Maxsurf Centre of Gravity

Discussion in 'Software' started by Trace, Sep 21, 2017.

  1. Trace
    Joined: Sep 2017
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    Trace New Member

    Hello everyone,

    I have designed a sailboat in Maxsurf (NURB hull surface, keel and rudder)
    I have put the model through Stability, but I don't understand how to calculate the position of the centre of gravity of the hull. The program displays the CG of the solid, but the position it gives seems wrong (it's exactly on the waterline, which seems an odd coincidence, and forward of the zero point, which seems wrong, considering the shape of the hull.)
    Can anyone tell me how to find the CG of different surfaces? (hull and keel primarily) Is that calculated in Modeller or Stability?
    One last thing, when I open the model in Stability, I have to recalculate the hull sections. The option to keep the existing data is greyed out.

    Thanks a lot!
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Forgive my ignorance, but the COG can't be established in the vertical plane from the lines, surely ? That depends on the structural elements. But obviously it will be somewhere along a vertical line passing through the centre of buoyancy.
     
  3. Trace
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    Trace New Member

    I am assuming no structural elements. I want to know the position of the CG of the hull shape, I don't understand why it couldn't be computed by the software. Once at equilibrium, it will be aligned with the LCB, but first we have to determine where the centre of gravity of the ship lies.
     
  4. TANSL
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    TANSL Senior Member

    The picture shows where you can find the menu to calculate the properties of the surfaces. But to calculate the CoG of the ship it is necessary to calculate the weight and the centers of gravity of each one of its elemengtos, not only of the hull. There is no choice but to do it by hand, although you can help with a spreadsheet.
    Snap22.jpg
     
  5. Trace
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    Trace New Member

    Thanks a lot, I'll have a look tonight.
    Have a good day everyone!
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The COG will be directly over/under the COB, simples ! Can't be anywhere else. How far above, or below, depends on the structure
     
  7. Trace
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    Trace New Member

    Hello again!
    Thanks for your help, it was indeed in Area calculations.
    Mr efficiency (cool name btw :), my problem was defining where the CG of the boat is, so that equilibrium calculations can be done correctly.
    You are right in saying that the COG is over/under the COB, but only at equilibrium. If I place the CG at the wrong place to begin with, it will make the calculations, the COB will be under/above it, but in the wrong place.
    Tell me if I'm not being clear.
    Have a good day everyone, happy designing!
     
  8. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Trace, the Center of Gravity depends on the shape of the boat, the construction of the boat, the weight/density of the materials used to build the boat, and the location and weight of equipment, location and passengers. During the preliminary stages of design the designer may guess/assume a CoG location based on experience, comparison with other boats, and/or simplified calculations of weights and moments. Then as the boat design progress the designer will do more detailed calculations to see if the CoG location will be close enough to the initial assumed CoG. If it is not close enough then some aspects of the boat will be revised so that it is close enough.
     
  9. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    This only gives you the centroid of the surface, not the CoG.

    As DCockey noted above, you need to produce a list of all items onboard and table them against their locations against a fixed datum. You do this in all 3 axis, to obtain the LCG, VCG and TCG. No hull modelling program will ever give you that.

    If you cant estimate the CoG from listed out each item, take an educated guess, and perform a simple set of KN curves, where the CoG is independent. To obtain the stability characteristics, one uses the KN curves and corrects for any value of KG. If all this is too much for you....then you should stop and seek professional advice.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the "zero point" referred to in the opening post ?
     
  11. bhnautika
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    bhnautika Senior Member

    Trace you have to set up the zero point position in the data menu first go to “frame of reference” set up base line and dwl. Then go to “zero point” and move it to “forward perpendicular” (for example) and pick your vertical position. Save this to your model. Now when you open the model in the stablity section that information goes with it you then need to run the “upright Hydrostatics” to get the displacement and the LCB. For the “Large angle stability” you need to open a new “Load case” , just copy in the disp into the weight and the LCB into the longitudinal arm cells then run the “Large angle stability”. Vertical arm cell is set at waterline by default.
     

  12. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Maxsurf says that he calculates the centers of gravity and there is no reason to doubt it.
     
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