Draft surveys are used to determine the weight of bulk cargo such as iron ore, coal, steel scrap, grain, and some specific quantities of liquid cargo loaded onto, or discharged from, a vessel.
Draft surveys are convenient and economical means of ascertaining the quantity of cargo loaded or discharged from a ship by reading the vessel’s drafts, measuring the ballast water, fuel and diesel oil and other liquids on board, and calculating the final quantity by using the vessel’s approved deadweight scales and tank tables, etc.
In principle, draft surveys require only a measurement of the water displaced by the vessel before and after the cargo is transferred, along with a measurement of the water’s density. Water displacement is measured through draft marks on the ship and converted to a volume using draft tables. The weight of displaced water is calculated by multiplying its volume (displacement) by its density. The difference between the weights of water displaced before and after the cargo transfer will equal the weight of the cargo within measurement accuracy limits.