The days are gone when the marine industries were very small. The ships were small with very limited cargo-carrying capacity. Things have changed, and the marine industry has become more versatile. Large ships and vessels carry tons of loads with ease. Different sizes of ships bring a discussion on gross tonnage vs net tonnage vs deadweight tonnage.
Interestingly, most people, especially beginners, think of tonnage as weight. However, tonnage doesn’t always refer to weight. Instead, it sometimes indicates a ship’s space or volume rather than its weight. In this article, I’ll explain the differences between gross tonnage, net tonnage, and deadweight tonnage. So, let’s get started!
Difference between Gross Tonnage (GT), Net Tonnage (NT), and Deadweight Tonnage (DWT)
Knowing GT, NT, and DWT is crucial if you’re in the marine industry. They help understand the financial potential and safety of ships and vessels. Remember, these parameters vary from ship to ship depending on their size and dimensions. Let’s discuss each of these tonnages and understand their importance and differences.
1- What is Gross Tonnage (GT)?
Gross tonnage refers to the internal volume or space of the ship. This term does not involve the weight of any component or portion of the vessel. Instead, it refers to spaces such as the engine room, crew accommodation, fuel tank compartment, and other storage sections. Remember, ship bathrooms are also counted in gross tonnage. In simple terms, it is usable space inside the ship.
Any spaces or compartments outside the ship are not included in the ship’s gross tonnage. This gross tonnage remains constant for a specific vessel. For example, a ship’s cargo or passenger load does not change it. Whether loaded or empty, the gross tonnage will remain the same. Two ships of the same size can have different gross tonnages.
For example, if one ship’s internal space is compact and has less volume due to a confined layout, its gross tonnage would be low. Ships with a vast internal layout would have higher gross tonnage. This GT number is an integral part of a ship’s official documentation. Ships with missing documents can face legal challenges.
Importance of Gross Tonnage
Generally, ships pay taxes and port fees during their stay. Ports typically charge vessels based on the gross tonnage. That’s the key reason why different ships pay different port fees. Harbors or ports check documents, verify GT, and determine vessel size. This helps them estimate the size and then charge the ship fees.
A vessel with higher gross tonnage will pay more, and vice versa. Remember, this GT does not indicate the cargo-carrying capacity of the ships. Additionally, each harbor or port may have rules or regulations for different types of ships. They make those rules based on ships’ gross tonnage. Moreover, ports allocate specific berths to ships in the harbor based on their GT.
Gross tonnage is also helpful in determining the number of crew members on a specific ship. A ship with a higher GT will need more crew members. That’s because the ship will have more internal space, and more crew will manage it better. This parameter is also used as a standard when ships are bought or sold. In simple words, GT has nothing to do with safety and is useful for ports and regulatory authorities.
2- What is Net Tonnage (NT)?
As I said earlier, gross tonnage includes all the enclosed space of a ship. However, the net tonnage is more specific to the areas of the ship that earn money. For example, ships consist of multiple sections. The cabin crew section, fuel storage section, or even bathrooms, these parts don’t make money, right? So, these areas of the vessels are not included in the net tonnage.
Some ships are only dedicated to cargo transportation and are known as cargo ships. In these ships, the area or space where cargo is loaded is called net tonnage. However, many vessels carry both passengers and cargo. In these ships, the areas where passenger seats or cargo are loaded are included in the net tonnage. Similarly, the tonnage of a passenger ship is measured only by the seating area where passengers sit.
The net tonnage indicates the areas of the ship that help generate revenue. Both cargo-carrying areas and passenger seating spaces bring financial benefits. It’s noteworthy that net and gross tonnage are always related. The net tonnage is determined from the gross tonnage and is always lower than GT. To calculate NT, you must know the GT of that specific ship.
Importance of Net Tonnage
Net tonnage is crucial for shipowners in estimating a ship’s profitability. As mentioned above, NT refers to the area that actually makes money. If this area is big, the ship’s earning potential is higher, and vice versa. For example, a ship with higher NT has greater cargo or passenger carrying capacity. More passengers or cargo simply means more money to the ship owners.
The net tonnage also helps shipowners plan their routes. For example, a ship with lower NT earns less money. So ship operators use shorter routes to reduce fuel consumption. On the other hand, ships with higher net tonnage earn more money. So, ship operators can use longer, safer routes instead of relying on shortcuts. In summary, the NT is useful to ship owners because it relates to the financial potential of the vessels.
3- What is Deadweight Tonnage (DWT)?
In the previous article, I discussed everything about the deadweight tonnage. Both gross and net tonnages refer to space or volume. However, deadweight tonnage indicates the weight of the ship itself. Each ship has a maximum weight-carrying capacity, known as deadweight tonnage. This includes the weight of cargo, passengers, crew, fuel, and other equipment.
A ship typically carries multiple pieces of equipment, such as rubber fenders, マリンホース, and so on. Deadweight tonnage is the weight of each piece of equipment. Interestingly, the weight of the ship itself is not counted in this tonnage. In other words, if the ship is empty, it will have zero deadweight tonnage. The cargo, passengers, equipment, water, and fuel storage tanks, etc, make the deadweight tonnage.
Importance of Deadweight Tonnage
Deadweight tonnage (DWT) is a key parameter that ship operators must understand. This helps keep the ship safe during the voyage. If the DWT is too high, it can make the ship unstable and lead to deadly accidents. Operators generally manage the load to stay within the DWT limit to ensure an efficient voyage.
If DWT is too high, ship operators prefer to carry more fuel on the long journey. The reason is that vessels consume more fuel when they’re loaded heavily. So, this extra fuel ensures the ship doesn’t run out of fuel midway. Additionally, understanding the DWT is crucial in determining which route a ship will take.
When DWT is too high, the ship can become unstable and even sink in harsh weather or large waves. So ship operators keep track of the weather and choose routes where conditions are safe. This helps avoid big waves and ensures ships with higher DWT remain safe. Simply put, Deadweight tonnage is important for the safety and stability of ships or vessels.
Comparison of Gross Tonnage (GT), Net Tonnage (NT), and Deadweight Tonnage (DWT)
Each of these parameters indicates a different aspect of the ships. However, each of them is equally important for the successful sailing of the ships and large vessels. Here is a table showing the comparison among GT, NT, and DWT:
| Feature | GT | NT | DWT |
| Measures | Total internal space | Cargo & passenger space | Max carrying weight |
| Unit | No Unit | No Unit | Tons (weight) |
| Includes | All enclosed areas | Only earning spaces | Cargo, fuel, water, supplies |
| Excludes | None | Engine/crew/fuel | Ship’s own weight |
| Use | Port fees | Commercial capacity | Safety & load planning |
よくある質問
Is DWT the same as GT?
No. DWT is deadweight tonnage, which refers to the maximum weight a ship can carry. On the flip side, GT or gross tonnage indicates the volume or space of a vessel. GT is not weight, but a space.
What are NT and GT?
Net tonnage indicates the space on a ship that generates revenue. These spaces include the cargo-carrying section and passenger seating areas. On the other hand, GT includes all the internal volume, including areas that do not make money, e.g., the fuel compartment.
How to calculate the deadweight tonnage of a ship?
Calculating the deadweight tonnage of any ship is very easy. This DWT refers to the weight load on an empty ship. So, you can use the following formula to calculate it:
DWT = Loaded ship weight – Empty ship weight
結論
In summary, NT, GT, and DWT are very important parameters in the maritime industry. Using these parameters, ships and vessels can achieve safe, smooth voyages. Generally, these parameters are considered the ship’s incorrect weight. NT and GT are not weights, but indicate the space of the ship. On the contrary, DWT is the total weight a ship can carry during its voyage. In this article, I’ve discussed key distinctions among these three parameters.