Draft Survey calculation sheet Date / 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 =
Initial
Final
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 to 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 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 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. This weight will be mentioned in the stability booklets. 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 (P/S), 2 x midships (P/S) and 2 x at the stern (P/S). The draft readings are averaged out in the 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 survey 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 t/m³ (salt water), corrections to the determined draft must be made in order to be able to find the correct displacement of the vessel. Make sure at the time of measuring the density the density meter is free floating. Displacement correction In order to find the correct displacement we will correct the displacement straight away, instead of correcting the drafts at first. We will use 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.
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 draft marks are not always placed on the perpendiculars, so the true draft of the vessel does not always correspond with the drafts found during the survey. The hydrostatic tables are mostly based on the length between perpendiculars and not to the length between the draft marks on the hull. 1st Trim Correction
=
TRIM(cm) x TPC x LCF -------------------------------LBP
Please 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 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 the vessel in trimmed condition. The position of the LCF in the ship’s hydrostatic tables are given for a ship on even keel only, but if the ship is trimmed the waterline has changed, causing a second movement to the LCF. The second trim correction is always positive and fairly small, for example with a loaded Panamax at a one metre trim the correction should only be approximately 12 tonnes, with a two metre trim approximately 50 tonnes. This second trim correction is calculated by the following formula:
2nd Trim Correction:
Trim²(m) x ΔMTCT x 50 -------------------------------------------LBP
In this formula the ΔMTCT is the difference of MTCT values over a range of draft 50 cm above and 50 cm 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 correct for the movement of 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 not strictly true for large trims. Please note that the second trim correction is rather small for small trims, and rarely exceeding 30 tonnes for large vessels. It is often ignored but for completeness sake the second trim correction should always be calculated. Density correction If the vessel is in a dock water density then she is sitting differently in the water than she would be if in sea water so we are reading a different 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. (1.025 – dock density) x Displacement
Density Correction:
--------------------------------------------------1.025
And of course this is subtracted from the displacement if the dock water density is below 1.025 t/m³ It 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 1.030 t/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 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. Corrected displacement When 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 two draft surveys is at first to find the vessels constant and then include this in the known weights 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 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:
=
x x -----------------------------LBP
=
tonnes
2 Trim (m) x MTCT Diff x 50 ----------------------------------LBP
2nd Trim Correction:
=
x x 50 ----------------------------------- = LBP
tonnes (always add)
(1.025 – dock density) x Displacement --------------------------------------------------1.025
Density Correction:
=
x -------------------1.025
=
tonnes
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 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 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 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. A copy of a zone-chart must be on board in order to assure that the vessel is not overloaded. Fresh water marks make allowance for the fact that the ship will float deeper in fresh water.
