Origin of the 1,4 factor in dynamic stability weather criteria?

Discussion in 'Stability' started by Paavo, Feb 21, 2024.

  1. Paavo
    Joined: Feb 2024
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    Location: Germany

    Paavo New Member

    Hello,

    I was wondering if someone would know more regarding the use of the factor 1,4 for the energy balance in weather criteria. This is currently used by, at least, the DNV and the earliest I can trace it back to is 1996 GL criteria, as in the attached figure. It resembles the 1962 weather criteria from the US navy which also uses a 40% margin, but it is not applied in completely the same fashion.

    What I am interested in is the reason for the, seemingly arbitrary, use of 1,4. It would be wonderful if someone could direct me to the right paper/book or would know an answer. I have tried to find earlier topics about it on this forum but was unsuccesfull.

    Thank you so much,
    Paavo
     

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  2. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Paavo,
    Welcome to the forum.
    I'm pretty sure you will not find a definitive answer to this. It is 1.4 because it works within the limits of underwriting. Many things in the class rules have filtered down into law and codes because in the overall scheme of things it is more "good" than "bad". For specific hull forms "1.4" is insufficient, but for most "normal" hullforms 1.4 is sufficient. YMMV.

    As a side note, this is one of the major issues I faced in my career: based on the "energy" in the seaway, how much "measurable static margin" is needed to protect against a "significant" single event..... ahhhh...this is where we put on our pointy hats and bash on each other....
     
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  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    To echo what JEH has written, there is often no real definitive reason why some things are - it can usually just be a "gut feeling" at that time of the investigation.
    And that "fudge factor" is copied and continued and becomes folklore...

    For ref, I did a search of all my papers of this 1.4, and going back many decades to the original paper that presented this, and at various forums within SNAME and the US Defence Dept.s etc, along with the subsequent discussions of the paper and the R&D conducted. And yet, no one asked that simple question...why 1.4?

    Hence we shall probably never know, but continue to accept it, ... as so far - it has proven to be very effective.
     
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  4. jehardiman
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    jehardiman Senior Member

    Yep...I was once asked "why a 150% load test for rigging?".... "Because it works..."

    Edit to Add: And if you have ever worked a back deck at +/- 0.5G you know why...
     

  5. Paavo
    Joined: Feb 2024
    Posts: 2
    Likes: 1, Points: 3
    Location: Germany

    Paavo New Member

    Thank you all. This corresponds with what I have found so far but it is good to have received some confirmation on it.
     
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