What are the different types of weights in a ship?

Exploring the Various Types of Weights in a Ship

The International Maritime Organisation considers safety regulations to be maritime laws that protect life at sea and minimise the risks of cargo damage. These laws are applied to all ports across the globe as safety regulations for shipping.   One of the essential factors in coining these regulations and taxing shipping companies is the types of weights in a ship. Depending upon the tonnage, the shipping companies plan for weight distribution to ensure that the integrity of the vessel is not harmed while at sea. There are different types of cargo ships in the maritime industry; therefore, their tonnage can vary from one ship design to another. Let us learn more about ship weights. 

What is the structural weight of a ship? 

The sea vessels operating in the maritime industry are made of steel to give structure to the ships and add sturdiness and malleability. Maritime vessels are made of steel and built on the lower part of the hull, responsible for floating the ship on water. Manufacturing companies can calculate the structural weight of a cargo ship by adding its components, including hull, decks, and superstructure. There are different types of cargo ships, and their weight may vary depending on their design and materials used for construction.  

Architects use Archimedes’ principle to calculate the weight of the ship. The ship’s weight exerts pressure on the water but keeps floating due to buoyancy (upthrust force exerted by the water on the ship that keeps it floating). Therefore, a ship’s design, construction and weight are optimised to match its buoyancy. The general weight of an average-sized cargo ship is somewhere around 165,000 tons, while smaller vessels weigh about 50,000 tons and larger vessels approximately 220,000 tons. 

What is Ship Tonnage? 

The different types of weight in a ship are expressed in terms of tonnage. The ship’s tonnage is a measuring unit that defines a sea vessel’s cargo capacity and is expressed in terms of volume. Different types of ships accommodate different numbers of containers or passengers; thus, their tonnage varies based on their design and carrying capacity. Tonnage is an essential factor that port authorities and customs consider when assessing and levying duty fees. The ship tonnage is also critical when complying with the International Maritime Organization (IMO) safety regulations. The SOLAS convention regulates the safety of maritime vessels and the crew on board, where the ship tonnage is considered. 

Types of weights in a ship 

  1. Gross Tonnage (GT)– It measures the volume of closed spaces in a vessel, including the engine rooms, crew accommodation compartments, area for the cargo hold, etc. 1 gross tonnage is expressed as 100 cubic feet. The formula for calculating gross tonnage in ships is GT = K1V, where K1 is a constant expressed as 0.2 + 0.02 log10 V, and V is the volume of enclosed space and is expressed in cubic metres. The value of gross weight is essential while drafting regulations such as SOLAS ( Safety of Life at Sea). 
  2. Net Tonnage (NT)– As opposed to gross tonnage, net tonnage measures a vessel’s cargo-carrying capacity. It is calculated by subtracting the volume of spaces such as crew, machinery, and fuel accommodation alongside the engine room space by the gross tonnage value. Net weight is beneficial in assessing a vessel’s efficiency and profitability.
  3. Deadweight Tonnage (DWT)– The overall capacity of a ship to carry weights, including cargo, crew, water, fuel, and machinery, is expressed as deadweight tonnage. Different designs of vessels vary in deadweight tonnage. It is a crucial parameter for defining a ship’s efficiency in carrying cargo without exceeding its safety limit. 
  4. Cargo Tonnage– It describes the cargo space in the ship where shipping companies load the container according to the vessel’s stowage plan. It is the sum of all sorts of cargo with their containers, vehicles and equipment for loading and unloading.  
  5. Lightweight Tonnage– It describes the weight of a well-built empty ship and is calculated using the Archimedes principle. The amount of water displaced by an empty ship is the calculation of its lightweight tonnage. While measuring this, no fuel, water, cargo, or passengers must be on board. It is, therefore, the measure of the actual weight of the ship with all its structures intact. 

How is tonnage taxed, and which authority issues certification? 

Shipping taxations do not lie under the jurisdiction of traditional corporations but are instead subjected to tonnage taxes. It is less than conventional government taxes and is a marine subsidy the government offers. Shipping companies use information regarding the various types of weight in a ship to calculate the taxation. The tonnage tax is charged to the shipping companies and is taxed based on the ship’s net tonnage. It is a helpful way of paying off their taxes where shipping companies, instead of paying taxes on the profits they made, pay based on their net tonnage of the ships. Tonnage taxes are more predictable and profitable for shipping companies than conventional taxing. 

All ships must possess an International Tonnage Certificate (ITC) issued by the flag states. The International Maritime Organisation, in collaboration with the flag states, certifies if a ship is ITC-approved. The certificate has an expiry date that must be kept in mind. Revision of ITC is necessary in case of any conversions in the vessel structure. 

What common materials used in ship construction contribute to a ship’s weight? 

The various types of weights in a ship are somewhat impacted by using different types of materials in its construction. 

  1. Steel or aluminium– The ship structure’s construction uses steel or aluminium. Steel is a cost-effective and tensile material for building parts such as thickness stringer plates, bottom plating, sheer strake, bilge strake, etc. Since steel is relatively high in weight, manufacturers also use aluminium for its lighter weight, high corrosion resistance, and low maintenance cost.  
  2. Shell plating– To keep the skin of the cargo ship watertight and offer longitudinal strength to the ship structure, various flat and curved steel plates are welded together. It adds to the total ship weight. 
  3. Superstructures– Passenger or crew accommodation requires compartmentalisation of the ship’s hull, which can be done using steel or aluminium. Installation of beds and other amenities facilitates crew accommodation. 
  4. Propellers– A ship’s propellers are dipped inside saltwater and, therefore, need to be constructed from an anti-corrosive element. Metal casting alloys such as brass, zinc and copper are key for constructing and plating propellers. 

What is the role of weight distribution diagrams in ship design? 

A weight distribution diagram in ship is also known as a loading diagram. It helps in planning how the shipping company must distribute the weight of a ship’s cargo, equipment, fuel, water, and other essential components throughout the vessel. A weight distribution design is formulated by analysing the ship’s different weights. It helps in- 

  1. Assessing stability– Depending on the height of the ship, its lightweight tonnage, and its cargo tonnage, the weight distribution diagram for different sea vessels is prepared. Ships rolling on the water often face turbulence and bad weather conditions, and therefore, weight management is necessary to avoid damage to ships, cargo, or life at sea. 
  2. Stowage plan– Loading diagrams assist shipping companies and workers in planning the loading and unloading of cargo and passengers, keeping in mind the weight distribution. It helps in complying with the safety standards of maritime shipping and secures the cargo. 
  3. Compliance with regulation– The SOLAS (Safety of Life at Sea) and MARPOL (Marine Pollution) conventions drafted by the International Maritime Organisation (IMO) to save the environment, goods, life at sea, and shipping vessels from accidents and damages. It states that all maritime vessels must comply with the regulations to sail. All shipping companies must exercise weight distribution and stay within the weight limits. 

These were a few types of weights in a ship and their uses throughout the maritime industry for various purposes. 

LOTUS Containers is a leading marketer of shipping container solutions and has partnered with 300+ container depots to sell and lease various types of shipping containers across the globe.

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