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.
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.