All About Ships' Tonnage Have you joined us in"Kawasaki-san"? The first leg of a knowledge-oriented travel might be a little too long. Hopefully, there might be many readers enjoying it. This time you are carried to a world where you can have access to something so fundamental. Instrumentally, it teaches us that many methods are invented to calculate ship's weight and loading capacity. Historically, let's stop a moment to think our safe and trustworthy work is sourced way back to Archimedes (287? - 212 B.C.) of the Ancient Greek Age.

Today Mr. Kawasaki was on the bridge from morning, listening to the Captain explain about ships' tonnage, a subject that Kawasaki was not entirely clear about.

"Kawasaki,we often hear people talk about a 10,000-ton general cargo ship or 200,000-ton tanker. Do you know what'ton'is referring to? "

"Well, ton must mean weight, right? "

"That's right, but the weight of what? "

"The weight of the ship, of course. No, wait, is it the weight of the cargo that ship can carry?"

"Hmmm. Can you think of anything other than weight? "

"Well, let me see. It could be the ship's size or its outward size."

"1n other words, the ship's volume, right? "

"Yes, that's right."

"Very good. Everything you said was correct."

"What? How can it a11 be correct? "

"I asked you a trick question,"the Captain laughed, "Actually, the word'tonnage'can refer to a number of different things. The most common usage is gross tonnage, or GT for short. For example, it's expressed as 10,000 GT. It's used as a basis for calculating port charges, pilotage and tug boat towing charges. It's also often used for various statistics."

"So it's like the representative tonnage ? "

"That's right. And this gross tonnage also indicates a ship's capacity-in other words,it's size."

"It does? "

"Yes, but there is also net tonnage(NT), which also indicates a ship's size-namely, the size of the space available for carrying cargo or passengers. Net tonnage is mainly used for tax collection."

"You mean it's the size of the cargo hold space? "

"In the case of a merchant ship that doesn't carry passengers, you can think of net tonnage as the size of the holds."

"So how do you express the maximum cargo weight that a ship can carry?"

"That's deadweight tonnage(DW or D/W). That's the tonnage that those of us in the shipping business see most often."

"Now that you mention it,l can recall hearing people use the expression'deadweight 100,000 tons, for example."

"That's Deadweight tonnage indicates how much a ship sinks down into the water when it is fully loaded as opposed to when its empty. It represents the weight of the water that would fill the volume of the submerged portion of the hull."

"Oh,you're talking about Archimedes' principle ? "

"Yes. Have a look at this drawing. When the ship is floating on the water, it displaces an amount of water equivalent to the volume of the ship below the water line. The weight of the displaced water is equivalent to the ship's weight. This amount of displaced matter is called the displacement. The weight that causes a ship to sink down to its maximum allowable depth (load line) is called the full load displacement."

"That would be part of the ship marked with diagonal lines in diagram(a), right? "

"That's right. And a ship's displacement when it's not carrying people, cargo, fuel, water, or anything else is called its light weight or light condition."

"That would be the weight of just the ship itself, right."

"It's the weight of the ship equipped with just its engines, equipment and legally required supplies, as shown in diagram (b)."

"So the weight of the cargo the ship can carry-its deadweight (c) - is the difference between(a)and(b), right ? "

"Strictly speaking, deadweight tonnage includes the weight of fuel, Water, lubricant oil and other such things on board. 0nce you subtract this weight from the deadweight tonnage, you have the weight of the cargo the ship can carry."

"Roughly what percentage of deadweight tonnage can a ship carry as cargo? "

"I can't give an unequivocal answer because the amount of water and fuel on board a ship varies depending on the length of its voyage. But it's safe to say that a ship can carry a load of cargo weighing roughly 90% of its deadweight tonnage."

"I see. The word tonnage certainly does mean quite a few different things, doesn't it? "

"But still, Captain, why did people start using the word 'ton' to refer to a ship's weight and size? "

"1t seems that a long time ago, a Ship's weight was expressed in terms of how many tuns it could carry, so ton came to be used as the unit of expressing a ship's weight and cargo capacity. Back in 15th century England, the volume of the tuns carried by ships was 40 cubic feet. When filled with liquor,a tun would weigh 2,240 pounds (1,016 kilograms), which became a standard unit of measure. So 40 cubic feet of cargo came to be called one ton. This later became known as a measurement ton while 2,240 pounds of weight came to be called a long ton."

"Wow, is that so? By the way, Captain, what's our ship's gross tonnage? "

"Her international tonnage, which means gross tonnage based on a international convention, is 50,000 tons. 0riginally, gross tonnage was calculated by taking a Ship's gross interior volume(enclosed volume)and subtracting the volume of the double bottom and any spaces above the upper deck that were related to navigation, safety or sanitation.In this calculation, each 100 cubic feet of volume was one ton. However, different countries had different rules about the spaces that were excluded from the calculation. This gave rise to inconsistencies, like a ship having a gross tonnage of 10,000 tons under Japanese regulations but only 7,000 tons under U.S. regulations."

"So the method of calculation differed between countries ? "

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