Monohull sailboat hull shape

Discussion in 'Hydrodynamics and Aerodynamics' started by Ivanthefool, Aug 2, 2014.

  1. Ivanthefool
    Joined: Aug 2014
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    Ivanthefool Junior Member

    Go easy on me here, I know very little. I have read that the hull shape with the most volume and least wetted area is a semi circle. And the bow has to have some fineness in it to pierce the water. I have seen lots of hulls with a vertical transom. So my question is, other then increasing internal volume, is there any reason why you would not have a hull with identical buttock lines all the way from the transom up to where you start tapering to the bow? Is there any reason hulls narrow down from the fat part near the back and the stern is narrower? Talking about a monohul sailboat, or maybe main hull of a trimaran?
     
  2. gonzo
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    gonzo Senior Member

    Yes, they are many reasons. For example, the initial stability of a cylinder is zero. Another is that wave resistance in the bow and turbulence/negative pressure in the stern are not equal, so it is possible to have more displacement aft. The theory gets rather involved. A book like Gerr's "The Nature of Boats" is a good starting point for an overall view of the concepts.
     
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Ivanthefool's question is very interesting because, indeed, if the heeling the boat does not substantially change the profile of the water line, that can be beneficial. Thus it would be important to know the reasons, or some, why this theory is not used.
    I have always believed that the designer was trying to get the lowest possible variations in the water line, shape and área, when the boat heels.
    I'm not sure I understand what is meant by"initial stability" and, whatever it is, I don´t understand that it can be null. That boat probably would have a bow and stern like any other. So I do not understand the phrase "in the bow wave resistance and turbulence / negative pressure in the stern are not equal." Any further explanation?
    Is an issue, again, interesting. Any expert can give reasons, certain, pro and con?. Thanks a lot.
     
  4. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    I didn't understand the question. :confused:
    A picture or a drawing would be useful.
     
  5. Ivanthefool
    Joined: Aug 2014
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    Ivanthefool Junior Member

    Rough sketch

    Here is a drawing. Obviously there has to be a keel, rudder etc and the hull should get finer at the bow end in more of a curved transition but Paint is not that sophisticated. Idea is to build in aluminum using rolled plate for hull before the waterline, probably on a 1.5 meter radius
     

    Attached Files:


  6. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Ok, now I get it, I hope. You are talking about two distinct issues there:
    1) the difference between a canoe stern and a wet-transom stern. What you have drawn is a wet-transom stern (meaning: submerged).
    2) the best shape of curve of areas for the assumed design speed.

    A short answer to your question is that the lower the design speed and/or available power, the gentler-sloped curve of areas will be required. It means that the transition from the midship to the stern will have to be gradual and smooth. Conversely, the higher the speed, the more straight the buttock lines are required in the aft parts of the hull.

    The hull you have designed will have a high drag at low (displacement) speeds, but will likely perform better at high speeds, when (IF!) the transom becomes dry. So it is a matter of design goals, what you want to optimize - including the construction costs and complexity. The blanket will always be too short.

    Check this excellent primer on hull design written by Eric Sponberg: http://www.sponbergyachtdesign.com/THE DESIGN RATIOS.pdf - it will help make some things clearer for you. After you have finished with that, you might want to read these discussions:
    http://www.boatdesign.net/forums/hy...nce-factors-planing-hull-low-speed-35799.html
    http://www.boatdesign.net/forums/multihulls/submerged-transoms-give-higher-speeds-22541.html
    http://www.boatdesign.net/forums/boat-design/planing-hull-disp-speeds-31464.html
    http://www.boatdesign.net/forums/bo...t-hull-hp-displacement-hull-speeds-40536.html
    And then, of course, books about boat design! :)
    Cheers
     
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