Inspections to determine the amount of cargo that has been loaded or that has been discharged, by comparing the displacements at the beginning and at the end of the operations. In these inspections the measurements of drafts should be very careful, since a few centimeters of error can mean a considerable amount of money. Both the buyer and the seller of the cargo can contract this type of inspection to safeguard their interests.
«Our surveyors will ensure a correct calculation protecting the interest of our customers»
Accuracy and errors in the practice of Draft Survey
On large vessels when the Draft Survey is correctly performed, an error of 0.5% or less can be expected. However, sometimes happens that the accuracy of a Draft Survey is difficult to determine, especially when the ship’s loaded or unloaded cargo shows a very different weight than that determined by the balance.
Unfortunately, the parties involved may doubt the resulting weight of the Draft Survey, since they have been relying on scales for many years. As the responsible person, the Draft Surveyor must resolve this issue to prevent any future demands.
Where do the doubts in the accuracy of the Draft Survey come from?
1 – Information on the hydrostatic characteristics of the ship with errors.
2 – Wrong drafts marks.
3 – Incorrect calibrations of the tanks.
4 – Mud and / or rust crust in the ballast tanks.
5 – Increase of the incrustations in the hull lining.
6 – Perturbations in the water, waves of swell, traffic near the ship.
7 – Variations in the density of seawater.
8 – Static Sinkage of the vessel.
9 – Asymmetric hull flexes.
10 – Flexion due to the sun.
11 – Use of approximate methods for corrections.
12 – Remote readings of draft.
13 – Anchors and chains.
Some of the common problems that conspire against accuracy during the Draft Survey are:
- Unused sounding tubes, that is, inaccessible; blocked or broken.
- Badly calibrated ballast tanks.
- Spaces on board that can not be sounding.
- Use of a density meter of doubtful quality.
- Loaded vessel supporting on the bottom before the beginning of the unloading.
- Ship supporting in the background after finish the cargo.
- Listed vessel, which has an effect on the displacement and soundings of liquid cargos.
- Weights on board (with the exception of cargo, ballast, or consumables) that are not the same at the beginning and at the end of operations.