Seakeeping prediction - Bale Formula

Discussion in 'Boat Design' started by anaorsolini, Oct 1, 2008.

  1. anaorsolini
    Joined: Sep 2008
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    anaorsolini New Member

    Hi everyone,

    I'm still a naval architecture student and I'm trying to make a new ship's project and I want to analyse how safe he will be. As I see, it has to do with the ship's seakeeping charateristics, but I guess, by now, using a software like SeaKeeper or Shipmo is too advanced for my project, I want to do a preliminar study first. Searching the Internet, I found something that's called Bale's formula, that, according to the source, "provides a quick and accurate estimate of general seakeeping for ordinary hulls" because it "produces a number, R, whose value indicates a rank for seakeeping. The scale is from 1 to 10 where 1 is a poor seakeeper and 10 is an excellent seakeeper".

    This formula can be found in http://imarine.mit.edu/ilearn/Marine-engineering courses/13.412/Seakeeping/Control/design.htm , but I tried to find other sources that mentioned it and I didn't find anything. As I had never heard about it, I was wondering if I could trust it or not. Has anybody already heard about it? What would you advise me?

    Thank you!
     
  2. daiquiri
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    daiquiri Engineering and Design

    The Bales' Seakeeping Rank Factor is the result of a research work done by Bales N.K. and presented in 1980 at the 13th ONR Symposium on Naval Hydrodynamics, in Japan. The title of the paper is "Optimizing the seakeeping performance of destroyer type hulls". It is now one of commonly used methods for the evaluation of seakeeping performances of ships.

    I don't have the original text of the research, but these are llinks to some works which explain or use Bales' Rank Factor:
    http://faculty.nps.edu/nlmiller/Fatigue/HSISymposium/cdr_pdfs/indexed/1a_3.pdf
    http://imarine.mit.edu/ilearn/Marin...2/Seakeeping/pdf_files/Seakeeping_Control.pdf
    http://www.gim.yildiz.edu.tr/alkan/gemihar/seak-bales-nps.ppt
    http://www.hydrocompinc.com/knowledge/techexchange/tex0701.pdf

    But I guess it will be possible for you (through university libraries) to find the Proceedings of the 13th Symposium on Naval Hydrodynamics, where you will find Bales' original work. ;)
    Hope it will help. :)
     
  3. anaorsolini
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    anaorsolini New Member

    Thank you, Daiquiri!

    Now, I was wondering, does this formula apply to any kind of ships?
    Bale made this study based on military ships (frigates, destroyers), right?
    Do you think it would be ok to use it to predict a bulk carrier or a sailing boat's seakeeping characteristics for exemple?
     

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

    daiquiri Engineering and Design

    Good question - I forgot to mention that. :D
    All empirical methods are created from the measurements performed upon a limited number of observed cases. If you have a hull which falls outside of that boundary, you have to use another method or accept the risks...
    Now, I have never really deepened too much my knowledge on Bales' method because I didn't have a need to use it. I can tell you that some of the limits are discussed briefly in the 4th link I gave you (from Hydro Comp site). Just to cite some of them:
    L/B from 8.0 to 9.8
    B/T from 2.5 to 3.7
    Cvpf > 0.68
    Cwf < 0.70
    If you're inside these parameters, you can use it safely. If you're not, then the quantitative results are not guaranteed but you can still make some qualitative evaluations about the various parameters involved.

    But I do know that there have been some extensions to the Bailes' original method. If I manage to find something, I'll let you know.
     
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