Draft Survey

RaetsMarine Insurance B.V.

DRAFT SURVEY GUIDELINES. Draft surveys are made in order to determine the quantity of cargo loaded, carried and discharged. This is done by measuring the vessels draft and calculating its displacement prior loading a cargo and after loading of this cargo, taking variables such as weight lightship, ballast water, fuel and stores into account. In order to obtain a reasonable accuracy in this draft survey calculation corrections to the draft readings and it initial corresponding stability values and parameters must be made. General definitions; Displacement of a vessel is the actual total weight of a vessel. It is expressed in metric tons, and is calculated by multiplying the volume of the hull below 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 the tropics, where water is warmer and hence less dense.) Deadweight of a vessel (often abbreviated as DWT for deadweight tons) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like Displacement, 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. Gross tonnage or Gross Register Tonnage is the total internal volume of a vessel, with some exemptions for 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 on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume 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 the Gross Tonnage minus the volume of spaces that do not hold any cargo (e.g. engine room, bridge, crew spaces, 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 midships and 2 x at the stern. The draft readings are averaged out in a mean draft. Via calculations this draft reading leads us to the vessels present apparent displacement at the time of our survey. Via on board, ship’s specific hydrostatic tables and stability curves. The water density at the time of the draft measurement is also determined by means of a density meter. A very common instrument for the direct measurement of the density of a liquid is the hydrometer, 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 vessel in water with a standard density of 1.025 (salt water), corrections to the determined draft must be made in order 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 correct the 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 density but again a tonnage correction and not a draft correction.

Draft Survey guidelines, 2010.1

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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 must be accomplished with the drafts at the fore & aft perpendiculars and at the amidships point of the hull. Note that the true draft at the vessel’s perpendiculars does not always correspond with the place on the hull were the draft signs are placed and read. The hydrostatic tables, however, are based on the vessel with a length corresponding to the perpendiculars and not to the length between the draft marks on the hull.

1st Trim Correction

=

TRIM(cm) x LCF x TPC -------------------------------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 of the mid point, or the trim is by the head or stern. Always draw a sketch to ensure you apply this trim correction 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 caused because of the irregular hull shape of a vessel in trimmed condition. The position of the LCF in the ship’s hydrostatics is given for a ship on even keel only but if the ship is trimmed the waterline must have changed causing a second movement to the LCF. Always positive this second trim correction is fairly small, for example with a loaded Panamax at a 1 metre trim the correction should only be about 12 tonnes, with a 2 metre trim about 50 tonnes. This second movement is corrected by the following formula:

2nd Trim Correction:

2 Trim(m) x MCTC Diff x 50 ----------------------------------LBP

Where MCTC Diff is the difference of MCTC values over a range of draft 50 cm above and below 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 allow for the movement of the LCF with the change of trim. It was deduced by Mr Nemoto after observations of a bulk carrier that he was involved in building. The theory on which it is based only holds true for small changes of trim, so it is not strictly true for large trims. However, as with so much in draft surveys, it is better than nothing. The correction is small for small trims, and rarely exceeds 30 tonnes for large vessels. It is often ignored but for completeness 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 she would be if in salt water so we are reading a larger displacement from the hydrostatic tables which are derived for salt water only. The following formula is simply the dock water formula converted to a tonnage figure, not a millimetre value.

Density Correction:

(1025 – dock density) x Displacement --------------------------------------------------1025

And of course this is subtracted from the displacement if the dock water density is below 1025. Draft Survey guidelines, 2010.1

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It is possibly worth mentioning at this point that although we always consider the density of salt water to be 1.025 tonnes per cubic metre, if you were to take a reading in mid ocean you would probably find it to be around the 1.030 to 1.033 mark! But remember the ship’s standard hydrometer is not an accurate instrument when it comes to using it to determine tonnage for a draft survey. It makes no allowance for the temperature of the water which will change 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. Don’t use this same density when determining how much you can submerge your loadline 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 the subtraction of all the known weights on board and the subtraction of the ships light displacement, what remains 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 the constant in the final survey, subtract the constant from the initial figure and you have the weight of cargo discharged.

TRIM(cm) x LCF x TPC -------------------------------LBP

1st Trim Correction:

=

=

tonnes

=+

tonnes (always add)

2 Trim(m) x MCTC Diff x 50 ----------------------------------LBP

2nd Trim Correction:

=

x x 50 ------------------------------

(1025 – dock density) x Displacement --------------------------------------------------1025

Density Correction:

=


x x ------------------------------

x -------------------1025

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=

tonnes

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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 safely loaded. 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 they were deliberately overloaded in the hope of collecting insurance money. Ships carrying emigrants from Europe to America were also lost. The British social reformer and politician Samuel Plimsoll advocated improved 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 the classification society that has surveyed the vessel's load line. The initials used include AB for the American Bureau of Shipping, LR for Lloyd's Register, and NV for Det Norske Veritas. These season and zone marks are used to ensure adequate reserve buoyancy for the intended area of operation. Ships encounter rougher conditions in winter as opposed to summer, and in the North Atlantic as opposed to tropical waters, for example. Fresh water marks make allowance for the fact that the ship will float deeper in fresh water.


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Example sheet draft survey calculations

Draft forward Draft aft mean Draft fore & aft mean Draft midship PS Draft midship SB Draft midship mean Draft mean for means Draft corr. for deformation Displacement 1.025 Trim bow / stern Trim in cm LCF LPP t/cm ETM Diff Density Trim correction 1 6 Trim correction 2 7 Correction for Density 8 Consumable weights HFO GO LUB Fresh water Ballast Constant Total known weights Corrected displacement Light ship Cargo weight total


Before loading 1a 1b 1c = 1a + 1b / 2 2a 2b 2c = 2a + 2b / 2 3 = 1c + 2c / 2 4

After loading

5 (at draft 4)

Trim x (0.5LPP-LCF) x t/cm / LPP Trim2(m) x 50 x ETM / LPP (1.025 – density) x displ.1.025 / 1.025

9 10 = 5 + 6 + 7 - 8 11 12 = 10 – 9 – 11

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Draft Survey

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