Draft Survey Guidelines,January,2010

8/3/2019 Draft Survey Guidelines,January,2010

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RaetsMarine Insurance B.V.

Draft Survey guidelines, 2010.1 www.raetsmarine.com 1 of 5

DRAFT SURVEY GUIDELINES.

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 its

displacement prior 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;

Displacementof a vessel is the actual total weight of a vessel. It is expressed inmetric tons, and iscalculated by multiplying the volume of thehullbelow the waterline (the volume of water it is displacing) by

the density of the water. (the density will depend on whether the vessel is in fresh or salt water, or is in thetropics, where 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, andstores. LikeDisplacement, it is often expressed in long tons or in metric tons

Lightship measures the actual weight of the ship with nofuel, passengers,cargo, water, etc. on board.

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.engineroom, 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, 2 x midshipsand 2 x at the stern. The draft readings are averaged out in a mean draft. Via calculations this draft readingleads us to the vessels present apparent displacement at the time of our survey. Via on board, ships specifichydrostatic tables and stability curves.

The water density at the time of the draft measurement is also determined by means of a density meter. Avery common instrument for the direct measurement of the density of a liquid is thehydrometer, a floater.The density 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 (salt water), corrections to the determined draft must be made inorder to be able to find the correct displacement of the vessel.

Displacement correction, so we will make this correction but instead of correcting the draft we will correctthe displacement (at 1.025) that we found from the hydrostatic tables with the mean draft as entry value,These calculations are called the trim corrections. Likewise we will also make a correction for water densitybut again a tonnage correction and not a draft correction.
http://en.wikipedia.org/wiki/Displacement_%28fluid%29http://en.wikipedia.org/wiki/Displacement_%28fluid%29http://en.wikipedia.org/wiki/Tonnehttp://en.wikipedia.org/wiki/Tonnehttp://en.wikipedia.org/wiki/Tonnehttp://en.wikipedia.org/wiki/Hull_%28watercraft%29http://en.wikipedia.org/wiki/Hull_%28watercraft%29http://en.wikipedia.org/wiki/Hull_%28watercraft%29http://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Storeshttp://en.wikipedia.org/wiki/Storeshttp://en.wikipedia.org/wiki/Storeshttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Cargohttp://en.wikipedia.org/wiki/Cargohttp://en.wikipedia.org/wiki/Cargohttp://en.wikipedia.org/wiki/Enginehttp://en.wikipedia.org/wiki/Enginehttp://en.wikipedia.org/wiki/Enginehttp://en.wikipedia.org/wiki/Crewhttp://en.wikipedia.org/wiki/Crewhttp://en.wikipedia.org/wiki/Crewhttp://en.wikipedia.org/wiki/Hydrometerhttp://en.wikipedia.org/wiki/Hydrometerhttp://en.wikipedia.org/wiki/Hydrometerhttp://en.wikipedia.org/wiki/Hydrometerhttp://en.wikipedia.org/wiki/Crewhttp://en.wikipedia.org/wiki/Enginehttp://en.wikipedia.org/wiki/Cargohttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Storeshttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Hull_%28watercraft%29http://en.wikipedia.org/wiki/Tonnehttp://en.wikipedia.org/wiki/Displacement_%28fluid%29


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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 the true draft at the vessels perpendiculars does not always correspond with the place on the hullwere the draft signs are placed and read. The hydrostatic tables, however, are based on the vessel with alength corresponding to the perpendiculars and not to the length between the draft marks on the hull.

TRIM(cm) x LCF x TPC1st Trim Correction = --------------------------------

LBP

Where LCF is the distance between the midship point and the position of the LCF.

This correction is added or subtracted to the displacement depending whether the LCF is forward or aft ofthe 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 correction,This 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 a vessel in trimmed condition. The position of the LCF in the shipshydrostatics is given for a ship on even keel only but if the ship is trimmed the waterline must have changedcausing a second movement to the LCF. Always positive this second trim correction is fairly small, forexample with a loaded Panamax at a 1 metre trim the correction should only be about 12 tonnes, with a 2metre trim about 50 tonnes.

This second movement is corrected by the following formula:

2Trim(m) x MCTC Diff x 50

2nd Trim Correction: -----------------------------------LBP

Where MCTC Diff is the difference of MCTC values over a range of draft 50 cm above and below thecorrected mean draft.This second trim correction is always added to the displacement.

The second trim correction, sometimes called the Nemoto correction, is intended to allow for the movementof the LCF with the change of trim. It was deduced by Mr Nemoto after observations of a bulk carrier that hewas involved in building. The theory on which it is based only holds true for small changes of trim, so it is notstrictly true for large trims. However, as with so much in draft surveys, it is better than nothing. The correctionis small for small trims, and rarely exceeds 30 tonnes for large vessels. It is often ignored but forcompleteness it should always be used.

Density correction, If the vessel is in a dock water density then she is sitting deeper in the water than shewould be if in salt water so we are reading a larger 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.

(1025 dock density) x Displacement

Density Correction: ---------------------------------------------------1025

And of course this is subtracted from the displacement if the dock water density is below 1025.


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It is possibly worth mentioning at this point that although we always consider the density of salt water to be1.025 tonnes per cubic metre, if you were to take a reading in mid ocean you would probably find it to bearound the 1.030 to 1.033 mark!

But remember the ships 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. Always use the density given by the draft surveyor if available,they have usually gone to considerable lengths to find the apparent density which is weight of water in air.

But a word of warning. Dont use this same density when determining how much you can submerge yourloadline in dock water, i.e. dock water allowance. For this you need density in vacuum.

Corrected displacement, Now that the true displacement of the vessel is known it only requires thesubtraction of all the known weights on board and the subtraction of the ships light displacement, whatremains is either the vessels constant if in an empty condition or the cargo weight if in a loaded condition.

The reason for the two draft surveys is firstly to find the vessels constant and then include this in the known

weights when calculating the actual cargo on board after loading.

If taking a draft survey on discharge the cargo weight plus constant are found in the initial survey and theconstant 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

2Trim(m) x MCTC Diff x 50

2nd Trim Correction: -----------------------------------LBP

x x 50= ------------------------------ = + tonnes (always add)

(1025 dock density) x Displacement

Density Correction: ---------------------------------------------------1025

x= -------------------- = tonnes

1025


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Many commercial ships have a symbol called a Load line painted on each side of the ship. This symbol,also called an International load line, or Plimsoll line, 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 politicianSamuel Plimsolladvocatedimproved safety standards, particularly at sea, and the mark bears his name in his honour.

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 Atlantic

Letters may also appear to the sides of the mark indicating theclassification societythat has surveyed thevessel's load line. The initials used include AB for theAmerican Bureau of Shipping, LR forLloyd's Register,and NV forDet 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.Fresh water marks make allowance for the fact that the ship will float deeper in fresh water.
http://en.wikipedia.org/wiki/Samuel_Plimsollhttp://en.wikipedia.org/wiki/Samuel_Plimsollhttp://en.wikipedia.org/wiki/Samuel_Plimsollhttp://en.wikipedia.org/wiki/Classification_societyhttp://en.wikipedia.org/wiki/Classification_societyhttp://en.wikipedia.org/wiki/Classification_societyhttp://en.wikipedia.org/wiki/American_Bureau_of_Shippinghttp://en.wikipedia.org/wiki/American_Bureau_of_Shippinghttp://en.wikipedia.org/wiki/American_Bureau_of_Shippinghttp://en.wikipedia.org/wiki/Lloyd%27s_Registerhttp://en.wikipedia.org/wiki/Lloyd%27s_Registerhttp://en.wikipedia.org/wiki/Lloyd%27s_Registerhttp://en.wikipedia.org/wiki/Det_Norske_Veritashttp://en.wikipedia.org/wiki/Det_Norske_Veritashttp://en.wikipedia.org/wiki/Det_Norske_Veritashttp://en.wikipedia.org/wiki/Det_Norske_Veritashttp://en.wikipedia.org/wiki/Lloyd%27s_Registerhttp://en.wikipedia.org/wiki/American_Bureau_of_Shippinghttp://en.wikipedia.org/wiki/Classification_societyhttp://en.wikipedia.org/wiki/Samuel_Plimsoll


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Example sheet draft survey calculations

Before loading After loading

Draft forward 1a

Draft aft mean 1b

Draft fore & aft mean 1c = 1a + 1b / 2

Draft midship PS 2a

Draft midship SB 2b

Draft midship mean 2c = 2a + 2b / 2

Draft mean for means 3 = 1c + 2c / 2

Draft corr. fordeformation

4

Displacement 1.025 5 (at draft 4)

Trim bow / stern

Trim in cmLCFLPPt/cm

ETM Diff

DensityTrim correction 1

6Trim x (0.5LPP-LCF) x

t/cm / LPP

Trim correction 27

Trim2(m) x 50 x ETM/ LPP

Correction for Density8

(1.025 density) xdispl.1.025 / 1.025

Consumable weights

HFOGOLUB

Fresh waterBallast

ConstantTotal known weights 9

Corrected displacement 10 = 5 + 6 + 7 - 8

Light ship 11

Cargo weight total 12 = 10 9 11

Draft Survey Guidelines,January,2010

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