more on diagonals

Discussion in 'Boat Design' started by BASIL J WALL, May 12, 2012.

    Joined: Nov 2011
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    BASIL J WALL designer

    Hi folks...
    I have a hypothesis...
    This applies to a displacement hull with D/L ratio about 300 and LWL about 40' ...It questions the shape of diagonals in designing the fastest hull all else being equal...
    If diagonals are set up so that they are at their greatest curvature at midships and then straighten out towards the ends ...and assuming that the hull is fair...then this will produce the fastest hull of this type compared to a similar hull with the greatest curvature aft of midships or forward of mid ships...What do you think?
    Another related question...How well does the shape of diagonals approach the actual flow of water around the hull? Does anyone have some scientific evidence for this?
  2. Tad
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    Tad Boat Designer

    Problem is it never is..........;)

    What is "fastest", do you mean having the least resistance? At what speed? Combinations of differing characteristics will provide a hull that is "fastest" at differing speeds. No one hull does it all.

    Your hypothesis is built on old wives tales, namely that diagonals (a surface fairing tool) actually have some hydrodynamic value.....They do not.
  3. philSweet
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    philSweet Senior Member

    There is some basis in theory on that one. But has nothing to do with diagonals per say.
    It turns out that a parabolic waterline works out well with regards to Michell's integral for wave resistance, and you can pretty much get a decent hull by revolving a parabola about the boat's centerline and squishing it as you see fit (to generate a fore/aft symmetric paraboloid). That gets you reasonable prismatic coefficients as well. Not quite ideal, but not far off. The theory ignores near field waves though. Leo L. could fill you in on that business.

    (I hate it when it does that... I'm in spell check and the darn thing posts when I edit a word. Anyone else have that problem???)

    edit... So it's not a paraboloid, rotation is about the wrong axis. I don't know what that shape is called. That's what happens when the bloody spell checker preempts me.:(
    Last edited: May 13, 2012
  4. gonzo
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    gonzo Senior Member

    Not really, you can have sharper curves and more displacement aft than forward. Also, the diagonals don't indicate what the water flow in the hull is. Unless you have experience with the type of hull you are designing, the diagonals alone won't give you much information.

  5. PAR
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    PAR Yacht Designer/Builder

    I disagree, diagonals (a fairing tool) if read by someone with considerable experience looking at diagonals, can offer quite a bit of information about the qualities of the hull.

    My issue are the original poster's premises. It has a a high D/L and is constrained by hull form (which assumes one is established), limiting it to speeds governed by it's LWL. Knowing this; select a set of coefficients that will permit this speed, possably with potential for more, as the penalty for being too low, is you'll never get to the desired speed or only so, at the very end of her abilities, which is an easy way to kill propulsive efficiency.
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