can be lifted off the wind to reduce drag.

    The junk rig is highly under rated IMO. On my boat it can point to 50 degrees from true wind, the same as most una rigged dinghys. Also it gives a great safety advantage over the sloop rig on your design, as it can be reefed to any point, and does not require people to go out to the ends of the hull, ever.

    The junk rig can carry more sail area due to its easy reefing, this makes up for it's lack of efficiency in light wind. It can be shunted in about 20 seconds.

    Your design to me looks like it will perform well but won't be much fun to handle. Will probalbly also be rather expensive.

    Check out my website at: jbyachtdesigns.co.nz
    and see some onboard videos here:http://www.youtube.com/watch?v=3sDfiajL9gk&feature=channel_page
     
  13. sigurd
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    sigurd Pompuous Pangolin

    James - nice boat!

    Tcubed: tension limited? you could perhaps use a single transverse compression element, with struts sticking out perpendicular from it - holding rope for shear web and for tension "flanges".
    edit: It could be perhaps be lighter than a stiff beam, because spectra is stronger to weight than carbon, and it does not need epoxy. Wood is good, carbon is usd18/lb.

    Timothy . Cool idea. did you think, with a single, or double sail? the hoops looks like they would fit around a mast. Double skin junk
     
  14. sigurd
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    sigurd Pompuous Pangolin

    Here the center triangle is a backbone, the grey things are struts, and the blue stuff is rope
     

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  15. sigurd
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    sigurd Pompuous Pangolin

    one set of vertical struts, plus one transverse compression element. add ropes to your heart is content

    edit ofcourse, you must put longitudal battens over, and cloth over those, to decrease resistance.
     

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