fast but lightweight cruiser

Discussion in 'Multihulls' started by Gary Baigent, Dec 25, 2013.

  1. hump101
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    hump101 Senior Member

    Years ago I built a "tortured ply" hull that was actually carbon laminate skinned. The carbon skin hardly compounds at all, and it wasn't so successful as a hull, but nevertheless I had predicted the degree of compounding using an FE model of the flat skin and distorting it. The FE model was very accurate in predicting the tortured shape, I should have just believed it.

    I've meant to do a similar exercise with plywood, as I am sure it would give equally good predictive results, and then allow accurate flat panel shape development, but I've not built a plywood hull since. If someone wants to give it a go, I'm happy to do the FE side.
     
  2. rapscallion
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    rapscallion Senior Member

    It really is amazing how much stiffer and stronger thin sheets of plywood are when they are "tortured." The Kurt Hughes Cylinder Mold approach seems easier to pull off than trying to do both hull halves at once. I too would love to make a super light tortured ply racing tri, but for right now I'm working on F16 hulls. The first hull just exploded-flew apart in all directions. I'll give it another go, but these "stealth" hull shapes look like a good compromise if I can't get the stress ply to work for me.

    http://www.catsailingnews.com/2014/12/building-stealth-plywodd-foiler-by.html
     
  3. LP
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    LP Flying Boatman

    Another form of torture that I was experimenting with involves applying stringers to a panel and glueing them while the panel is bend the wrong way. Stringers are attached at panel edges on one side and along the middle of the panel on the other side. When the panel is straighten and bent the right way, the centroid of the materials try to align and force compound curvature into the panel.

    Here is a thread where I was working through the process. Pages 2 and 3 show the meat of the matter.

    http://www.boatdesign.net/forums/wooden-boat-building-restoration/15-lapstrake-kayak-40486-2.html

    I was playing with a lap strake idea, but the the process could be applied to simple panels as we'll. The edge pieces would need to be removed post assembly into the hull, so that adds a level of complexity or they could be worked into the design as spray rails and bump strips. I think that this process could work easily into the design that Rapscallion posted and would add the stiffness of compound curvature into the mix.

    Roughly speaking, the centroids of the stringers are attempting to align so I believe that 1" high stringers will put close to 1" cross panel curvature into the panel.
     
  4. Russell Brown
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    Russell Brown Senior Member

    I'm surprised to see how much interest and knowledge the is about tortured plywood.
    I need the encouragement as my beginning attempts haven't been going so well.
    After several tries I finished a scale model of a Tornado hull last night. Later while taking a shower I heard a loud bang and knew just what it was. Before it broke it was pretty impressive, so I'll keep at the model building.
    According to Meade, the models both scale up accurately and bend the same (using the table of thicknesses in the book, so I'm following the book as I get started.
    Predicting the panel shapes by computer is something I'm interested in as well, but not having computer skills limits me.
    I did (with my friend Brandon) digitize the tornado hull, lengthen it, and then pump it up in volume to come near the displacement I need in a main hull.
    We then unrolled a hull side in a program called Squash (I think) that's meant for predicting formed metal parts.

    Hump, I'd like to know what FE is and how you used it.
    I'll learn a lot from the models about what's possible, but maybe I could also learn what the limitations and uses of CAD are for predicting panel shapes.

    Really glad you liked the video Gary. We live in a good spot too, but just had 5 days of snow. We don't do much sailing in the winter here.
    Wish I had met you when I was in Auckland. I was on a mooring in Devonport and working at the AC base. I think it was early 02.

    Thanks again for the encouragement,
    Russell
     
  5. waikikin
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    waikikin Senior Member

    Hi Russell,
    That Tornado power cat is really cool, like "Jetsons" style from the future. Are the ears for noise or cold or are they headphones? Beautiful area!
    Regards from Jeff.
     
  6. hump101
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    hump101 Senior Member

    FE is finite element structural analysis software. The process I used (still subject to refinement) is to define a flat panel with the appropriate anisotropic properties, then deform it into the shape required, applying appropriate boundary conditions (I used no membrane tension, but I think you could live with some membrane tension on the keel line).

    At this point you need to review the stress and strain distributions in the panel, and you should find a "neutral axis" of zero in-plane shear. You now need to iterate by moving the displaced locations on the panel (that is equivalent to changing the flat panel shape) until, in its deformed state, the sum of energies in the panel above and below the neutral axis are equal, so the internal forces are in equilibrium. Once this is achieved, the starting panel shape will be the required cutting shape, and you should end up with even compounding and no twisting or shearing in the hull.

    If you try to make a shape that won't compound, then you will see kinking in the edges of the panel between supports, or the minimum bending radius of the panel will be exceeded, or the model won't solve, so you can easily determine what is feasible and what is not.

    We use Ansys FEA software, which has its own programming language (APDL). This is really handy as you can code some automation in moving the points around, otherwise the iteration stage is really tedious and error-prone if done manually.

    As a practical analogy, the above is like starting off with a big rectangular panel, bending it around a former, then gradually trimming it back bit by bit and re-bending it until you have the desired shape. You have to do it iteratively because the excess material around the edge changes the compounding characteristics of the panel. With the FE model you can skip one half of the iterations as the panel size is always correct at each iteration, just not distributed correctly.
     
  7. Russell Brown
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    Russell Brown Senior Member

    I'm starting to get my head around the tortured plywood thing.
    I have made one successful scale model of a Tornado hull after a couple of failures.
    I wanted to start with the Tornado hull as I could start with a known panel perimeter shape and a known finished hull shape. At the scale I'm working at (the model is about 1 m long), I'm seeing an accurate shape using 1 mm plywood.
    Next, I'll start pushing things around to see if I can get the displacement I would need in a trimaran main hull.
    Hump, I would like to try reverse designing for predicting the panel shapes. If I made a model I liked and could digitize the panel profiles and finished hull shape I could then see which program would most closely predict the panel profiles. Does this sound good?
    It's going to take time as I have a business to run and am building a boat for the race to Alaska (as a job, not for me) at the same time.
    Russell
     
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  8. Richard Woods
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    Richard Woods Woods Designs

    bit of a give away! Is this the Turnpoint boat?

    No snow down here in Mexico

    Richard Woods (you don't have to answer!)
     
  9. Gary Baigent
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    Gary Baigent Senior Member

    What type of boat will succeed in the race to Alaska, Russell? Combination sailing trimaran rowing design?
     
  10. hump101
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    hump101 Senior Member

    Yes, that would be my preferred approach - validate the theoretical prediction with a known successful hull. I was going to try it with the Tornado hull, as the data already exists, but never got around to it.

    I'm also running a business (I'm an NA/Structural consultant in the offshore energy industry) so also can't promise a rapid response, but will do my best.
     
  11. rayman
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    rayman Senior Member

    Gary, are there any new developements on the "fast backpacker cruiser" or has it been shelved?? I have made a folder of SID's build pics, you make it look so easy but it's hard to get a perspective of size just with the old hen for comparison. I can see the two sheet joins with a small piece spliced on the bow and the sheet width at the ring frame so about what is the hull depth at the frame and under the beam?? about 3ft?? and what will the new cruiser's volume be in that area, I hope you have'nt given up on it. (the cruiser that is)
     
  12. Richard Woods
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    Richard Woods Woods Designs

  13. Russell Brown
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    Russell Brown Senior Member

    I have no idea what will succeed in the race to Alaska. That's one thing that makes this race so interesting is that no one knows. There will be a steep learning curve in the first race for sure and hopefully enough different boats to show us what works.

    I do think the trimaran is the ideal platform Gary, because of the human power element.
    Having the displacement and wetted surface in one hull has got to be better at very low speeds. I picture a fair amount of light air sailing and motor sailing too. By motor sailing, I mean using human power to augment drive from the sails, which can be magic in really light airs.
    It seems like petal drive units are going to be the preferred method of power in the multihulls.

    The boat I'm building for the race is a Bieker designed plywood proa, but it doesn't look like Paul or myself will be able to race this year, so a couple of I-14 sailors will likely sail this race.

    Hump, (is that what I should call you?), thanks for validating my thoughts on the reverse designing from models to CAD. I would very much to see the results.

    Regards,
    Russell
     
  14. hump101
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    hump101 Senior Member

    Hump is fine, though my name is Guy Hawkins. Hump is a nickname from the days before powerful laptops when I used to travel around clients with a workstation built into a backpack.
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    For you Russell. This is a drawing I did a couple of decades ago for Jim Young of a design he was doing for Digby Taylor, after he retired from Round the World racing, to take tourists out on Lake Rotorua or Rotuma or one of the mid North Island lakes. The catamaran was built and is still in use, I think. I helped Digby set up the frame moulds. Just thought I'd inform you that Digby and Jim beat you to it. Just joking. Your power cat is a beauty.
     

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