How to decrease the inclining oscillation and increase the stability on the boat

Discussion in 'Stability' started by Nickname123, May 30, 2017.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe not the only hope, but a solution that adds beam at the waterline seems sorely needed to my eye. Why it wasn't designed in, in the first place, who knows. I doubt they care much about Ad Hoc's GZ curves !
     
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  2. TANSL
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    TANSL Senior Member

    I totally agree with you. What happens is that for the existing ship you have to ask for some other solution.
    The GZ curve of Ad Hoc, very nice, very useful for many things, very typical of a naval architect who wants to prove that he knows many difficult things, but totally unhelpful in this case. They will do well to ignore it, to solve this problem.
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    No you don't.
    Don't know about you...but i was trained before computers and having to draw these graphs by hand on graph paper. It is easy to do. And, considering the nature of the vessel, one is not talking about absolutes either!
    However, if the GZ curves has been done, then the GMt will already be known anyway too and plotted on the graph as a double check!
     
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  4. Ad Hoc
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    Ad Hoc Naval Architect

    Simple answer yes. However the formulae for calculating that are well known, but, require endless years of mathematics to understand properly how to use them. But, it is still only theory and as yet, does not fully align with observation, but close enough. However, what you are after, a simple one liner, nope...doesn't exist.

    Also, no such thing as hull speed. Misnomer.
     
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  5. Jed233
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    Jed233 Junior Member

    Thanks. I guess there is nothing to do except go to a straight drive, quadruple the power with the 3 hp 2000 watt motor and see what happens.
    I only want to achieve 10 mph as simply as possible, with a run time of about 4 hrs on 120 ah. I really thought a 24v, 86 ft lb thrust trolling motor would do it. Didn't even get close. 10 mph doesn't sound like much but it would half the run time to get to the spots I like.
     
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  6. gonzo
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    gonzo Senior Member

    The GZ curve shows, as usual, that at 0 angle = 0. The first thing to look at, is what will be the maximum safe angle of heel and if the vessel won't heel more than that with all the weight, wind force, etc. acting on it.
     
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  7. TANSL
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    TANSL Senior Member

    Sometimes that's not the case.
    How can we quantify the "maximum safe angle of heel"? I do not see how the GZ curve can determine that value whatever that means.
    The stability criteria establish minimum GZ and DN values, for certain angles and conditions, according to the type of boat, but I would not call that "maximum safe angle of heel".
     
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  8. Mermaid Co
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    Mermaid Co Junior Member

    Here vessel's VCG is already in question..If designed and constructed properly,on the first hand this problem would not arise...who knows if Inclining test is perfect and it is giving accurate results?. Many times small errors in inclining test produces significant error in VCG. So we can not rely much on theoretical GZ curve unless we are very sure of VCG.. KN curve is hull's intrinsic property but GZ rely on KN and VCG...will have to go for solid ballast,keel weight or superstructure weight reduction...Later proper inclining test shall be done to determine VCG and final stability booklet shall reflect that boat is in safe zone.
     
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  9. TANSL
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    TANSL Senior Member

    Excuse me, what problem are you referring to?
     
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  10. Mermaid Co
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    Mermaid Co Junior Member

    referring to excessive roll problem and high vcg of lightship
     
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  11. TANSL
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    TANSL Senior Member

    OK thanks. I was a bit clueless with the inclininng test and the stability booklet, etc. In these circumstances (boat already finished) it is not so much where the c of G is but how to lower it and/or how to decrease the wind impact on the dead area.
     
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  12. Mr Efficiency
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    Mr Efficiency Senior Member

    The OP has not returned, hopefully all is well.
     
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  13. TANSL
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    TANSL Senior Member

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd say the boat itself has already put the "frighteners" into him.
     
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  15. Nickname123
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    Nickname123 Junior Member

    Hello all,

    Thank you very much for your all replies, I were so busy yesterday so I can't reply all of your questions.

    And then, there are some more information for your reference:

    1) The boat were built by GRP Composite, basic description (you can see on my drawing attached in the first post):

    - Lmax: 8 m
    - Bmax: 2.35 m
    - Hmax: 2.77 m
    - Hfloor: 0.6 m
    - Draft: 0.54 m
    - Deadrise in transom: 22 degree
    - Deadrise in middle: 27 degree
    - Dead weight: 2.25 tons (including 550 kg concrete in the keel)
    (even we put the permanent concrete in the keel, the inclining oscillation still terrible)
    - Passenger: 10 persons
    - Crew: 2 persons
    - Engine: 200HP outboard engine.

    2) Please find the Pantokaren curves in the attachment, I also attach the calculation sheet for your reference (we use Autohydro to calculate it), it written by Vietnamese so please use Google translate to translate it into your language if you don't understand something.

    If you have any question, please ask me. We really need your comments to fix the boat.

    Thank you very much.
    Duong.
     

    Attached Files:

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