Placement of chines in multi-chines

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

  1. gonzo
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    gonzo Senior Member

    Unfortunately I can't really understand most of the text. From the first graph, looks like the resistance curves intersect at about 27 knots, and from there the single chine has less resistance.
     
  2. HJS
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    Location: 59 45 51 N 019 02 15 E

    HJS Member

    Gonzo

    The attached file shows the required horsepower for the two boats compared.
    Both boats had the same weight and internal volume.
    This has been reported in detail in an earlier thread.

    js
     

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  3. makobuilders
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    makobuilders Member

    HJS, that chart is quite interesting because even though it is a planing boat, but down at displacement speeds (say 5 knots or less) the double chine is indicating about 33% less power required.
     
  4. gonzo
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    gonzo Senior Member

    Do you have graphs with axis titles in any other language?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    One is 13 degrees, the other 20. SO the difference is not just single vs double.
     
  6. HJS
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    HJS Member

    The attached graphs show trim and the vertical acceleration in oncoming waves. The trim angle is measured in relation to the keel as both boats have 1.5 degrees inclination to be subtracted from the values shown. Both boats have the same internal volume, payload and the total weight.

    js
     

    Attached Files:

  7. MikeJohns
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    MikeJohns Senior Member

    For a hard chine from tank testing it's been shown that turbulent flow will stay attached without forming a separation vortice when the waterlines at the chine intersect form an angle of no less than 165 degrees. That's both fore and aft. I always try and get it far less pronounced fwd since it will still trip laminar flow. Although laminar flow usually trips after around 1 to 1.5m in real world conditions.
     
  8. jesdreamer
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    jesdreamer Junior Member

    Chines to help Planing??

    I have found this thread to be extremely interesting. It appears that a multi-chine hull outperforms a single chine (V bottom) and that chines clearly affect performance. I have been thinking of skegs which are really more pronounced chines -- a single skeg is popular along hull centerline to aid stability. Would 2 parallel skegs do a better job stabilizing under side heeled condition??

    But of greater interest, might 2 straight, and separated, but non-parallel skegs converging toward stern give us some increased hydrodynamic "support" and actually aid planing or allow planing at lower speeds?? (slight increased drag due to skegs not parallel to flow lines overcompensated by the pressure increase on the vastly greater hull area between the converging skegs)
     
  9. PAR
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    PAR Yacht Designer/Builder

    The idea that a skeg is really a big chine is an incorrect assumption. Twin skegs add more drag, but more importantly, often cause a single, centerline prop to vent or cavitate in turns. Pigeon toeing the skegs will dramatically increase drag and places increases in the pressure wave laterally, which isn't going to do anything except slow you down. Lastly, your assumption about multi's being better than single chine, is also not correct. You'd be best advised to study basic hydro and flow dynamics. There are many "decisions" that need to be made when electing to employ the various features discussed here. None are any better than others in general conversation, though some may be prefered given a specific SOR compliance. This is the real crux for the designer, how to best address the SOR, to hit targeted goals.
     
  10. DCockey
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    DCockey Senior Member

    All VPP software which I'm aware calculates resistance based on "curve fits" of various hull measurements and does not consider the exact hull shape. I don't know of any which would consider the shape of chines.

    To calculate the effects on resistance of chine shape CFD software would be needed which calculates the details of the flow field based on physics, not interpolation and extrapolation.

    The effects of chines on resistance for displacement speeds can be significantly reduced but rounding the chines with a large radius.
     

  11. Barry
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    Barry Senior Member

    You have left out perhaps the most important parameters with your notes regarding these two hulls. Ie chine width and deadrise.

    The upper chine on the hull with the "twin" chines will probably not even be in contact with water as the water will leave the hull off the lower chine.

    You could add a couple of more chines above the lower chine and not see any change in drag

    What this graph clearly shows and is not relevant to this thread, is that with higher deadrise you get higher drag at planing speeds.

    Any drags differences due to the differences in deadrise at displacement speeds should also be ignored as the OP was asking a question about a displacement hull. Not a planing hull operating at displacement speeds

    (ignoring the beam of the lower chine)
     
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