Ship Tonnage Explained - Displacement, Deadweight, Cargo, Gross, Etc.
Introduction
The article "Ship Tonnage Explained," published on June 28, 1932, offers a detailed explanation of the concept of ship tonnage, a crucial aspect of maritime operations. It clarifies the different types of tonnage measurements used in the shipping industry, such as gross tonnage, net tonnage, and deadweight tonnage, and explains their significance in the context of ship design, cargo capacity, and maritime regulations. The article aims to demystify these technical terms for a broader audience, providing both historical context and practical applications.
Everyone who has looked at specifications for steamships is often bewildered by the many different tonnages used for the same vessel.
An ocean liner may have different gross tonnage, depending on which country's rules were used in determining the weight.
Below is an article from 1932 that provides a good explanation on just what the tonnages for ships really are.
There are five kinds of tonnage in use in the shipping business.
They are:
- Deadweight,
- Cargo,
- Gross,
- Net and
- Displacement Tonnages
Plan of Tonnage Deck and Elevation for Measuring Tonnage of Ships. E. W. Blocksidge in a Paper Read at the Spring Meetings of the Sixty-Sixth Session of the Institution of Naval Architects, London, April 1925. GGA Image ID # 1fe2d8613b. Click to View Larger Image.
Tonnage Defined
- Deadweight Tonnage: expresses the number of tons of 2,240 pounds that a vessel can transport of cargo, stores, and bunker fuel. It is the difference between the number of tons of water a vessel displaces "light" and the number of tons it displaces when submerged to the "load line." Deadweight tonnage is used interchangeably with deadweight carrying capacity. A vessel's capacity for weight cargo is less than its total deadweight tonnage.
- Cargo Tonnage: is either "weight" or "measurement." The weight ton in the United States and in British countries is the English long or gross ton of 2,240 pounds. In France and other countries having the metric system a weight ton is 2,204.6 pounds. A "measurement" ton is usually 40 cubic feet, but in some instances a larger number of cubic feet is taken for a ton. Most ocean package freight is taken at weight or measurement (W/M) ship's option.
- Gross Tonnage: applies to vessels, not to cargo. It is determined by dividing by 100 the contents, in cubic feet, of the vessel's closed-in spaces. A vessel ton is 100 cubic feet. The register of a vessel states both gross and net tonnage.
- Net Tonnage: is a vessel's gross tonnage minus deductions of space occupied by accommodations for crew, by machinery, for navigation, by the engine room and fuel. A vessel's net tonnage expresses the space available for the accommodation of passengers and the stowage of cargo. A ton of cargo in most instances occupies less than 100 cubic feet; hence the vessel's cargo tonnage may exceed its net tonnage, and, indeed, the tonnage of cargo carried is usually greater than the gross tonnage.
- Displacement: of a vessel is the weight, in tons of 2,240 pounds, of the vessel and its contents. Displacement "light" is the weight of the vessel without stores, bunker fuel, or cargo. Displacement "loaded" is the weight of the vessel plus cargo, fuel, and stores.
For a modern freight steamer the following relative tonnage figures would ordinarily be approximately correct:
- Net tonnage 4,000
- Gross tonnage 6,000
- Deadweight carrying capacity 10,000
- Displacement, loaded, about 13,350
Registered Tonnage
A vessel's registered tonnage, whether gross or net, is practically the same under the American rules and the British rules. When measured according to the Panama or Suez tonnage rules most vessels have larger gross and net tonnages than when measured by British or American national rules.
Bibliography
Article appearing in an American Export Lines Passenger List from June 28, 1932.
Illustration Fig. 4: E. W. Blocksidge, "Tonnage Legislation and Its Application to Measurement of Ships," in Marine Engineering and Shipping Age, New York: Simmons-Boardman Publishing Company, Vol. XXX, No. 6, June 1925, p. 347.
Key Points
- Gross Tonnage (GT): Refers to the total internal volume of a ship’s enclosed spaces, measured in "tons" where one ton equals 100 cubic feet. It is a measure of the overall size of the ship and is used primarily for regulatory purposes.
- Net Tonnage (NT): Represents the usable volume of a ship’s cargo spaces, excluding areas not used for cargo, such as the engine room, crew quarters, and navigation spaces. Net tonnage is essential for determining the ship’s earning capacity.
- Deadweight Tonnage (DWT): Indicates the maximum weight a ship can safely carry, including cargo, fuel, passengers, crew, and provisions. It is a critical factor in ship design and safety.
- Displacement Tonnage: Refers to the weight of water displaced by the ship when it is floating, equivalent to the ship’s weight. It is used to describe the overall weight of a vessel.
- Historical and Regulatory Context: The article also discusses the historical development of tonnage measurement systems and their role in international maritime regulations, such as the calculation of port fees and compliance with safety standards.
Summary
The article "Ship Tonnage Explained" provides a comprehensive guide to understanding the different types of tonnage used in the maritime industry. It breaks down complex technical terms like gross tonnage, net tonnage, deadweight tonnage, and displacement tonnage, explaining their purposes and how they are calculated. The article also explores the historical evolution of these measurements and their importance in ship design, cargo capacity, and compliance with international maritime regulations. Through clear explanations and examples, the article demystifies these concepts for readers, highlighting their practical significance in the shipping industry.
Conclusion
"Ship Tonnage Explained" serves as an essential resource for anyone looking to understand the intricacies of maritime operations. By clarifying the various types of ship tonnage and their applications, the article provides valuable insights into the factors that influence ship design, cargo capacity, and regulatory compliance. This knowledge is crucial for professionals in the maritime industry, as well as for enthusiasts and scholars interested in the technical aspects of shipping.
For a more detailed discussion on Ship Tonnage and related computations, see:
