Draft survey calculation_sheet_2010.1
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  • RaetsMarine Insurance B.V. Draft Survey calculation sheet Date / Initial Final 1a Draft forward 1b Draft aft mean 1c = 1a + 1b / 2 Draft fore & aft mean 2a Draft midship PS 2b Draft midship SB 2c = 2a + 2b / 2 Draft midship mean 3 = 1c + 2c / 2 Draft mean for means 4 Draft corr. for deformation 5 (at draft 4) Displacement at sg 1.025 Trim bow / stern Trim in cm LCF LBP TPC (t/cm) MTCT Diff Density 6 Trim correction 1 7 Trim correction 2 8 Correction for Density Consumable weights HFO GO LUB Fresh water Ballast Constant/Others 9 Total known weights 10 = 5 + 6 + 7 - 8 Corrected displacement 11 Light ship 12 = 10 – 9 – 11 Cargo weight total Cargo loaded/discharged = Our contact details: RaetsMarine Contact: Jeroen Ebbeling/Robbert Beekman Fascinatio Boulevard 622 2909 VA Capelle A/D IJssel, The Netherlands Telephone: .. +31-10-24 25 000 Faxnumber: . +31-10-24 25 088 E-mail: j.ebbeling@raetsmarine.com E-mail: r.beekman@raetsmarine.comDraft Survey Calculation Sheet, 2010.1 www.raetsmarine.com 1 of 6
  • RaetsMarine Insurance B.V.Draft surveys are made in order to determine the quantity of cargo loaded, carried anddischarged. This is done by measuring the vessels draft and calculating itsdisplacement prior to loading a cargo and after loading of this cargo, taking variablessuch as weight lightship, ballast water, fuel and stores into account.In order to obtain a reasonable accuracy in this draft survey calculation corrections tothe draft readings and it initial corresponding stability values and parameters must bemade.General definitions;Displacement of a vessel is the actual total weight of a vessel. It is expressed in metric tons, and iscalculated by multiplying the volume of the hull below the waterline (the volume of water it is displacing) bythe density of the water. The density will depend on whether the vessel is in fresh or salt water, or is in thetropics, where the temperature of the water is warmer and hence less dense.Deadweight of a vessel (often abbreviated as DWT for deadweight tons) is the displacement at any loadedcondition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Likedisplacement, it is often expressed in long tons or in metric tons.Lightship measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board. Thisweight will be mentioned in the stability booklets.Gross tonnage or Gross Register Tonnage is the total internal volume of a vessel, with some exemptionsfor non-productive spaces such as crew quarters. There are therefore different gross tonnage measurements(Suez/Panama).Tonnage measurements are now governed since 1994 by an IMO Convention (International Convention onTonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. Inaccordance with the Convention, the correct term to use now is GT, which is a function of the mouldedvolume of all enclosed spaces of the ship.Net Weight or Net Register Tonnage is the volume of cargo the vessel can carry; in other words its theGross Tonnage minus the volume of spaces that do not hold any cargo (e.g. engine room, bridge, crewspaces, depending which country is making calculations (Suez/Panama)).During a draft survey the vessels draft is measured. The draft is measured at 6 points, 2 x bow (P/S), 2 xmidships (P/S) and 2 x at the stern (P/S). The draft readings are averaged out in the mean draft. Viacalculations this draft reading leads us to the vessels present apparent displacement at the time of oursurvey via on board, ship’s specific, hydrostatic tables and stability curves.The water density at the time of the draft survey is also determined by means of a density meter. A verycommon instrument for the direct measurement of the density of a liquid is the hydrometer, a floater. Thedensity of the water in which the vessel is surveyed directly influences the draft/dept of the vessel.Considering that the hydrostatic tables and stability curves are pre-calculated for displacements of a vesselin water with a standard density of 1.025 t/m³ (salt water), corrections to the determined draft must be madein order to be able to find the correct displacement of the vessel. Make sure at the time of measuring thedensity the density meter is free floating.Displacement correction In order to find the correct displacement we will correct the displacement straightaway, instead of correcting the drafts at first. We will use the hydrostatic tables with the mean draft as entryvalue. These calculations are called the trim corrections. Likewise we will also make a correction for waterdensity but again a tonnage correction and not a draft correction.Draft Survey Calculation Sheet, 2010.1 www.raetsmarine.com 2 of 6
  • RaetsMarine Insurance B.V.First trim correction,It is necessary to correct the fore & aft drafts to the true draft at the perpendiculars.This can be achieved either by calculation or draft correction tables if available. The main calculations mustbe accomplished with the drafts at the fore & aft perpendiculars and at the amidships point of the hull.Note that draft marks are not always placed on the perpendiculars, so the true draft of the vessel does notalways correspond with the drafts found during the survey. The hydrostatic tables are mostly based on thelength between perpendiculars and not to the length between the draft marks on the hull. TRIM(cm) x TPC x LCF1st Trim Correction = -------------------------------- LBPPlease note that the LCF is the distance between the midship point and the position of the LCF.This correction is either added or subtracted to the displacement depending whether the LCF is forward oraft of the mid point, or the trim is by the head or stern. Always draw a sketch to ensure you apply this trimcorrection the correct way.Second trim correctionThis is a little more difficult to grasp. It is because there is in fact a second movement of the LCF causedbecause of the irregular hull shape of the vessel in trimmed condition. The position of the LCF in the ship’shydrostatic tables are given for a ship on even keel only, but if the ship is trimmed the waterline haschanged, causing a second movement to the LCF. The second trim correction is always positive and fairlysmall, for example with a loaded Panamax at a one metre trim the correction should only be approximately12 tonnes, with a two metre trim approximately 50 tonnes.This second trim correction is calculated by the following formula: Trim²(m) x ΔMTCT x 502nd Trim Correction: -------------------------------------------- LBPIn this formula the ΔMTCT is the difference of MTCT values over a range of draft 50 cm above and 50 cmbelow the corrected mean draft.This second trim correction is always added to the displacement.The second trim correction, sometimes called the Nemoto correction, is intended to correct for the movementof the LCF with the change of trim. It was deduced by Mr Nemoto after observing a bulk carrier being built.The theory on which the second trim correction is based only holds true for small changes of trim so it is notstrictly true for large trims. Please note that the second trim correction is rather small for small trims, andrarely exceeding 30 tonnes for large vessels. It is often ignored but for completeness sake the second trimcorrection should always be calculated.Density correction If the vessel is in a dock water density then she is sitting differently in the water than shewould be if in sea water so we are reading a different displacement from the hydrostatic tables which arederived for salt water only. The following formula is simply the dock water formula converted to a tonnagefigure, not a millimetre value. (1.025 – dock density) x DisplacementDraft Survey Calculation Sheet, 2010.1 www.raetsmarine.com 3 of 6
  • RaetsMarine Insurance B.V.Density Correction: --------------------------------------------------- 1.025And of course this is subtracted from the displacement if the dock water density is below 1.025 t/m³It is worth mentioning at this point that although we always consider the density of salt water to be 1.025 t/m³,if you were to take a reading in Mediterranean Sea ports you would probably find it to be around the 1.030t/m³ to 1.033 t/m³ mark!Please note that the ship’s standard hydrometer is not an accurate instrument when it comes to using it todetermine tonnage for a draft survey. It makes no allowance for the temperature of the water which willchange the density as temperature changes.Corrected displacement When the true displacement of the vessel is known it only requires the subtractionof all the known weights on board and the subtraction of the ships light displacement. What remains is eitherthe vessels ‘constant’ if in an empty condition or the cargo weight if in a loaded condition.The reason for two draft surveys is at first to find the vessels constant and then include this in the knownweights when calculating the actual cargo on board after loading.When taking a draft survey upon discharge the cargo weight plus constant are found in the initial survey andthe constant in the final survey, subtract the constant from the initial figure and you have the weight of cargodischarged. TRIM(cm) x LCF x TPC1st Trim Correction: -------------------------------- LBP x x = ------------------------------ = tonnes LBP 2 Trim (m) x MTCT Diff x 502nd Trim Correction: ----------------------------------- LBP x x 50 = ----------------------------------- = tonnes (always add) LBP (1.025 – dock density) x DisplacementDensity Correction: --------------------------------------------------- 1.025 x = -------------------- = tonnes 1.025Draft Survey Calculation Sheet, 2010.1 www.raetsmarine.com 4 of 6
  • RaetsMarine Insurance B.V.Many commercial ships have a symbol called a Load mark painted on each side of the ship. This symbol,also called an International load line, or Plimsoll mark, marks the level to which the ship can be safelyloaded. As cargo is brought on board, the ship floats lower and the symbol descends farther into the water.Before these symbols were made compulsory, many ships were lost due to overloading. Sometimes theywere deliberately overloaded in the hope of collecting insurance money. Ships carrying emigrants fromEurope to America were also lost. The British social reformer and politician Samuel Plimsoll advocatedimproved safety standards, particularly at sea, and the mark bears his name in his honor.The letters on the Load line marks have the following meanings: • TF - Tropical Fresh Water • F - Fresh Water • T - Tropical Seawater • S - Summer Seawater • W - Winter Seawater • WNA - Winter North AtlanticLetters may also appear to the sides of the mark indicating the classification society that has surveyed thevessels load line. The initials used include AB for the American Bureau of Shipping, LR for Lloyds Register,and NV for Det Norske Veritas.These season and zone marks are used to ensure adequate reserve buoyancy for the intended area ofoperation. Ships encounter rougher conditions in winter as opposed to summer, and in the North Atlantic asopposed to tropical waters, for example. A copy of a zone-chart must be on board in order to assure that thevessel is not overloaded.Fresh water marks make allowance for the fact that the ship will float deeper in fresh water.Draft Survey Calculation Sheet, 2010.1 www.raetsmarine.com 5 of 6
  • RaetsMarine Insurance B.V.Draft Survey Calculation Sheet, 2010.1 www.raetsmarine.com 6 of 6