Koike Marine Survey Inc.

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Outline of Draft Survey

A large amount of dry bulk cargo is transported from country to country by vessel. The loaded quantity of the cargo is calculated by measuring the weight of the liquid (generally seawater) of which the vessel eliminates before and after loading. It uses Archimedes' principle that the weight of the liquid eliminated by the vessel equals the buoyancy. Although it is a primitive method, it is a method of measuring at one time with a large container such as a vessel, so if its working environment is improved, high accuracy can be obtained.
The procedure of the draft survey is as follows.
@ Draft mark reading：There are a total of 6 draft marks (each on the port side and starboard side of the bow, center and stern) on a vessel. We read the draft marks and calculate the average draft.
A Measurement of seawater density：We measure the density of the seawater on which the vessel is floating. The weight of the entire floating vessel is calculated by multiplying its density to the volume of seawater eliminated by the vessel.
B Measurement of the ballast water：The vessel's draft is adjusted by taking seawater into the ballast tanks of the vessel. It is generally conducted that when cargo is loaded in full condition, the ballast tanks will be almost in empty condition, and when nothing is loaded, they will be almost in full condition. We measure the ballast water, which has been taken into each ballast tank, and calculate the weight of the ballast water.
C Confirmation of belongings such as fresh water and fuel oil ：We check how many tons of the belongings such as fresh water and fuel oil are loaded on the vessel.
D Calculation of the displacement：We calculate the displacement equivalent to the average draft by the displacement table belonging to the vessel.
E Calculation of the loading cargo weight : We calculate the displacement of the vessel before and after loading, and then deduct the ballast water, fresh water, fuel oil, etc. which are fluctuated, respectively. The difference in the deducted quantity before and after its loading will be equal to the loaded cargo weight.