What is Ship Tonnage? All Types and Use Explained!



We’ve all wondered how much a ship weighs and may have come across phrases like gross tonnage, net tonnage, displacement, lightship weight, and so on. We will have a better knowledge of all of these concepts and learn what ship tonnage truly means in this article.

Ship tonnage in shipping refers to a ship’s carrying capacity measured in terms of volume or weight. Tonnage in shipping is defined as the total number of tons registered or the overall carrying capacity of the vessel.

Tonnage originated from the practice of charging dues on ships and is also used to calculate the duty or cost for crew rules, registration, port taxes, and other similar things. These fees were calculated depending on the number of ‘tons’ that a vessel could carry.

Previously, tonnage was determined using the interior volume of the ship. However, the measurement technique has evolved to one in which the cargo’s weight is regarded as the unit of measurement. The conventional unit of measurement for this capacity is the ton. Tonnage has been defined and computed for ships in a variety of ways.

Let us get into the article to find out more.

History Of Tonnage

King Edward I introduced the concept of tax on ships in 1303, based on tonnage laden. King Edward III then imposed a tax on imported wine, measuring each “tun” as 252 gallons (1.146 m3) of wine weighing around 2,240 lb (1,020 kg), or what is now referred to as a long ton or imperial ton.

For tax purposes, a ship’s estimated capacity in “tuns” was expressed using dimensions in feet (Length x Beam x Depth)/100, where depth refers to the hold’s depth below the main deck. The numerator of this equation gives the ship’s volume in ft3, and the denominator is a “tun” that is thought to be equal to 100 ft3.

The Builder’s Old Measurement, often known as burden or burthen and abbreviated as BOM was used to determine a ship’s cargo capacity from around 1650 until 1849 and was then abandoned after much clearer means of calculations was introduced.

The measurements were again in feet, with the length measured from stem to sternpost and the tonnage computed as [Length – (Beam x 3/5)] x [Beam x]/2]/94.

How Was Tonnage Calculated Before Standardization?

Looking back to history, to measure a ship’s tonnage, different countries utilized different standards. George Moorsom created a different way of measuring ships in 1854.

According to this system, the ship’s volume was considered to serve as a better indicator of its size, and this was used to calculate how much to pay for services rendered by ports.

The term “gross” and “net registered tonnage,” which are both abbreviated as “grt” and “nrt,” were used since the unit of measurement had always been “tons” in Britain’s first British Shipping Act (1854).

The Royal Thames Yacht Club created the Thames Tonnage in 1855 as an alternative to the Builder’s Old Measurement and it was designed for yachts and other comparable vessels.

The majority of maritime nations started using the Moorsom technique in the early 1900s, although the results were not always directly comparable due to differing interpretations by various nations, including the precise terminal points of physical measurement and other measurable adjustments.

International efforts to standardize tonnage calculations began in 1925, but the IMO International Convention on Tonnage Measurement of Ships did not become effective until 1969.

This first became effective for new ships on July 18, 1982, and it became completely effective for all ships on July 18, 1994, except warships, boats shorter than 24 meters in length, and some lake and river vessels. IMO terminology replaced GRT and NRT with gross tonnage (GT) and net tonnage (NT).

What Are The Different Types Of Ship Tonnage?

There are five kinds of ship tonnage in use in the shipping business: Gross Tonnage, Net Tonnage, Deadweight Tonnage, Displacement Tonnage, and Lightship Weight Tonnage. Let’s take a closer look at each type of ship tonnage used in shipping.

Gross Tonnage

The overall size of the vessel, stated in terms of volume, of the enclosed area within the ship, from the keel of the ship to the deck, is known as gross tonnage.

The following formula provides a straightforward way to calculate gross tonnage:

GT = K1 multiplied by V

Where k1 = 0.2 + 0.02LogV and V = total number of enclosed cubic meters

Net Tonnage

The total molded volume of the ship’s cargo compartment is known as net tonnage. Only the vessel’s available capacity for loading cargo is taken into account. About 30% less than the gross tonnage is anticipated for the net tonnage.

The procedure of determining the net tonnage is made easier by the following formula:

Net tonnage = k2 x Vc x (4d/3D)2 + k3 x (N1 + N2/10).

In this case, K2 = 02. + 0.2 LogVc

Vc is the total cubic volume of the cargo spaces.

Summer load line draught is calculated as K3 = 1.25 x (GT + 10,000)/10,000 d in meters. D is the measured molded depth between ships.

N1 = Total number of people using up to eight beds in the cabin. N2 = Amount of additional travelers.

The total number of people that the ship is permitted to transport by its passenger certificate is equal to N1 + N2. N1 and N2 should be treated as zero if N1 plus N2 is less than 13. GT stands for the ship’s gross tonnage.

Deadweight Tonnage

A ship’s contents, including its cargo, fuel, passengers, crew, food, and water, are measured by its “deadweight tonnage.” It is measured in tons which approximately is equal to 1,016 Kilograms.

Displacement Tonnage

The volume of water displaced by a ship when it is at sea determines its displacement. The unit of measurement is Cubic meters. The displacement tonnage refers to the amount of water a ship can move when floating, fully fueled, with all sources on board. The unit of measurement is metric tonnes.

Given that a floating vessel displaces the same volume of water as it weighs, it is the real weight of the vessel. On the other side, a ship’s lightship displacement is the volume of water it displaces when it is empty of all people, crew, cargo, fuel, water, and food.

Lightship Weight Tonnage

The weight of a ship plus its permanent equipment is referred to as lightweight tonnage. Fuel, water, and any other supplies on board the ship are not included in the lightweight tonnage.

What Is The Difference Between Ship Tonnage And Displacement?

In the maritime industry, displacement is a relatively common measurement for vessels. The weight of water that a vessel displaces is known as displacement weight.

The weight of a ship is measured in displacement weight. The displacement tonnage and displacement weight of a ship are the same.

The volume of a ship’s interior is measured in terms of gross and net tonnage. Net or gross tonnage is primarily used for registration and revenue collection purposes.

How Is Tonnage Taxed?

In place of traditional corporation taxation, shipping businesses are subject to tonnage tax. The tax is frequently seen as one of the most significant marine subsidies provided by the government in recent years since it is typically lower than conventional government taxes. Based on the overall tonnage value of the fleet of vessels operated by a single firm, the tax is assessed.

For instance, it makes no difference if the freight is carrying its maximum 10 tonnes of load; the tonnage is what matters. Another benefit of this taxing technique is that it does not depend on business profits or the number of all shipments, making tax calculation simple and efficient for shipping businesses and authorities

Which Authority Issues The Tonnage Certification For Vessels?

Solely based on the tonnage of the ships, manning restrictions, registration costs, port dues computation, and safety norms are developed.

All ships must possess an International Tonnage Certificate (ITC), which is provided by the flag states by the International Convention of the International Maritime Organization (IMO) and the Tonnage Measurement of Ships 1969.

Before delivery, the necessary calculations are performed by the vessel’s classification society. Additionally, it issues the certificate on the flag state’s behalf. Although the certification includes an expiration date, it must be updated if the structure of the vessel changes or if the vessel is subjected to any conversions.

Ship Tonnage Summary

Ship tonnage is an important measuring unit for commercial vessels as it helps the government to impart taxes and duties which serves as an important means of revenue.

Ship tonnage can also be used as an indicator in terms of the total volume enclosed in the vessel.

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