ISO 12217-3 simplified offset-load test

Discussion in 'Stability' started by mc_rash, Jun 14, 2024.

  1. mc_rash
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    mc_rash Senior Member

    Hey all,

    for my 3.3 m open boat design (design category D) I am following the ISO 12217-3 standard for stability evaluation.
    The design has a crew limit of 4 persons (all sitting, one forward, two midships, one aft).

    upload_2024-6-14_12-3-42.png

    I followed 6.5.2 simplified procedure for offset-load test. Since the design is still preliminary the light ship values are estimated, ref point is aft perpendicular at keel line:
    Item - Mass - LCG - TCG - VCG
    Lightship - 60 - 1.342 - 0 - 0.23
    Outboard - 21 - (-0.15) - 0 - 0.6

    As crew area I used the whole boat area from forward to aft over the whole beam.
    Crew values are:

    LC1 - LCG @ 75 % crea area length (3.3 m)
    4*85 kg - 2.325 - 0 - 0.5

    LC2 - LCG @ 25 % crew area length (3.3 m)
    4*85 kg - 0.675 - 0 - 0.5

    (Edit: Due to ref point position LCG is 0.15 m offset)

    I calculated the GZ curves in Maxsurf (large angle stability, free to trim) and from that the righting moment. Also, the crew heeling moment with the ISO formula 6.5.2.4.
    In the plot it is clearly visible that the design does not pass the requirements as I guess the crew heeling moment has to intersect the righting moment at one point.
    upload_2024-6-14_12-24-53.png

    Setting the trim in Maxsurf to fixed instead of free to trim would raise the righting moments a little but I assume this is not correct and the curves still wouldn't intersect.

    The critical parts are a) the crewlimit is to high and b) the beam of crew area is to large. Lowering b) to roughly 0.7 m (instead of 1.4) would solve the problem but this is not desired in this design.

    Edit: a) lowering the crew limit is not possible due to design requirements

    Any ideas or recommandations on fixing the stability assesment?
     
    Last edited: Jun 14, 2024
  2. TANSL
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    TANSL Senior Member

    Check the units used in each calculation.
    The ISO formula for the heeling moment uses N.m. What units is Maxsurf using?
     
  3. mc_rash
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    mc_rash Senior Member

    Maxsurf is using m and kg, I calculated righting moment with GZ*mass*9.806
     
  4. TANSL
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    TANSL Senior Member

  5. mc_rash
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    mc_rash Senior Member

    So solving this issue would be lowering the crew heeling moment curve and/or increasing the righting moment curve. Lowering the calculated crew heeling moment could be achieved with a smaller crew area width.

    I struggle a little bit with this ISO statement (12217-3, 6.5.1.7)
    upload_2024-6-16_0-25-46.png
    from which I would assume the whole breadth of an open boat must be used for the calculation.

    From the following rules the crew area width could be reduced (although I am not sure if I understand the word 'ledges' correctly):
    upload_2024-6-16_0-28-39.png

    Would my section drawing still apply to open boats from note 2?
     
  6. mc_rash
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    mc_rash Senior Member

    This would be the crew heeling moment curve with the 'new' crew area width with the assumptions I made above. Still too much but better than the 'old' curve
    upload_2024-6-16_0-42-31.png
     
  7. mc_rash
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    mc_rash Senior Member

    I am also wondering about specifying the VCG of the crew, ISO 12217-3, 6.5.2.2.

    In my first calculation I assumed the crew sitting (~400mm above baseline) and according to ISO the VCG should be set 100 mm above the crew's seats. At the specified LCGs of the loadcases are actually no seats (this does not mean there are no combinations where the crew can sit on seats resulting in a total LCG where no seat is located). Assuming there is no seat at the specified LCG - shall crew VCG be set to 100 mm above the surface they stand on or above the seats?
     
  8. TANSL
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    TANSL Senior Member

    Indeed, that is another of the many inconsistencies of ISO standards. I take the CoG of the personnel 700 mm above the plane on which they are situated, sitting or standing. If you want to be very careful, take 800 mm. On the other hand, check that in the chosen area there is enough space for people, do not consider more than 4 people per square meter.
     
  9. mc_rash
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    mc_rash Senior Member

    Thanks @TANSL for your replies.

    I'll try figure out the full procedure for offset-load test, it looks more straight-forward to me and also a more realistic scenario than the simplified method.
     
  10. TANSL
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    TANSL Senior Member

    Except for the inconsistency that we have mentioned (position of the crew 0.1 m above the surface on which they are standing), it seems much more comfortable to me to use the simplified method (6.5.2.). In it you only have to calculate the GZ curve for two loading conditions (LC1 and LC2) in a completely similar way to how it would be calculated for any other loading condition. That is, no "strange" calculations need to be performed. The heeling moment curve is drawn on these curves, according to 6.5.2.4, and the point of intersection of both curves is determined.
    I think this is much simpler than the full procedure 6.5.3.
     
  11. mc_rash
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    mc_rash Senior Member

    Yes, it is no strange calculation but I can't see how my design should satisfy the simplified offset procedure.

    I mean, there are many boats with comparable specs which do have the CE mark.. (No, I do not plan to sell the boat. I just want to use it privately but I want to design it according to standards)
     
  12. TANSL
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    TANSL Senior Member

    As said before, I think the simplified method is more convenient to check.
    If you have any questions, please feel free to ask.
     
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  13. mc_rash
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    mc_rash Senior Member

    I appreciate @TANSL
    I made a readable worksheet so others can follow my work a little, see the excel file attached.

    Again, I followed the simplified procedure for the stability assesment power boat < 6 m, category D. I adjusted the seat height to 0.3 m above the keel height which results in 0.4 m for the VCG of the crew. This change also leds to slightly different, but still not satisfying, results. I also calculated the crew heeling moment curve for a crew limit = 1 and = 2. Even a crew number of CL = 1 would only partially agree with the rules and does not satisfy the requirements for the crew LCG at 75% crew area length.

    Edit: the righting moment for CL = 1 and CL = 2 should have been apart calculated with the mass of CL = 1 respectively 2, I did'nt which is not correct.

    upload_2024-6-19_10-24-11.png

    The results are clearly for me and the boat cannot be testet with the simplified offset-load test and comply with SOR and ISO, or I am doing something completely wrong with the GZ calculation. If someone wants to check my calculation I can provide a Maxsurf model or IGES of the hull.
    I assume I have to follow the full offset-load procedure or I have to asses the boat with option 3 (the latter would not be my favorite).
     

    Attached Files:

    Last edited: Jun 19, 2024
  14. TANSL
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    TANSL Senior Member

    In rows 64 to 72, columns F and H, of the spreadsheet you are working on, you calculate some values in kg*m while the value of the heeling moment, rows 89 to 91, the value obtained is (see ISO standard cap 6.5.2.4) in N *m. If you use the same units in all calculations you will see that the situation improves.
     

  15. mc_rash
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    mc_rash Senior Member

    In row 64 to 72 I calculated the total LCG, TCG, VCG of the loadcase. These caculations can be ignored.
     
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