Help needed- metacentric height and other restrictions

Discussion in 'Stability' started by Alfonso, Nov 30, 2010.

  1. Alfonso
    Joined: Sep 2008
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    Alfonso Junior Member

    Hello,

    in advance, thank You for Your time!

    I need some literature or formulas that would help me to exclude some of generated projects due to stability criteria.
    I know there are regulations for minimal metacentric height, but what is with maximum?? There is a formula that says:

    GMmax=(B*C/T)^2,

    where:
    B-width
    c-coefficient
    T-roll period (min. some 7-12 sec)


    and you can give roll period that You want, but it does not suits me. Because, for the same ship I get GMmin=0,3m and GMmax=0,39 (for roll period 8 sec)???

    Is there any criteria for minimum KM, or BM.... Or something like that??

    Type of ship is: fishing vessel

    Thank you
     
  2. Guillermo
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    Guillermo Ingeniero Naval

    Well, asuming your FV has traditional displacement forms, and height of the center of gravity such that the vessel complies with the general rule of thumb of the roll period in seconds being equal to the breadth in meters or slightly lower, for an adequate stability both from the point of view of safety and working platform comfort (full load condition), you should have a breadth of around 8 - 8,5 metres for an 8 seconds rolling period.

    If you are not running your forms in a computer hydrostatics program, and you need to do a rough approximation to KM, you can do it by using the following:

    KB = (0,843 - Cb/3*Cwp)*Tcb
    BM =(0,07032*Cp+0,01039*Cp^2*Cwp^2)*Bwl^3*Lwl/Vol

    Then obtaining KG by means of a weights analysis you can get GM.

    Note:
    Cb = block coefficient
    Cwp = waterplane coefficient
    Tcb = body draught
    Cp = prismatic coeficient
    Bwl = waterline beam
    Lwl = waterline length
    Vol = displacement volume


    As you know GMo has to be over 0,35 m as per IMO's intact stability code. There is not a rule to limit maximum GM, but it shall be such that accelerations are not too high so working aboard can be done safely. That's why period should be not much shorter in seconds than the breadth in meters, as said above.
     
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  3. Alfonso
    Joined: Sep 2008
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    Alfonso Junior Member

    Yes

    What I did, was very similar to what you described.

    I didn't use formulas You gave, although I'm well aware of them. I used regression coefficients from ''Evaluation of cross curves of fishing vessels at
    the preliminary design stage'' by Huseyin Yılmaza and Abdi Kukner. I think they are maybe more precise.

    I did get KG by weight analysis. The problem is small interval of GM. Becouse I get

    GMmin (given by IMO)= 0.35 m

    GMmax=0.37 m - using upper formula

    But I used period of 8 sec. I didnt know that period should be similar to breadth or smaller?? Where can I find some literature about that to confirm it. Not that I dont believe You, just I need to list references... It would give higher GMmax if I used that condition. B=period.

    Any theory about that? Any thoughts?

    Thank You very much! If you want I can send You that article with regression coeff!
     
  4. Olav
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    Olav arch. nav.

    Too true! An example of the possible results of very high GM can be seen in this Investigation Report by the German Bundesstelle für Seeunfalluntersuchung (Federal Bureau of Maritime Casualty Investigation) concerning a lethal accident of an 8,750 TEU container vessel. Even though it says a metacentric height of 7.72 m was unavoidable in this particular case (cargoless ship on ballast voyage) it clearly shows that too much stability can be as bad as too little.

    However, I've never heard of a fishing vessel suffering from too much stability - quite the opposite!

    Just my two cents...
     
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  5. Alfonso
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    Alfonso Junior Member

    Olav, thank You

    I think that ''too much stability'' in fishing vessel case means that crew need to work in somehow ''hostile enviroment'' due to too high accelerations. If You consider that fishing is rather dangerous and unpleasant work itself, You shoud do everything in Your power to asure that they are comortable at least when they are working or sleeping.

    Too much stability would make work unpleasant and dangerous due to possible moving of objects on a working deck because of large accelerations.

    Thank You for your reply and Investigation report!
     
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  6. MikeJohns
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    MikeJohns Senior Member

    The classic naval architecture dangerous GM scenario were the ore carriers.
    As Guillermo said a good rule of thumb for boats is for the roll period to match the beam in meters.
     
  7. Guillermo
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    Guillermo Ingeniero Naval

    Alfonso,
    You can find a reference to the rule of T in seconds being more or less the beam in meters at the reccomendations for the determination of the approximate initial stability for small FV by means of a rolling period test.

    Annex 29 to the report of the Maritime Safety Committee on its eighty-seventh session (MSC 87/26, 4 June 2010): SAFETY RECOMMENDATIONS FOR DECKED FISHING VESSELS OF LESS THAN 12 METRES IN LENGTH AND UNDECKED FISHING VESSELS

    From there:
    3.3.3 A rolling period test – option 1**
    A rolling period test* should be conducted when the vessel is loaded according to the operating condition as specified in 3.8.1.1. The stability is deemed satisfactory if the rolling period (Tr), in seconds, is less than the breadth of the vessel (B), in metres.


    Such reccomendation was quite widely used in the past, even for relatively big vessels, when detailed stability calculations were more difficult to perform. Now it is being limited to the small ones, but can still be an useful guide for a rough approximation.

    I attach the document here.

    Regards.
     

    Attached Files:

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  8. Alfonso
    Joined: Sep 2008
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    Alfonso Junior Member

    Guillermo,

    I can't thank You enough! This is exactly what I was looking for. And this isn't the first time that You helped me!

    Thanks!
     
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