Draft survey Question & answer by MMD surveyor

Draft survey Question & answer by MMD surveyor Have you seen a truck weighing bridge ? Do you know how it works ? Weigh bridge It weighs the empty weight of the truck and then the loaded weight. The difference is the weight of the cargo on that truck. Well draft survey uses a similar principle to measure the cargo loaded on board ships. With draft survey we measures the initial weight (displacement) of the ship and it measures the final weight (displacement) of the ship after loading. The difference plus all the weights that were taken out (like ballast) will be the cargo loaded. Why Draft survey ? On tankers, the measurement of cargo quantity loaded is simple. We know the density of the cargo and we know the volume. It is easier to know the weight of the cargo loaded. But with cargoes like coal, we cannot measure the weight by just measuring the the height of the hold to which cargo is loaded. The only difference between weight measurement in case of truck and ship is that for later the calculation is not that simple. Calculating the cargo loaded with draft survey is the most appropriate way. But it isn’t only with solid cargoes. Sometimes we have to measure the cargo quantity of liquids by draft survey. One such cargo is Mollasses loaded on chemical tankers. This cargo has air in it and thus the density of this cargo is not uniform. Calculating the way we calculate the weights for other liquids will give wrong quantity. Draft survey is the answer in this case too.o which cargo is loaded.