Boat Hull Design Question

Discussion in 'Boat Design' started by Matthew777, Sep 17, 2021.

  1. Matthew777
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    Matthew777 Junior Member

    Below are the drawings of three boat hulls. The left two are intended to be made from alloy plate and have straight sides (Specialized Marine and COPE) . The one on the right is the Marissa Eco 18 a boat intended to be built from plywood.

    My question regards the round curvature of the sides of the Marissa. Is this primarily for ascetic reasons? Is there some hydrodynamic reason he designed his boat with that curvature? The only thing I can think of is that it would have slightly less whetted surface area, but at the expense of stability. I would think that if you were turning hard on the Marissa the non uniform chine would create unexpected turning results. Can someone give insight to this curvature?



    hull profile.PNG
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Are there any better drawing available ? Hard to make much sense of.
     
  3. jehardiman
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    jehardiman Senior Member

    Oh, help me Lord! As Mr E stated, not much to make sense of, but I'll take the off bet that the volumetric Froude Number to installed horsepower are very different between the hulls. And therefore Bob's your Uncle.
     
  4. wet feet
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    wet feet Senior Member

    I agree with the comments about the small amount of information to be gleaned from the screenshot.I suspect we will arrive at a discussion about conic shapes.
     
  5. TANSL
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    TANSL Senior Member

    I agree that little can be argued with the available information, but it must be said that none of the three options seem suitable for plywood.
     
  6. fallguy
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    fallguy Senior Member

    ...chine flats aren't going to cause any issue done right...drier ride sometimes

    ...vessels with finer entry typically ride better in a small chop

    but not sure I am willing to make any major guesses from looking at these pics

    The likely idea is once she is up on plane, the boat with the wider chine flats may be riding on a narrower section which can be done on less power (as indicated by hardiman), but they do not reduce the boat's volume for the exchange.

    To suggest Marisa has less stability is incorrect on the facts known. Are you assuming that less hull in the water at speed creates instability? I'd say that is a leap. There are too many factors to consider, deadrise being one. Planing boat stability is needed at rest; not withstanding stability underway of course, but a narrow(er) boat at rest is typically less stable, so the chine flats help keep her from rocking as well when drift fishing

    corrections welcomed
     
  7. fallguy
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    fallguy Senior Member

    I disagree. Plywood is versatile and would work fine for the 3rd one. But disagreements happen easily here on the wide open pandora box threads.
     
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  8. TANSL
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    TANSL Senior Member

    I do not think that the bow area on the side or bottom can be easily achieved by bending plywood but, indeed, the discrepancies are good and something positive always comes out of them.
    And a doubt assails me. Why could the third hull be made from plywood and the other two couldn't?
     
  9. fallguy
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    fallguy Senior Member

    Well, it is a ply design and used for a course on ply building. The chine flats are added build fun/complexity.

    As for the other two; not sure.
     
  10. TANSL
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    TANSL Senior Member

    Do you mean gluing thin plywood boards, one on top of the other, as if they were the layers of a FRP laminate?
     
  11. HJS
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    HJS Member

    Show us a normal line drawing and all data. Then we can make fair judgments.
    JS
     
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  12. Matthew777
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    Matthew777 Junior Member

    Fall Guy says: "The chine flats are added build fun/complexity."

    I thought the point of those chine flats was to allow the boat to get on a plane easier.

    What I am asking is the curvature of the hull on the right boat as compared to the two boats on the left. Looking from above the boat makes a nice swoop like the curve of a woman's hip. But the two boats on the left are dead straight.

    Is there any reason other than looks to make that curving swoop on that hull shape?


    Inkedhull profile_LI.jpg top.PNG front bottom.PNG
     
  13. fallguy
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    fallguy Senior Member

    Well, the internal volume of the Eco, ceteris parabis, could be larger that way, with a fuller bow. But it is largely conjecture. I don't think your curve depiction, red line, aft, is really accurate. A planing hull would not taper back.

    The chine flats are multi-purpose. Yes, they provide lift, but they also decrease wetted sa on plane and for the same wsa on plane, they increase stability at rest. Also, reduce spray.
     
  14. TANSL
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    TANSL Senior Member

    Thinking about the construction with plywood, I can see that the shapes of the side have improved and seem to be more suitable, but the twist that seems to exist in the bottom is a difficulty for plywood construction. Furthermore, from the middle of the length aft, it would be better to keep a deadrise and a chine width, constant. Imo.
     

  15. bajansailor
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    bajansailor Marine Surveyor

    Surely on all three of the hull forms shown you would have to do a conic projection to put some curvature in to the frame shapes below the chines so that the plywood will wrap nicely around these frames?
    Or cut out the frames as straight lines (rather than curves), and then fill in the gaps afterwards once the plywood has been wrapped around?
    Predicting plywood bending behavior for stitch and glue design https://www.boatdesign.net/threads/predicting-plywood-bending-behavior-for-stitch-and-glue-design.24551/page-2

    The Marissa Echo 18 looks like a nice seaworthy boat.
    Marissa Eco 18 Plans https://bandbyachtdesigns.com/marissa-eco-18-plans/

    B & B Yacht design - Marisa Echo 18.jpg
     
    Last edited: Sep 17, 2021
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