Exact definition of Draft Mark?

Discussion in 'Stability' started by Dibin_marino, Jul 14, 2014.

  1. Dibin_marino
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    Dibin_marino Junior Member

    Dear All,

    Please help me a little clarification on taking draft marks of ships, can we take draft marks including the thickness of keel plate and compare that draft marks in the hydrostatic tables ???? or do we need to minus the thickness to get the exact displacement in hydrostatic tables???

    Is there any relevant documents supporting the same?????
     
  2. TANSL
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    TANSL Senior Member

    Depends on how you have calculated the hydrostatic values, if you have taken the baseline on the outside or inside of the hull. Both possibilities are correct but the designer must indicate how it was calculated.
     
  3. NavalSArtichoke
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    NavalSArtichoke Senior Member

    The draft markings on the side of a vessel can have several different reference points, depending on the design of the particular vessel.

    For vessels with a straight keel which runs parallel to the baseline, the bottom of the mark indicates the draft of the vessel as measured to the bottom of the keel. For vessels which have drag to the keel, i.e., where the keel line is not parallel to the baseline, but is deeper aft, the marks are still referenced to the bottom of the keel at its lowest point along the length of the vessel.

    By US regulation, these standards for draft marks are followed:

    http://www.law.cornell.edu/cfr/text/46/196.40-10

    You do have to be careful when using drafts taken from draft marks to enter a hydrostatic table or curves of form. Printed tables should indicate if the drafts used are molded or keel drafts, as should the curves of form. Sometimes, a vessel's stability book will include a diagram showing the location and reference points for the draft marks.

    Vessel with drafts measured using imperial units have marks which are 6" high with a 6" spacing between the top of the lower mark and the bottom of the upper mark. For vessels with drafts measured using metric units, there are several different marking standards.
     
  4. DCockey
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    DCockey Senior Member

    From the link NavalSArtichoke provided:

    (a) All vessels must have draft marks plainly and legibly visible upon the stem and upon the sternpost or rudderpost or at any place at the stern of the vessel as may be necessary for easy observance. The bottom of each mark must indicate the draft.

    (b) The draft must be taken from the bottom of the keel to the surface of the water at the location of the marks.

    (c) In cases where the keel does not extend forward or aft to the location of the draft marks, due to raked stem, or cutaway skeg, the datum line from which the draft shall be taken shall be obtained by projecting the line of the bottom of keel forward, or aft, as the case may be, to the location of the draft marks.


    So if the keel has drag draft marks use the bottom of the keel (or the projection of the bottom line of the keel) at the location of the draft marks.
     
  5. Yobarnacle
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    Yobarnacle Senior Member

    Off topic maybe, but for clarification, the loadlines amidship, including Plimsol mark, measure freeboard rather than draft. The reference point for loadlines is the deck edge.
     
  6. athvas
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    athvas Senior Member

    Draft marks are measured from the keel line (including keel thickness). But as TANSL said when you refer the hydrostatic table you must be sure that the displacement values are given for drafts from the Base Line or from the keel Line and you have to calculate the displacement accordingly.
     
  7. Dibin_marino
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    Dibin_marino Junior Member

    Dear All,

    Thank you for your assistance regarding this matter

    As per the draft marks drawing of a 150 pax catamaran, its mentioned that draft marks are taken from baseline of the ship, the problem is that the designers has not provided any exact definition for the baseline, whether it is including thickness or not, also the trim and stability booklet the hydrostatics are prepared with reference to Draft at AP, As its the issue of Draft survey if i took draft marks with including keel thickness and without minuzing keel thickness
    the vessel will get a displacement more than 2% of the original light ship displacement.

    I am little bit confused in taking the draft marks. it would be really great full if anyone can assist me.
     
  8. TANSL
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    TANSL Senior Member

    While others may have a different opinion I think that if the hull is GRP (and has been built with female mold) baseline will be located outside of the liner. Otherwise, it is normal to be baseline inside lining.
    In any case, see the difference you have in the displacement, considering the baseline outside and inside, and as likely to be very small, forget the problem and do what is most comfortable for you and for which subsequently , will take the drafts.
    Now that nobody reads us, let me tell you that reading drafts often becomes far more error that the error your marks may enter.
     
  9. athvas
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    athvas Senior Member

    Find the displacements with the drafts excluding the keel thickness and check it is within the limits or not.
    Class allows deviation limit of 1% for ships of 160 m or more in length and 2% for ships of 50 m or less in length and as determined by linear interpolation for intermediate lengths or a
    deviation from the lightship longitudinal centre of gravity of 0.5% of Ls , Otherwise vessel should be inclined.
     
  10. Dibin_marino
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    Dibin_marino Junior Member

    While calculating the Displacement and LCG we have found that the vessel is within the recommended limit of 2% of Displacement and 1% of LCG. but my actual concern is that witnessing surveyor is not sticking on to my points.he want any supporting documents from IMO or any other rules to make him convience him.
     
  11. athvas
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    athvas Senior Member

    LCG allowable limit is 0.5% not 1% .... Can i have a look at your results of displacement and LCG of the sister vessel. Also the Reference values of the Lead ship. what is the Length of the vessel (Rule Length)?
     
  12. TANSL
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    TANSL Senior Member

    In my opinion, you have no choice but to figure out how they have calculated the hydrostatic, where they have considered is the baseline.
     
  13. Dibin_marino
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    Dibin_marino Junior Member

    Light ship survey report

    Hi,

    i am attaching a Preliminary Light ship survey report of a vessel which i have done recently, kindly look into it and advise if found any mistakes
     

    Attached Files:


  14. TANSL
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    TANSL Senior Member

    We can not do much with the data that you bring except check if you multiplied well, which, I suppose, is not what you wanna do.
    I only have two comments:
    - The density of seawater is about 1,025 tons/m3
    - To calculate the displacement, hydrostatic interpolating in, you have to use the mean draft of the ship and hydrostatic with the trim that has the ship at the time of the study. If not, you must make a correction for trim.
     
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