50 Common Errors in Powering Predictions

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, May 13, 2011.

  1. DMacPherson
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    DMacPherson Senior Member

    But were these same tools used for the "ones that suffer(ed) the consequences of erroneous speed/powering predictions"? In other words, could you attribute hull characteristics, speed ranges, or processes to the errors?
     
  2. BMcF
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    BMcF Senior Member

    Hard to say without quite specifically examining each case. Further, its really a kit bag full of defined methodologies that we are all talking about here. 'Tools' that render those myriad methods to software apps may or not have been involved in any event.

    For example, there is an entire body of technical literature that Bob Wilson authored that is specifically focused on the proper methods of model testing SES and accurately predicting full scale performance. But who uses that information? How do/did they use it? Did they create software apps based on that information (in many cases, yes they did. Marintek, in Norway, for one example).

    Some use established methods and tools based on those methods..others, not so much.;)
     

  3. DMacPherson
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    DMacPherson Senior Member

    Understood. And I agree that it is one thing to take a publication and toss together a spreadsheet, or even produce a "tool" for others to use. It is something else altogether to vet the equations and use them in a coherent methodology.

    We see the same type of thing with conventional ship resistance expansion to full scale. Many implementations of drag prediction methods incorrectly mix friction lines, for example. The CR coefficients of the ubiquitous Series 60, as an example, were determined using the Schoenherr CF line on models of some 16 ft length (if I recall correctly). I have seen tools that mix this Schoenherr-derived CR with the ITTC57 line. (It is not hard to keep things compatible, by rebuilding equivalent model scale CT and then expanding, which is how we do it in our commercial tools.)

    Rigorous adherence to proper methodology is vital. One may not prefer the methodology that is employed, but at least you know what you get - and then can "correct" it as you see fit. This is my big gripe with the USN Taylor Standard Series "worm curve" approach. The multipliers applied to the TSS CR values for a prediction are subjective. When I have asked in the past how the worm curve multiplier values were selected, the answer was: "from experience". This has always been unsettling...
     
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