To ensure the stability and safety of the ships, we need to weigh them. But our ships weigh approximately 200,000 tons. I doubt that there is no such device to calculate this amount of weight. So, how do you do that? We actually don’t weigh them. we use a combination of scales and Archimedes principle. Okay, but why Archimedes principle? How Archimedes principle is related to weighing a ship? What scale is used and how it is used? What are the various weighing terms used in the shipping industry? We will look after all of them in a short and crisp manner in this article. So, let’s get started.
You might have come across this term at least once in your lifetime. If not, let’s make it simpler. “The weight of the displaced water is equal to the weight of the ship”. Precisely this concept is used to weigh the ships. But what if the displaced water gets mixed with the surrounding water, how do we separate the displaced and surrounding water? if this is your doubt, then you are on the right track. Before clarifying your doubt, let’s look into the different weighing terms used in the shipping industry.
DEAD WEIGHT: It is the total weight of all cargo, fuel, crew, passengers etc.
LIGHT WEIGHT: It is the actual weight of the ship without cargo, fuel, crew, passengers etc.
DISPLACEMENT OF SHIP: It is just the total weight of the volume of water, displaced by the ship. In simple,
Displacement of ship= Dead weight + Light weight
GROSS TONNAGE: It is the measure of ship’s overall internal enclosed volume/space from keel to funnel including cargo space.
NET TONNAGE: It is the total volume of the ship’s cargo spaces only.
Draft /Draught Scale marks: If you have been close to any of the ships, you might have noticed few markings on them as shown in the figure below. They are used to measure how low the ship is sitting, and there is a table that shows how much cargo a ship can carry safely. The number starts from bottom of the ship, starting from 0 at the keel. Most of the ships will be having metric reading. Draft marks are located at six points around the ship.
Draft marks position:
Forward port and starboard.
Amidships port and starboard.
Aft port and starboard.
If you do read all 6 drafts, you could actually calculate the average. That average draft then allows you to work out the volume of water using the Bonjean curves.
Bonjean curves are used to calculate the underwater submerged VOLUME of water by the ship. We will see in detail about the Bonjean curves in our upcoming articles.
Finally, to convert the Volume into Weight, we use the below formula.
Weight= Volume × density
Sea water density is 1025 kg/m3(it varies at different locations)
The above calculated weight is the weight of the ship after loaded with cargo.
Now, to calculate the weight of the cargo/dead weight, we use the below formula.
Weight of cargo=Above Calculated Weight – Light weight
We actually can’t weigh every passenger, crew, or cargo before loading to check the safety limit of the ship. The above-mentioned method is the scientifically simplest method for us to weigh a ship. Hope you understood how ships are weighed before and after loading with cargo, the draft lines, Bonjean curves, and various weighing terms used in the shipping industry.
Authored By:- Cdt. Krishnasai Venkatesan, TMI