Date / Initial Final1a Draft forward1b Draft aft mean1c = 1a + 1b / 2 Draft fore & aft mean2a Draft midship PS2b Draft midship SB2c = 2a + 2b / 2 Draft midship mean3 = 1c + 2c / 2 Draft mean for means4 Draft corr. for deformation5 (at draft 4) Displacement at sg 1.025Trim bow / sternTrim in cmLCFLBPTPC (t/cm)MTCT DiffDensity6 Trim correction 17 Trim correction 28 Correction for DensityConsumable weightsHFOGOLUBFresh waterBallastConstant/Others9 Total known weights10 = 5 + 6 + 7 - 8 Corrected displacement11 Light ship12 = 10 – 9 – 11 Cargo weight total
Draft surveys are made in order todetermine the quantity of cargoloaded, carried and discharged. This isdone by measuring the vessels draft andcalculating its displacement prior toloading a cargo and after loading of thiscargo, taking variables such as weightlightship, ballast water, fuel and storesinto account. In order to obtain a reasonable accuracyin this draft survey calculationcorrections to the draft readings and itinitial corresponding stability values andparameters must be made.
General definitions; Displacement of a vessel is the actual total weight ofa vessel. It is expressed in metric tons, and iscalculated by multiplying the volume of the hull belowthe waterline (the volume of water it is displacing) bythe density of the water. The density will depend onwhether the vessel is in fresh or salt water, or is in thetropics, where the temperature of the water is warmerand hence less dense. Deadweight of a vessel (often abbreviated as DWTfor deadweight tons) is the displacement at anyloaded condition minus the lightship weight. Itincludes the crew, passengers, cargo, fuel, water, andstores. Like displacement, it is often expressed in longtons or in metric tons. Lightship measures the actual weight of the ship withno fuel, passengers, cargo, water, etc. on board. Thisweight will be mentioned in the stability booklets.
Gross tonnage or Gross Register Tonnage is thetotal internal volume of a vessel, with someexemptions for non-productive spaces such as crewquarters. There are therefore different gross tonnagemeasurements (Suez/Panama). Tonnage measurements are now governed since1994 by an IMO Convention (InternationalConvention on Tonnage Measurement of Ships, 1969(London-Rules)), which applies to all ships built afterJuly 1982. In accordance with the Convention, thecorrect term to use now is GT, which is a function ofthe moulded volume of all enclosed spaces of theship. Net Weight or Net Register Tonnage is thevolume of cargo the vessel can carry; in other wordsits the Gross Tonnage minus the volume of spacesthat do not hold any cargo (e.g. engine room,bridge, crew spaces, depending which country ismaking calculations (Suez/Panama)).
During a draft survey the vessels draft is measured. Thedraft is measured at 6 points, 2 x bow (P/S), 2 x midships(P/S) and 2 x at the stern (P/S). The draft readings areaveraged out in the mean draft. Via calculations this draftreading leads us to the vessels present apparentdisplacement at the time of our survey via onboard, ship’s specific, hydrostatic tables and stabilitycurves. The water density at the time of the draft survey is alsodetermined by means of a density meter. A very commoninstrument for the direct measurement of the density of aliquid is the hydrometer, a floater. The density of thewater in which the vessel is surveyed directly influencesthe draft/dept of the vessel. Considering that thehydrostatic tables and stability curves are pre-calculatedfor displacements of a vessel in water with a standarddensity of 1.025 t/m³ (salt water), corrections to thedetermined draft must be made in order to be able to findthe correct displacement of the vessel. Make sure at thetime of measuring the density the density meter is freefloating.
Displacement correction In order to findthe correct displacement we will correct thedisplacement straight away, instead ofcorrecting the drafts at first. We will use thehydrostatic tables with the mean draft asentry value. These calculations are called thetrim corrections. Likewise we will also makea correction for water density but again atonnage correction and not a draftcorrection.
First trim correction, It is necessary to correct the fore & aft drafts to thetrue draft at the perpendiculars. This can be achieved either by calculation or draftcorrection tables if available. The main calculationsmust be accomplished with the drafts at the fore & aftperpendiculars and at the amidships point of the hull. Note that draft marks are not always placed on theperpendiculars, so the true draft of the vessel does notalways correspond with the drafts found during thesurvey. The hydrostatic tables are mostly based on thelength between perpendiculars and not to the lengthbetween the draft marks on the hull.TRIM(cm) x TPC x LCF 1st Trim Correction= --------------------------------LBP
Please note that the LCF is the distancebetween the midship point and theposition of the LCF. This correction is either added orsubtracted to the displacement dependingwhether the LCF is forward or aft of themid point, or the trim is by the head orstern. Always draw a sketch to ensure youapply this trim correction the correct way.
Second trim correction This is a little more difficult to grasp. It isbecause there is in fact a second movement ofthe LCF caused because of the irregular hullshape of the vessel in trimmed condition. Theposition of the LCF in the ship’s hydrostatictables are given for a ship on even keel only, butif the ship is trimmed the waterline haschanged, causing a second movement to theLCF. The second trim correction is alwayspositive and fairly small, for example with aloaded Panamax at a one metre trim thecorrection should only be approximately 12tonnes, with a two metre trim approximately 50tonnes.
This second trim correction is calculated bythe following formula:Trim²(m) x ΔMTCT x 50 2nd Trim Correction: ----------------------------LBP In this formula the ΔMTCT is the difference ofMTCT values over a range of draft 50 cmabove and 50 cm below the corrected meandraft. This second trim correction is always addedto the displacement.
The second trim correction, sometimes called theNemoto correction, is intended to correct for themovement of the LCF with the change of trim. It wasdeduced by Mr Nemoto after observing a bulk carrierbeing built. The theory on which the second trimcorrection is based only holds true for small changesof trim so it is not strictly true for large trims. Pleasenote that the second trim correction is rather smallfor small trims, and rarely exceeding 30 tonnes forlarge vessels. It is often ignored but forcompleteness sake the second trim correction shouldalways be calculated. Density correction If the vessel is in a dock waterdensity then she is sitting differently in the waterthan she would be if in sea water so we are reading adifferent displacement from the hydrostatic tableswhich are derived for salt water only. The followingformula is simply the dock water formula convertedto a tonnage figure, not a millimetre value.
(1.025 – dock density) x Displacement Density Correction: --------------------------1.025 And of course this is subtracted from thedisplacement if the dock water density isbelow 1.025 t/m³ It is worth mentioning at this point thatalthough we always consider the density ofsalt water to be 1.025 t/m³, if you were totake a reading in Mediterranean Sea portsyou would probably find it to be around the1.030 t/m³ to 1.033 t/m³ mark!
Please note that the ship’s standard hydrometeris not an accurate instrument when it comes tousing it to determine tonnage for a draft survey.It makes no allowance for the temperature of thewater which will change the density astemperature changes. Corrected displacement When the truedisplacement of the vessel is known it onlyrequires the subtraction of all the known weightson board and the subtraction of the ships lightdisplacement. What remains is either the vessels‘constant’ if in an empty condition or the cargoweight if in a loaded condition. The reason for two draft surveys is at first to findthe vessels constant and then include this in theknown weights when calculating the actual cargoon board after loading.
When taking a draft survey upon discharge thecargo weight plus constant are found in theinitial survey and the constant in the finalsurvey, subtract the constant from the initialfigure and you have the weight of cargodischarged.TRIM(cm) x LCF x TPC 1st Trim Correction: --------------------------------LBPx x = ---------------------------= tonnesLBP
Many commercial ships have a symbol called aLoad mark painted on each side of the ship.This symbol, also called an International loadline, or Plimsoll mark, marks the level to whichthe ship can be safely loaded. As cargo isbrought on board, the ship floats lower and thesymbol descends farther into the water. Before these symbols were madecompulsory, many ships were lost due tooverloading. Sometimes they were deliberatelyoverloaded in the hope of collecting insurancemoney. Ships carrying emigrants from Europe toAmerica were also lost. The British socialreformer and politician Samuel Plimsolladvocated improved safetystandards, particularly at sea, and the markbears his name in his honor.
The letters on the Load line markshave the following meanings: TF - Tropical Fresh Water F - Fresh Water T - Tropical Seawater S - Summer Seawater W - Winter Seawater WNA - Winter North Atlantic
Letters may also appear to the sides ofthe mark indicating the classificationsociety that has surveyed the vesselsload line. The initials used include AB forthe American Bureau of Shipping, LR forLloyds Register, and NV for Det NorskeVeritas.
These season and zone marks are used toensure adequate reserve buoyancy for theintended area of operation. Ships encounterrougher conditions in winter as opposed tosummer, and in the North Atlantic as opposedto tropical waters, for example. A copy of azone-chart must be on board in order toassure that the vessel is not overloaded. Fresh water marks make allowance for thefact that the ship will float deeper in freshwater.
A draft survey is a means of determining theweight of any materials being loaded into (ordischarged from) a vessel. SGS’s experiencedand highly qualified professionals can conduct adraft survey on your behalf and produce anindependent draft survey report. The vessel draft survey measures thedisplacement of the water both before and afterthe loading or unloading, with the resultingdifference between the two displacementsrepresenting the weight of the cargo. Oursurveyors work according to definedinternational guidelines when determining theweight of the cargo and consider several keyfactors while making their surveys. These factorsinclude:
Water density of the sea, river or otherwaterway Changes in the ballast quantity betweeninitial and final draft readings Changes in the vessel’s consumables (e.g.fuel oil, drinking water) between initialand final draft readings Trim and deformation corrections (we usedata from the vessel’s draft tables) The accuracy of a weight determined bydraft survey also depends upon the seaconditions (pitch and swell) at the time ofloading or discharge.