Finding the waterline of a boat

Discussion in 'Boat Design' started by Sandertn, Oct 15, 2015.

  1. Sandertn
    Joined: Sep 2015
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    Location: Norway

    Sandertn Junior Member

    Hi, I would like to try and calculate the draft of a boat. Im clueless how to do this. The boat is yet not build, thus I cant try it out in a real life scenario. The boat is designed in a 1:100 scale.

    The measurements I know are:
    The boat will be carrying 3-6 kilos. We can use 3 as a reference number.
    The boat is 115 cm tall, and 8,5 meters long. The design is somewhat similar to a traditional rowboat.
    Freeboat is the program Im using, so there`s some numbers there I dont know exactly what mean.
    KM=16.47
    LCF=35.09
    Displ=4529.03
    Lateral area=205.94

    Just tell me if I need to know anything more
     
  2. W9GFO
    Joined: Dec 2014
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    W9GFO Senior Member

    Simplest way in FreeShip is to specify the draft in the "project Settings" dialog. Then go to "Calculations" - "Design Hydrostatics" and it will tell you the displacement.

    Next easiest is "Calculations" - "Hydrostatics" and enter values for Start, end, step and trim and it will give you a table of values for each step of draft.

    Read through FreeShip's help file - that will help a lot too.
     
  3. Sandertn
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    Sandertn Junior Member

    Thank you, Im trying to use the Method Holtrop calculations, but i they dont seem to work for me. Any tips?
     
  4. W9GFO
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    W9GFO Senior Member

    I use almost exclusively the resistance calc for canoes and kayaks, if your design is like a rowboat, that may work for you.
     
  5. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    I don't know FreeShip.

    KM is normally the distance between the keel and the metacentre normally given in meters (may be there are other units to select).

    LCF the distance between the aft perpendicular and the centre of flotation (L = longitudinal).

    Displacement (Mass or Volume)

    Lateral area = the area of the immersed part of the section of the hull at midships plane
     
  6. DCockey
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    DCockey Senior Member

    Sandertn, are you trying to design a boat, or analyze an existing design?
     
  7. Sandertn
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    Sandertn Junior Member

    Im analyzing an existing design yes. :)
     
  8. TANSL
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    TANSL Senior Member

    Sandertn, for the LCF it is necessary to know the height of the water plane from the baseline (the draft and trim). Therefore, as you know LCF, these data may appear among those you already have.
     
  9. DCockey
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    DCockey Senior Member

    Two conditions for a floating vessel at rest:

    1) Archimedes Principle: The mass of the volume of water displaced by the hull equals the mass of the boat.

    2) Moment equilibrium: The Center of Buoyancy (centroid of the volume of water displaced by the hull) is on a vertical line with the Center of Gravity of the boat.

    So to determine the draft of the boat the mass of the boat and the longitudinal and transverse CG location of the boat must be known.

    The position of the boat including trim and heel as well as draft which satisfies the conditions above is found. This can be done manually by iteratively moving the boat until the conditions above are satisfied. Alternately some software can solve for the waterline after the mass and CG location as well as the shape of the boat are put into the software.

    There is a third condition which determines if a boat at rest is statically stable and will remain upright. It is that the metacenter is be above the CG.

    Are you a student?
     
  10. DCockey
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    DCockey Senior Member

    The LCF location is not needed to determine the draft of a boat.
     
  11. DCockey
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    DCockey Senior Member

    My understanding is the Holtrop Method is used for estimating resistance/drag, not static position of the boat. Am I mistaken?
     
  12. Heimfried
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    Heimfried Senior Member

    1 person likes this.

  13. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Of course, but it is difficult to estimate the LCF if you do not know the depth. So you will agree with me, I suppose, that if the LCF is known is because the draft is also known.
     
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