Boat stability

Discussion in 'Stability' started by Alexanov, Oct 23, 2019.

  1. Alexanov
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    Alexanov Senior Member

    Is it any practical way to quickly measure initial stability ( KMt ) of the small boat???
    Boat is on the water and we know only main dimensions.
     
  2. Alik
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    Alik Senior Member

    Apply ISO12217-1 or -3 standard. The testing methods described there are intended for on-water measurements. You don't need KMt for such boat; use offset load test and, residual heel angle and freeboard instead.
     
  3. Alexanov
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    Alexanov Senior Member

    Thank you. I will study it.
     
  4. bajansailor
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    bajansailor Marine Surveyor

    You could try the method described in the attached sheet - if you measure the roll period this can give you a good idea as to what the stability is like.
    I have tried it with 'small' (ie less than about 22' / 7 metres long), and it seems to work.
     

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  5. Alik
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    Alik Senior Member

    The problem with this method is that 'K' is dimensional, and its value strongly depends on many factors, say, size and inertia of boat, hull shape. etc. This method works for small boats ONLY if You have 'anchored' it to similar type of boats before, so You identified 'K' from other boats of same type.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    The actual correct formula for roll period, has - 2.Pi.kxx - on the numerator. Which is where the 'confusion' perhaps comes from.

    The "kxx" is merely the transverse radius of gyration of the vessel's mass.

    It is not that difficult to calculate if you have all the weights and centres, but just a bit tedious, and for this purpose, one doesn't have said data to begin with. Thus, IMO and ITTC suggest a value between 0.40 and 0.45B , and as such does cover all types. Although other work attempting to cover all types also suggested 0.35 - 0.45B.

    So roll period can be rewritten as = 2.Pi.(0.35 ~ 0.45B)/SRT(g.GM)

    When doing model testing, the radius of pitch gyration is also required. In the absence of this value, most test tanks default to 0.25L. In a similar way default to 0.4B for roll.

    The MCA has a slightly more methodical approach for rolling period, but essentially the same thing here. Where they suggest 15 rolls, rather than 5.
     
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  7. Alexanov
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    Alexanov Senior Member

    Yes, It work. I saw something similar in another book. Very practical.
     
  8. Alexanov
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    Alexanov Senior Member

    Found something more interesting.:)
     

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  9. Alik
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    Alik Senior Member

    It is not only boat's own gyradius, but also added inertia of water and roll damping. Thus, K factor will strongly depend on submersed shapes. Small boats have varieties of hull shapes and underwater appendages, and using above IMO recommendations for them would be inappropriate simplification. Say, we defined this K coefficient for number of small craft, and the range is 0.8 to 1.4.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    Yes, that's correct, Kxx = Sum Mass Moment Inertia + Add Mass (A44 )

    By the 'K' I assume you're referring to the added mass aspect of 'K'.
    Added mass for most vessels are in the 0.80 range...and could be less, if more slender. If you go into odd shapes that have plan forms which are greater at their base tn their waterplane, then this value can increase above 0.80. Swaths for example, can be anything from 1.0 - 1.40 at their extremes. But these are extreme shapes. It can also be affected by beam to draft ratios, in the cases of simple shapes like catamarans.

    Typical catamaran A44 is something like this:
    upload_2019-10-25_8-50-49.png

    Whereas a Swath is more like this:
    upload_2019-10-25_8-0-51.png

    The more you investigate, the more complex it becomes and the more factors seem to have an influence....yet while the values may fluctuate wildly, the overall net effect on roll is often not as much as one would image, only when looking at extreme hull shapes:
    upload_2019-10-25_8-24-49.png


    Hence the IMO's experiments to establish a "rough" baseline norm, in the absence of hard data. And it does, for most 'typical' vessels have a relatively narrow range for K, that noted as roughly 0.35 ~ 0.45B.

    Is that from your own experiments?
     

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    Last edited: Oct 24, 2019
  11. Alik
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    Alik Senior Member

    Yes, it is.
    Note that 'K' factor (in roll period - GM formula) also includes non-inertial roll damping.

    Say, many small craft have developed skegs, chines. Also developed superstructure and mast. Those are not 'odd shapes', those are normal shapes - for small boats... ;)
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    That is a wide variance 0.8 - 1.40m.

    Im not familiar with superstructures and mast having an influence on the A44 added mass of a vessel. o_O
     
  13. Alik
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    Alik Senior Member

    But they do have influence on inertia of a vessel.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Just as does: WTBs, Eng girders, a crane or a winch or windows or seats etc. etc. when calculating the mass moment of inertia.
    Everything has an influence, some minor some major...that is the point of the calculation to see what, if any, effect it has on the overall characteristics of the vessel.

    Removing a deckhouse to change the inertia doesn't help those in their seats when it rains though! :D
     

  15. bajansailor
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    bajansailor Marine Surveyor

    Alexanov, would you like to post a photo (or two) of the small boat mentioned in your original post?
     
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