Will this hull work for a 12ft beach cat?

Discussion in 'Multihulls' started by ahen, Mar 1, 2017.

  1. ahen
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    ahen Junior Member

    I made a forum post earlier giving some details about a catamaran I plan to build (http://www.boatdesign.net/forums/mu...ran-design-construction-57203.html#post797305)
    I posted a preliminary design and got some advice on how I should change it. I has some misconceptions, so I feel like making a new thread that is more clear.

    Right now my design is a double chine (I think), displacement hull that I plan to fabricate using stitch and glue with marine plywood. My design is vertical on all sides which I believe will be the easiest to build. Here are some specs:
    Length Overall and of the waterline: 12'
    Beam: 1'
    Draft: 18''
    Buoyant force: ~3000 N if fully submerged
    The prismatic coefficient is about .72, but that includes all the freeboard.
    Pics attached below

    Once I finish the hulls I plan on connecting them with 6' wooden beams that will be the main structure of the craft, from which the center board, rudders, and mast will be attached. I plan on moving forward with that stuff after the hulls are complete. I want to learn as much as possible so I have the greatest chance of success, so any tips are appreciated if you think I am missing something.

    Thanks
     

    Attached Files:

  2. upchurchmr
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    upchurchmr Senior Member

    That's a single chine design FWIW.

    You don't ever want to drive or load a catamaran where the hull is submerged to the deck line. It's too easy for a small wave to go over the bow and stop the boat immediately.
    Then the boat rotates around the bow and ends up upside down.

    So normally you would set a "waterline" where the boat will float at rest, including the sailor, etc.

    This will work, its a very old style design. Depending upon where the waterline is it may not turn very well.

    Will you have a centerboard for lateral resistance?

    Any reason for selecting 6' beam? Most beach cats are 8' because the width gives you more stability from being blown over by the sail.
     
  3. ahen
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    ahen Junior Member

    It is not meant to be submerged to the deck line, that would only happen if there was a mass of ~620 kg resting on both hulls. Looking at other catamarans of a similar size they had a mass of about 100kg and a sailor is about 70-80 kg at most, so the waterline shouldn't be that close to the deck.

    I'm thinking the centerboard will make sailing up wind and turning easier, so yes it is for lateral resistance.

    As for the beam I read that 2:1 length/beam ratio was a good approximation but I don't see a reason why it can't be 8' if that adds stability.
     
  4. Rastapop
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    Rastapop Naval Architect

    The longer you make the beams the stronger they'll have to be, and the stronger they are, the heavier they are.

    How fast are you expecting this to typically sail, realistically?
     
  5. ahen
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    ahen Junior Member

    Its not for racing, its just for fun so it doesn't need to be super fast. I don't have a set speed in mind, but faster is better.
     
  6. Rastapop
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    Rastapop Naval Architect

    I didn't ask how fast you want to go. Everyone agrees faster is better.
    How fast do you expect to go? Or even just plan to go?
     
  7. ahen
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    ahen Junior Member

    My expectations are low, I don't see it going very fast. I don't really have any plans for the result because I have never built a boat before; this will be a baseline if that makes sense.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Hardly seems worthwhile when well evolved designs already exist. Instead of spending time trying to work the bugs out of your design, you could be sailing.
     
  9. CT249
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    CT249 Senior Member

    The bow looks to be very fine in plan form and lacking in buoyancy. Small cats are notorious for nosediving and this appears to have characteristics of a boat that is very prone to it. For example, the fairly wide stern means that even if you move well back when sailing downwind in strong winds, the buoyancy at the back will not allow the stern to sink to lift the bow.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    The bow looks sharp enough to cut a swimmer in half !
     
  11. Rastapop
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    Rastapop Naval Architect

    Then I wonder where you came up with the hull shape. Why does it have a flat transom?

    If you build it, it will (most probably) float, and be able to move at a speed in a direction close to forwards, but beyond that there's very little positive to say about it.

    EDIT: after reflection, I regret sounding so harsh. To rephrase, or perhaps clarify, I'd say that there's been no thought or decision making put into the hull. It's just a vaguely "hullish" shape, without meeting any particular requirements.
     
  12. guzzis3
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    guzzis3 Senior Member

    But then what would be the point of any of us designing building modifying or restoring our boats ?
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Restoring an old beach cat, or building a proven design would be a far better idea, looking at the OP's boat, I'd say this will end in tears. Do you think it will "work" as the OP says, as a practical exercise, guzzi ?
     
  14. sailhand
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    sailhand Junior Member

    have a look at this set of measurements for an arafura cadet catamaran they will give you some ideas as to efficient sailing hull dimensions and one that is easily built in ply.

    http://arrowarafura.com/wp-content/uploads/2012/01/arafura-cadet-restrictions-current.pdf

    they are flat bottom hulls and they are made in ply. there are no small sailing cats these days with a sharply veed keel except maybe assymetrical hulled boats like hobie 14/16. with such a small boat displacement should be maximised and the best way to do that is a flat wide keel panel. I would suggest a max keel panel beam of around a twelfth of your length and lean the hull sides out at 10 degrees. 12:1 beam length ratio will carry weight well in a small boat and still perform ok. high performance hulls may go as skinny as 20:1 but will not carry very much weight. I suggest you use a free software package that is easy to use like freeship. it is very easy to use and will give you a fair hull with all your hydrostatic calculations especially displacement. you can just draw the port half of your hull and the software will give all the information you need. they have a simple tutorial available to teach you how to use it. my grand daughter learnt how to use it. also go to a sailing club or shop where they sell beach cats and look at some hulls for pointers. there are no abrupt steps in there hulls as they cause heaps of drag. pm me if you want some technical help with your project.
     

  15. sailhand
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    sailhand Junior Member

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