Placement of chines in multi-chines

Discussion in 'Hydrodynamics and Aerodynamics' started by makobuilders, Jul 24, 2016.

  1. makobuilders
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    makobuilders Member

    When deciding where to draw the chines in with a multi-chine bottom on a small boat, does one generally try to take into account the streamlines so as to minimize drag?

    Would you use advanced fluid dynamic modelling software or is there a tried-and-true old fashioned way of drafting them in?
     
  2. baeckmo
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    baeckmo Hydrodynamics

    Are you talking about a planing hull or a displacement dito?
     
  3. makobuilders
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    makobuilders Member

    This is for a displacement hull. An honestly, at a typical cruise speed of 7 to 9 knots I doubt that the drag could add up, but still, I'm wondering. Thanks.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    This subject has been broached before around here, there is not a great deal of difference in form resistance between a double chine and a round bilge of similar proportions, I'd imagine subtle differences in placement of chines would make little difference either.
     
  5. makobuilders
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    makobuilders Member

    Well how can I argue about hull efficiency with a guy named "Mr Efficiency"??? :)
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, there would be several who see that as a misnomer, but I'd say the difference would be getting to the stage of not being worth worrying about. You could confirm that with test models.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Perform a few model tests in a VPP or resistance program. It all really depends on what you're after. If this is a well burdened cruiser, then it really doesn't matter a whole lot, on the other hand if two hulls were built to similar dimensions, the round bilge would certainly have an advantage over the other, particularly in certain condisions. Chines are typically employed so you can employ sheet goods in the hull shell. Flow lines are considered, but more importantly is the ability to have a developed surface, so sheet goods can be hung easily.
     
  8. gonzo
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    gonzo Senior Member

    Chines, particularly aft, are good to help a boat plane. However, they have more drag at lower speeds. Boats are like sleeping under a short blanket: either your feet are cold or your head; you can't get both.
     
  9. TANSL
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    TANSL Senior Member

    Trying to get the thread not go unwanted ways, I think makobuilders is not asking what they are or what the function of the chines, he surely already knows, but "When deciding where to draw the chines ... "
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Fair enough, but as indicated by PAR it has to be drawn to be developable, (ideally) and there is not unlimited freedom to alter the trajectory of the lines without turning it into a different boat altogether.
     
  11. TANSL
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    TANSL Senior Member

    Thanks for trying, Mr Efficiency, I do not know how they are defined and where chines are placed and I would be very interested to know. So I adhere to OP in his question hoping someone clarify those doubts. At this time, if it is a developable surface or not, I do not worry much.
    We will have to ask in the forum applied hydrodynamics for their theoretical foundation, what really they serve for and why are placed in a boat. At the moment the question is "where" and perhaps why of its shape.
     
  12. gonzo
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    gonzo Senior Member

    The where is determined by the why. Depending on the speed, heel angle, speed/length ratio, etc. For example, if this is a small boat of 12 feet waterline length, a speed of 9 knots will require flat buttocks aft, a chine that will run somewhat parallel to the centerline, it will be submerged at low speeds or stop, the water should flow clean off the chine at the proposed speed. In short, what the usual small powerboat bottom looks like. The upper chine is largely a cosmetic choice. Another approach is the flat bottom pad used in powerboats for more lift and less draft. It introduces a chine that is submerged at all times.
     
  13. TANSL
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    TANSL Senior Member

    "The where is determined to by the why". Okay, let's talk seriously, about why. Are you prepared for it ?.
    I do not know what you call the "upper chine" but I'm sure no chine is a "cosmetic choice". If that were true, the answer to the OP would be: "You put the chine where you like better because the chine is just a decorative element." Is that the question Gonzo ?. I think not, that the shape, size and position of the chines have, technically speaking, reasons for being, much more serious and effective than the decorative elements.
    Is it possible, please, someone who really knows the subject, to answer?
     
  14. makobuilders
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    makobuilders Member

    To steer this towards what I'm wondering about, I'm referring to a displacement boat of say 40ft, multi-chine. So previously TANSL made reference to the flow lines. Now back in my aerospace days we'd either put a model in a wind tunnel with pieces of yarn on it or by blowing smoke streams. Later on some sophisticated modeling software was developed which I never took to learning.

    So at this point I'd like to learn if there are simpler, less high-tech ways of determining the placement of the "flow lines" on a hull. Logic seems that placing the chines along the flow might reduce drag an extremely minuscule amount, so perhaps this is a more theoretical question.

    Thanks.
     

  15. TANSL
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    TANSL Senior Member

    What are you looking for, 1 or 2?
     

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