Pod catamaran rocker?

Discussion in 'Multihulls' started by ChrisZ, Jul 23, 2019.

  1. ChrisZ
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    ChrisZ New Member

    Hello All,

    Long time lurker but first time poster, please forgive me if I'm asking a redundant question. I've been working on a Pod cat design for about a year and a half. I do product design and cnc manufacturing for a living and well it's getting time for me to start making some scale models of my pod cat. I've learned a lot in the past year or so but the bottom hull curve (I think it's called a rocker) still alludes me. From what I gather, it's not crucial but I would think a flat bottom cat would not surf well. I can draw what I think would work well but I would really like to be referring to some sort of proven formula or something. Does anyone have any links that discuss the rocker of a catamaran?

    Thank you in advance,
    Chris
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    A Pod cat ? Is what ?
     
  3. ChrisZ
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    ChrisZ New Member

  4. Mr Efficiency
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    Mr Efficiency Senior Member

    If you are going to have round hull sections throughout, rocker is pretty much inevitable, that give the most displacement for the least wetted area.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Rocker makes for easier tacking as well.
     
  6. ChrisZ
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    ChrisZ New Member

  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Designing boats is a holistic operation, a lot of balls have to be juggled in the air at once, and if you just concentrate on one "ball" (rocker), you risk dropping the lot. Boats have basically evolved to be fit for purpose, apart from the fashions of styling, the shapes are more or less dictated by practical considerations.
     
  8. ChrisZ
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    ChrisZ New Member

    No **** Sherlock. Now let me see if I can unsubscribe from my own post so I don't have to waste time reading your comments.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Point being that "rocker" is not an independent variable, it is just the description of a profile shape, nothing more.
     
  10. sailhawaii
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    sailhawaii Junior Member

    I'm no expert, but I would think having a higher CP would tend to surf more. That's why a high CP like .64-.70 tend to do better at speeds over hull speed and have more drag at less then hull speed. One of the ways to get a higher CP is to decrease your rocker (at a trade off of not tacking as quick). So to answer your question lower rocker will most likely surf better because the CP went up, but have more drag at low speeds. Unless your floats are double ended you will also need some rocker to get the stern out of the water. How much will be driven by your target displacement and beam to length ratio of your floats. So if you have targets for your CP, displacement and other ratios, you will not have a ton of flexibility on your rocker. So you may have to tweak your targets to get the draft and turning ability you want.

    Tony Grainger had a interesting article on rocker, but I can't seem to find it now.
     
  11. trip the light fandango
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    trip the light fandango Senior Member

    From what I can tell older designs have more rocker and seakindliness, rocker is dropped for speed, just get there quicker. The correct approach to ask a question is to give a set of requirements or whatever it's called so people can start juggling balls for you..Watson.. ha, or a question normally just sits unanswered, and welcome to the forum by the way. Intended use- sail grounds , voyage length is weight, and what your sea conditions you're prepared to sail in , all helps deduct a considered opinion.. Plowing into the wave in front of you can be an unpleasant experience, less rocker suggests lifting out of the hull speed earlier, hull shape,sail area dependant, hopefully the rudders are still in the water,,reserve forward/bow buoyancy above waterline.. then there's manoeuvrability required, yada , yada. The height of free board and how far above the waterline flare begins to dampen pitch and roll counteracts the need for rocker, but I could be wrong, ha, hope that helps.
     
  12. redreuben
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    redreuben redreuben

    Churlish and unnecessary.
     
  13. Doug Lord
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    Doug Lord Flight Ready

  14. InetRoadkill
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    InetRoadkill Junior Member

    How about a narrow front, round mid, and a flattish aft hull tapering back up to the waterline. From what I read, the asymmetry fore/aft helps reduce unwanted pitching. But I'm unclear if there's a significant drag penalty with such an arrangement.
     

  15. Richard Woods
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    Richard Woods Woods Designs

    startled by your comment to Mr Efficiency. So no point continuing the thread if you don't want to learn

    RW
     
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