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Draft Survey: Procedures and Calculation



marine cargo ship surveyors

The Draft Survey procedures and calculation ascertained as the following series :

  1. Reading the draftmark of the ship, which consist of six (6) points of draftmarks, i.e.; Fore, Midship, and After at both sides of the ship,
  2. Sampling and testing the sea water or dock water density at the place where the vessel floats,
  3. Determining of deductible weights by measuring and sounding of ballast tanks, fuel oil, fresh water that existing onboard at the time of survey,
  4. Using Hydrostatic Table provided onboard to calculation.

Reading the Draftmark of the ship

Commonly, all ship are designed with draftmark for working with Draft Survey to determined their actual weight. The draftmark could be find at six (6) points on the below places:

  • Forward Port Side (FP),
  • Forward Starboard Side (FS),
  • Midship Port Side (MP),
  • Midship Starboard Side (MS),
  • Aftward Port Side (AP),
  • Aftward Starboard Side (AS),

View the Draftmark:

marine cargo ship surveyors

Use the small boat to go around the ship and get as near as possible to the draft mark for best viewing. The surveyor should be read all above marks clearly, because reading the draftmark is the first and most essential process. I am not saying that other processses is not essensial, but this process is hard to do and involves many rules of conduct to gain the correctness and accuracy of Draft Survey itself (I will post it later). The draftmark read is recorded on the surveyor notebook, do not try to remember it or write down in your palm hand. Its useless and un-professional.

Sampling and testing the sea water or dock water density

After reading the draftmark, directly engage with the sampling of sea water or river water around the ship’s dock.  Why? Because the ship draft will not be the same at different water densities (at the lower density means the ship more sink and at the higher density means the ship more float).  Where as the water density is subject to changes which follow with water tide that carrying different water salinity and temperature on to the ship dock. The sea water density is indeed at density 1.025 and the fresh water at density 1.000. To determine the density of water, we need the instrument named Hydrometer or Density Meter. Inserted the Hygrometer on to the water sample on the Sampling Can or Tube, then we could check the scale pointed on the surface of the sampling water. Records the water dock density as survey data.

Determining of deductible weights by measuring and sounding

Deductible Weight could measure by sounding  the tanks which used the Sounding Tape or gauging the tank level by visual inspection. Any deductible weight such as Ballast Water, Fresh Water, Fuel and Diesel Oil, and Bilges is notify to check. Records all in the survey book includes with the density for Ballast and Bilges, and for Oil complete it with density and temperature . The Fresh Water was at density 1.000.

View the Sounding Pipe:

marine cargo ship surveyors

Using Hydrostatic Table provided onboard to begin calculation,

I think all necessary data was completed, so we could do calculation. The calculation is uses Displacement Table or usually called Hydrostatic Table. This table is included all data that we need to complete the calculation.

  • Raw Draft Calculation; Fore Mean or Fm = (FP+FS)/2, Mid Mean or Mm = (MP+MS)/2, and Fore Mean or Am = (AP+AS)/2. while Apparent Trim  or AT = Am – Fm. the Apparent Trim is the Trim that visually find.
  • Draftmark posision and correction to perpendicular. As the ship draftmark is not placed at the perpendicular, the Fore and After draft should be corrected with distance from the draftmark to perpendicular. The correction rules is: if the Trim by Stern, the Fore correction should be minus and After correction plus, and if the Trim by Head (stem), the Fore correction should be plus and After correction minus. The Midship correction is parallel with the fore correction with the same pattern. Some Hydrostatic table provided with these correction result. But if not the reference pattern is  for Fore Correction or Fc = (Fd x AT) : LBM and After Correction or Ac = (Ad x AT) / LBM. Where Fd = Fore distance to perpendicular, Ad = After distance to perpendicular, and LBM = Length Between Mark or Length between Fore and After draftmarks  or LBM = LBP – (Fd + Ad).
  • True Draft Calculation / Draft Corrected; Fore draft corrected or Fcd = Fm + Fc, Mid draft corrected or Mcd = Mm + Mc, and After draft corrected or Acd = Am + Ac.
  • True Trim or TT : Actual  Ship Trim after draft corrected or  TT = Acd – Fcd.
  • Fore and After Mean Draft or FAm = (Fcd + Acd)/2, Mean of Mean Draft or MM = (FAm + Mcd)/2, and Mean of Mean of Mean Draft  or MMM or Quarter Mean = (MM + Mcd)/2.
  • The above calculation is similar with : MMM = {(Fcd x 1) + (Acd x 1) + (Mcd x 6)}/8.
  • Coresponding to the MMM or Quarter Mean result, the surveyor could check the value of needed parameters on Hysdrostatic table like; Displacement, TPC, LCF, and MTC. Records them accurately.
  • Get the Displacement or Disp.
  • First Trim Correction or FTc = (TT x LCF x TPC x 100) / LBP. Could be plus or minus depend on LCF.
  • Second Trim Correction STc = (TT x TT x MTC x 50) / LBP. The result always plus (+).
  • Displacement corrected by Trim or DispT = D – (FTc + STc).
  • Density Correction or Denc = DispT x {(Aden – 1.025) / 1.025}. where the Aden is Actual Density that surveyor has taken sampling and testing previously. The density correction commonly in minus (-), due to the Actual Density is usually lower than 1.025 (fresh sea water). In case of at some port where the water salinity is high, the density correction could be plus (+).
  • And we have got the Displacement corrected by Density or DispDenc = DispT + Denc. (after corrected by density we will get the actual ship weight as per shown by Draft Survey)
  • Deductible Calculation. The same as draft, the deductible also need to corresponding to the table that named Tank Table / Tank Capacity Table. Refer to the sounding records that done before, the surveyor could be calculate the total deductbile existing onboard. Total Deductible or Deduct =  Ballast Water + Fresh Water + Bilges + Fuel Oil + Diesel Oil,  this total should be minus to the Displacement corected by Density.
  • The Net Displacement or NDisp = DispDenc – Deduct.
  • The Net Displacement is the actual ship weight after minus with deductible weight. For Unloading, to estimate the quantity of cargo onboard, the Net displacement should be minus with Light Ship and Constant.

I think difficult for the amateur writer like me to describe the complex narration of draft survey calculation, but to make it clear, the sample of Draft Survey Calculation will be post sooner or later.. :)

For video reference >> http://www.sevensurveyor.com/video/

Using draft survey software  link http://www.sevensurveyor.com/software/surveysoft-marine-surveyor-software/

Regards,
Surveyors



Author: Faisal Yusuf

Hi, my name is Faisal Yusuf, but you can call me Surveyor. I live in Jakarta, Indonesia and work as a surveyor.

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172 Comments

  1. Dear All,

    Kindly pls advise the rule for percentage of different between draft survey with shore scale/weigher.

    Thanks/Suhendar

  2. Alex,

    You have to consider the trim of tug boat to obtain the correction of sounding that you have done, due to on tug / vessel with trim the liquid on tank were at different level between sides (fore and aft sides of tank). The correction is made to flat it on the same level (being actual liquid level) for calculation.

    As your sounding quantity still in Cubic Meter, the density of fuel oil is needed to converted the Cubic Meter to Kilo Liter, and than from Kilo Liter to Metric Ton. ASTM Table 54b (for Volume Correction Factor or VCF) and Table 56 (for Weight Correction Factor or WCF).

    Regards,
    Faisal

  3. Dear Sir,

    What if we have to consider the trim of the tugboat and the density of the fuel oil?

    Please advise.

    Regards,
    Alex

  4. Thanks, that was really helpful.

  5. Dear Capt. Valter,

    Thanks to comment.

    Although in calculation the differences of both methods (1 and 2) in small quantities as well, but it could be a question to standard of draft survey that we have to use from the our customers.

    I am in opinion that the Displacement corrected to density should be the displacement after corrected by the 1st and 2nd trim correction. By mean, the vessel displacement should be free from all vessels un-box shape and un-even keel correction before correcting by density. This method is commonly use for bulk carrier and it recorded on vessel hydrostatic table.

    The UN ECE might be mean the same, the D they meant is D after 1st and 2nd trim correction, where we mentioned as DispT.

    Waiting for your next comments. :)

    Best Regards,
    Faisal

  6. Dear sir,

    I have find discrepancy between your way to calculate density correction and UN ECE recomandations (which is used as a standard for many customers). I’m asking You for the opinion becouse we have discousssed a lot among my coleagues.

    The difference is in Displacement where by you Displacement corrected for the trim should be used, while by UN ECE standard the displacement from the table (line 161) should be used.

    1. Your way and also my opinion is the same

    Denc = DispT x {(Aden – 1.025) / 1.025}

    2. UN ECE method

    Denc = D x {(Aden – 1.025) / 1.025}

    Please tell me your opinion.

    Regards

    Capt Valter Suban, M.Sc.

  7. Thanks that was helpful.

  8. Hi Milind,

    – Expect to read the Draft maximum sea vessels, although it is not possible or dangerous.
    – In circumstances like this, read the Draft and land next to the sea with the help of manometer, consisting of clear plastic hose, strong, mounted on both ends of the pipe to facilitate water level measurements.
    – If possible, pipes installed along the width of the entire ship – the size is taken from the overall width.
    – Reading Draft sea side obtained through adding to or subtracting from the land side of the manometer readings
    – The surface is measured from the same base line and taken to the same distance from both sides of the ship’s centerline.
    – All air must be expelled from the hose and the pipe surface is maintained at a higher elevation than the deck on the centerline.
    – The height of the water surface on both sides of the ship measured above deckline (h) or other fixed surface. The difference between the height divided by two and the result (d) is added to or subtracted from the readings Draft one to get the mean on Draft centerline.

    SAMPLE OF CALCULATION
    – QUESTION:
    Draft midships left = 8.00M, the right side of the manometer readings 8.50M on both sides of centerline.
    Full width of the ship = 20.85m. Draft midships Calculate the mean, the centerline Draft, if the difference between the 2 readings manometer manometer readings higher right = 25cm.

    – ANSWER: The width of effective 8:50 x 2 = 17.00M, with the triangle at 0:25: 17 = d: 20.85
    d = 20.85×0.25: 17 = 0.307m wide differences in management.
    Differences half-width = 0307: 2 = 0.153m. Centerline Draft = 0153 8:00 = 8.153m

    DRAUGHT SURVEY
    THE U.K. VERSION, 1994

  9. Hi Alex,

    Thanks, you have to use interpolation to get the mentioned quantity.

    Let’s assume 2.380 = A, 2.375 = B, 2.400 = C, 26.15 = D, and 26.50 = E.

    The Formula is: ROB = D + {((A – B) / (C – B)) x (E – D)}

    As per the above formula the ROB found 26.157 m3. You can try it. :)

  10. Dear Sir,

    I would like to know the formula to calculate the remaining bunker (MGO/Fresh Water/LO/GO/DO) on board based on the tank sounding table.

    For example:
    Sounding – 2.38m
    Tank sounding table shows:
    1) 2.375 – 26.15m3
    2) 2.40 – 26.50m3
    How to get the value (ROB) at 2.38m?

    Regards,
    Alex

  11. Can some one please guide me on the procedure of calculating overside drafts of a vessel with a list and where over side drafts cannot be successfully read.

  12. Hi Myo..

    Thanks for your comments.

    As I know, there is no differences for formula between the Ship and Hopper Barge Draft Survey. Draft Survey formula is for general uses.

    For references, you could download the complete Draft Survey Standard and Procedures at this LINK

  13. Dear Sr,
    Your posted were support me to learn about Draft survey.I’m a junior surveyor in our country,Union of Myanmar at South East Asia.
    I want to request you that to explain about Draft survey for Barge carrier and Hopper calculation.Because the technique in our country were very poor to learn.I want to know the formula to calculate the water content of Sea Sand cargo.
    Regard,
    Myo

  14. Hi James, thanks for your question..

    I have made try to adopt from Petroleum Calculation named Weight Formula. The formula is as follows:

    A = S + {(U – DF) x F}
    Where :
    – A : Adjusted Sounding at Aft Bulkhead Tank
    – S : Actual Sounding
    – U : Sounding point distance to Aft Bulkhead
    – D : Total Height of Tank
    – F : Trim Factor, where the formula is Trim divided by LBP.
    – DF = D x F.

    So after getting A, refers it to the Tank Capacity Table to find the quantity on Tank.

    For example No. 2 Water Ballast Tank Portside:
    Sounding = 1.27 meters,
    U = 3.00 meters,
    D = 19.00 meters,
    Trim = 2.50 meters,
    LBP = 182.00 meters

    So we can calculate,
    F = 2.5 : 182 = 0.014 and
    DF = 19.00 x 0.014 = 0.261
    Then,
    A = 1.27 + {(2.5 – 0.261) x 0.014}
    A = 1.27 + (2.239 x 0.014)
    A = 1.27 + 0.031
    A = 1.301. (check on Tank Capacity Table).

    Does anyone has the other correction ways? I would be plesant if you could submit reply to this comment.

  15. I found your brief on draught surveys very informative. Can you advise if there is a standard correction formula for asertaining the amount of ballast in a tank where there are no trim corrections available.
    Regards,

    James Barr

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