A draft survey is one of the most commonly used worldwide for determining the weight of seaborne bulk cargoes of low unit value. The accuracy of the draft survey may vary for several reasons. Discrepancies in draft survey results between the loading and unloading points may give rights to commercial disputes. Those disputes can be between the sellers, vessel operators, buyers, and surveyors, over which weight to accept as the basis for payment of the cargo, freight, port fees, and other charges.
Many cargoes are transported by sea in the world in bulk. The question of how much cargo is loaded on or removed from the ship will always be relevant. This amount can be determined both by coastal measuring systems and by ship draft – by draft survey method.
The organization of shore measurements can be cumbersome, and a compact draft survey will be a good alternative to shore measurements. There are no problems with the organization of weighing cargo on modern terminals. However, the draft survey is not a superfluous independent means of determining the amount of cargo on board.
The draft survey methodology is based on a principle that is more than 2000 years old and can be trackback to Archimedes’ law of buoyancy. Formulas further refined the application of this law of physics to ships.
What is the purpose of the draft survey and its calculation?
The purpose of the draft survey is to determine the weight of the cargo on board the vessel. The surveyor can calculate the ship’s weight in a fully loaded condition by:
- measuring the draft by using the ship’s cargo documentation and information on the calculation of the ship’s volume;
- Using the density of the water in the vessel’s location.
From this calculation, the surveyor subtracts the vessel’s weight and other weights on board the ship, which are not part of the cargo, the difference being the weight of the load.
Cargo loaded = Net Displacement final – Net Displacement initial
Most importantly, in practice, it should be borne in mind that the ship is flexible, and the hull always reacts to different forces from outside. The information of the ship’s builders about the ship varies. It is challenging to measure ships’ draft accurately to know the ballast’s actual weight.
What is the accuracy of the draft survey calculations?
Draft survey calculations accuracy of cargo may vary typically by between 0.5 and 1.0% of the cargo loaded on board. This comparison is between cargo on board, calculated by the draft survey, and shore figures of weighted cargo obtained from the port terminals’ scale. But the practice imposed by insurance companies and P§I Clubs requires a difference of 0.5% of the load on board.
However, in many cases, significant differences in the cargo weight on board according to draught surveys calculations and shore figures are obtained.
I want to emphasize some specific measures and measurements that every surveyor needs to bear in mind to increase the maximum of his draft survey calculations.
The commonly forgotten weights on board the vessel are:
– water in bilges in cargo holds and engine compartment, void spaces, and the chain lockers;
– water in the swimming pool, if it is such on board;
– if the draft survey is performed when the vessel is at anchor, the weight of the anchor and the cable into the water on the seabed must be excluded from calculations, either when the vessel is alongside, and an anchor has been deployed as part of the mooring arrangement;
– ballast water residues as silt and mud, which usually accumulated in the double bottom tanks of vessels regularly ballasting in rivers or estuaries. The extra weight coast mistake in cargo weight calculations.
1. Calculate precisely the ballast quantity on board.
To sum up, let’s highlight the surveyor’s main thing, which determines the amount of ballast onboard the ship:
- carefully read the plans for the location of ballast tanks;
- to make measurements of ballast tanks, using a roulette from steel with watermarking paste;
- determine the density of water in each tank;
- calculate the volume occupied by water in each tank, applying the necessary adjustments to the heel and trim;
- determine the weight of ballast water in each tank using the multiplication of volume and density.
2. Determination of ships’ constant weight.
The ship’s constant, contrary to the name, is not consistent.
It is the difference between the net displacement (ships’ displacement density corrected minus consumables onboard – ballast, freshwater, fuel, and lubricants, settling water, etc.) and the light vessel.
The constant includes the crew, ship’s supplies, paint, mud residues in the tanks, minor discrepancies in the marks of the ships’ scales, and inaccuracy in determining the vessel’s weight (light vessel).
During the initial draft survey on the ship in ballast, the surveyor determined the constant by calculation. For a small bulk carrier, the standard value of the constant is about 100 – 150 tons. Ships of older construction have a constant higher than vessels of new construction. The value of the constant will fluctuate with the change on board the number of lashing materials, stocks, and the appearance of ice and snow on the deck. Due to these indeterminate factors, light vessel weight can change by 60 tons.
In some cases, the surveyor receives a negative constant. This is usually a sign of an error. However, if the constant remains negative after repeated measurements and calculations, this value should be used.
3. Middle draft scale measurements.
The accuracy of middle draft measurements is vitally important. The surveyor must not make any compromises on accuracy. It is because middle draft calculations participate six times in the formula for calculating the middle draft twice corrected (mean of means). If there is a difference between the scale of the ship in the middle and the Plimsoll mark, it must be taken into consideration in calculations. The surveyor has to measure by gauge the distance and add or subtract following the same principle as corrections fore and aft (the difference between ships’ scales and ships’ perpendiculars fore and aft).
Middle Draft Calculations = (middle draft starboard side + middle draft port side divided) / 2
4. Water density of the basin (sea or river).
For calculations of displacement, it is vitally important to measure water density in the dock when the vessel is alongside. When the surveyor and responsible officer read draft marks, they must obtain overboard water samples as soon as possible. This is particularly relevant when the vessel is alongside it an estuarial or river port. The tide, the water’s density may be changing. The density should be checked quickly after obtaining the sample. As there may be temperature differences between the actual sampling of the dock water and the time of determination of its density, which may cause errors.
Samples of the water must be taken in the following way:
- between one and three samples must be taken depending on the vessel length;
- to overcome the problem of layering of the water. The samples should be obtained using a closed sampling at a depth of approximately half the vessel’s existing draught. If there is no can, using a bucket is also possible. Moreover, for maximum accuracy, when the ship’s draft is more than ten meters, the surveyor can obtain three samples. The samples have to be from the water surface, at depth at about five meters, and from the ships’ keel depth. Use measured samples and calculate the average value for draft survey calculations;
- in order to avoid parallax errors and errors due to the meniscus. As a result, this could happen when reading the hydrometer floating in the sample of water. The observer’s eye should be as close to the water level as possible.
5. Keel thickness.
Never forget to subtract keel thickness from the draft measurements. Pay attention to the ships’ tables, whether the keel is included in the draft or not. Do not rely on that keel thickness will appear in the ships’ constant. It is not correct. Remember – if the surveyor neglect keel thickness, it will harm the consignee.
When independent surveyors undertake draught surveys, the cooperation of the ship’s officers is essential. The survey sections should be undertaken with the vessel’s chief officer, chief engineer, or appointed deputies.
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