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

    I'd suggest you e-mail the Kapten Boat Collar people, include a link to this thread as well, he may be able to advise as to whether it is feasible to fabricate and fit something that suits your boat. Possibly not within their size range, but won't hurt to ask.
     
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  2. Nickname123
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    Nickname123 Junior Member

    Hello,

    Thank you for your feedback,

    I will try to connect them.
    Duong.
     
  3. Mermaid Co
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    Mermaid Co Junior Member

    better share inclining test results and lightship estimation data in english....check righting arm vs wind arm area ratios at different wind speeds 5,10,20,25 m/sec. Double check deadweight vcg in loading conditions. Crew vcg is mentioned as 1 however it looks more than 1.5...
     
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  4. TANSL
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    TANSL Senior Member

    Nickname123, not easy to interpret the calculations you've done but I seem to appreciate a few things:
    - Speed does not seem to be an imperative since you do not mention it in the list of characteristics of the boat.
    - I do not know if the wind criteria are well tested but it seems that they are not met, so you should greatly decrease the area of the area exposed to the wind. As the height of the superstructure is very large, you should try to reduce it. The placement of large bilge keels will help you meet these criteria and reduce roll.
    - The freeboard is close to 0.85 m, which is a very high value. This would allow you to increase the displacement, that is, to place more fixed ballast. If you place lead instead of concrete, you can get a lot more weight (three times more) with the same volume.
    All this refers to the boat that is already built. For the next boat, it would be necessary to revise the whole project and, almost certainly, to modify the mold, not just the beam but the deadrise (perhaps, to keep the same value of the transom to the center of the boat). But these, as Ad Hoc wisely says, are assumptions and the final solution can only be given after making the relevant calculations.
    Good luck.
     
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  5. FloodyBloody
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    FloodyBloody New Member

    Mermaid Co - completely agree on the 1.5 - thats how it appears.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Nickname123

    Thanks for the GZ data etc.
    See, with data like you have given, the problems and issues become immediately apparent.

    1) Roll period. A rough calculation from your data and the GM, indicates a roll period of around 2.4 seconds. Using other imperial methods confirms this value. This is rather low roll period. But for a small vessel, not unknown.

    2) When people are standing up, there is a shift in the KG. If they move to the side at the extreme, then there is a further movement.

    Looking at the GZ curve an angle of say 5 degrees, or a roll of 5 degrees, is roughly 0.05m. Thus the amount of weight, person(s), that is required to produce a roll, or rather a list, of 5 degrees is roughly 190 kg. That is only 3 people at most. So, not a lot and easily felt as stepping on board.

    However if the people are standing and yet to be seated, this moves the KG up, and thus reduces your GM and hence your GZ is also reduced by a similar amount. The VCG of a person is higher than that of one seated, so the KG of vessel increases, at roughly GG(shift) = 0.19m, assuming a persons KG is roughly 1.0m above the deck they are standing on.

    So your new GZ with people standing and the boat rolling/listing to 5 degrees is simply 0.05 – 0.6Sin.5 = -0.002m - negative!!

    So it is immediately apparent that when all passengers at sitting down, the boat has a low-ish roll period. Not ideal.

    However any movement of said persons, by standing up makes the stability of the boat a whole lot worse, the GZ almost becomes negative when all standing (not accounting for crew higher up moving about) and can be made much much worse when everyone is standing or on the foredeck (since many quick calc assumptions – but roughly indicative).

    So, ignoring the bleating by the arm chair designers…it is very clear that you have a very obvious stability issue that was not checked in the preliminary design phase. That of people standing and/or moving to the edge has a serious effect on the stability. The hull form and GA does not lend itself to such an application.

    Serious changes to the boat are required to correct this.
     
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  7. TANSL
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    TANSL Senior Member

    You are very right but to me, going on with the reasoning started by you, it seems more despicable bleating (magistral lessons) of pedants (even without arm chair).
    That already knew the OP. That is the reason, I am sure, to start this thread. It might be interesting to know the changes that, in your opinion, are necessary. Something new in relation to what has already been said?
     
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  8. Nickname123
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    Nickname123 Junior Member

    Hello Ad Hoc,

    Thank you very much for your reply.

    But, can you give me an advice how to repair this boat? I am really disappointed about the boat.

    Thanks,
    Duong.
     
    Last edited: Jun 6, 2017
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    To wit ? Have you shared your solution with him privately ? I think we'd all like to see it.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    If the objective remains the same - to carry those persons and crew onboard....then your only real option is to use a hull that has the required stability when all persons are standing up and moving about. Since without meeting basic stability requirements, the changes needed to the existing hull would most likely make the vessel unusable as it would not meet the other objectives required for the design.

    See above explanation.
     
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    [QUOTE="Ad Hoc, post: 805454, member: 27849 the changes needed to the existing hull would most likely make the vessel unusable as it would not meet the other objectives required for the design.



    .[/QUOTE]
    What changes are you referring to ?
     
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  12. Ad Hoc
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    Ad Hoc Naval Architect

    You mean you are unaware of what changes are required from the explanation above?...despite your protestations and bleating from the outset as knowing the solution?
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    The only "changes" I see specified by you, are to change to a boat that can do the job. What are these mystery changes that would solve the stability problem, but render it unable to meet the other objectives ?
     
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  14. Ad Hoc
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    Ad Hoc Naval Architect

    You have been bleating from the outset in this post that you know the answer and solution....so why are you asking me?
    You are the expert here...since you have waved your magic wand from the beginning with zero facts and data. Go wave your wand, impress us...
     
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  15. Mr Efficiency
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    Mr Efficiency Senior Member

    *yawn* Never mind my "bleating", give us your expert view of what changes would solve the stability issue, but be disabling in some other way.
     
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