Cargo shipping is a low margin business model that requires vessels to be fully loaded in order to sustain profitable operations. When a ship is in the design phase it is almost always structured in a specific classification of naval architecture and built to serve a specific route or purpose.
Vessels that are built to pass through specific bottlenecks while carrying the maximum amount of cargo are termed “-max”. For example a freighter designed to pass through the Panama Canal are called Panamax. This means that the ship will fit into a minimum bounding box that matches the dimensions of the smallest locks in the canal. A bounding box is measured in three dimensions and includes areas under water and above the ship in addition to maximum length and width.
In a maritime specific case the dimensions of the bounding box have some different but still familiar names. Draft is the measurement from the surface of the water to the bottom. Beam is the width of a vessel at its’ widest point. Length is measured as the overall length of a ship but in some cases maximum dimensions might consider the length at the waterline which can differ significantly from length overall (LOA) because of the Deadrise of the hull. The final measurement is Air Draft which is the measure of the maximum height above the waterline of any structure on the ship.
Other terms you will see are Gross Tonnage (GT) and Dead Weight Tonnage (DWT) and while many perceive this as a measure of weight it is actually described best as a measure of volume of the vessel’s hull. Weight only factors in when an equivalent weight of water displaced by the hull needs to be expressed.
Now let’s get to the definitions.
Ship Size Definitions
Most of these definitions pertain to cargo vessels but they can be applied to any kind of ship. Military and cruise ships can also be classified under these definitions but the most common usage concerns cargo ships.
Aframax – This classification almost always refers to an oil tanker although it is occasionally applied to other bulk commodities. These vessels serve oil producing areas with limited port resources or where man made canals lead to terminals that load raw petroleum products.
The size limitations in this class are few. The main restriction is the beam of a vessel which in this case cannot exceed 32.3 Meters or 106 feet. Tonnage of this type of vessel is approximately 120,000 DWT.
Capesize – Here is one of the instances where the naming scheme is different but the concept is the same. A Capesize class of ship is limited by the depth of the Suez Canal which is currently 62 feet or about 19 meters. The soft geology of the region has allowed the canal to be dredged to a greater depth since it was first built and it possible the canal will be dredged again in the future so this classification may change its maximum draft limit.
Capesize vessels are large bulk carriers and tankers that get their name from the route they must take to bypass the Suez Canal. This route takes the past the Cape of Good Hope in Africa or Cape Horn off of South America depending on the final destination of the ship.
The displacement of these vessels can range from 150,000 to as much as 400,000 DWT.
Chinamax – Chinamax is a little bit different since it is determined by the size of port facilities rather than physical obstructions. This term is not only applied to ships but also to port facilities themselves. Ports that can accommodate these very large vessels are referred to as Chinamax compatable.
These ports do not necessarily need to be anywhere near China they only need to meet the draft requirements of dry bulk carriers in the 350,000 to 400,000 DWT range while not exceeding 24 meters or 79 feet of draft, 65 meters or 213 feet of beam, and 360 meters 1,180 feet of overall length.
Malaccamax – Here is another situation for naval architects where the main restriction is draft of the vessel. The Strait of Malacca has a depth of 25 meters or 82 feet so ships of this class must not exceed this depth at the lowest point of the tidal cycle.
Vessels serving this route can gain capacity in the design phase by increasing beam and length at the waterline in order to carry a greater capacity in a limited draft situation.
Panamax – This class is the most commonly recognized to most people since it refers to the Panama Canal which is quite famous in its own right.
The current size limitations are 294 meters or 965 feet in length, 32 meters or 106 feet of beam, 12 meters or 39.5 feet of draft, and 58 meters or 190 feet of air draft so vessels can fit under the Bridge of the Americas.
The canal opened in 1914 and by 1930 there were already plans to enlarge the locks to pass larger vessels. In 2014 a third larger set of locks will begin operations and define a new class of vessels called New Panamax.
New Panamax has size limitations of 366 meters or 1200 feet of overall length, 49 meters or about 160 feet of beam, and a draft of 15 meters or 50 feet. The air draft will remain the same under the Bridge of the Americas which is now the main limiting factor for large vessels passing through the canal.
Seawaymax – This class of vessels is designed to achieve the maximum size for passage through the Saint Lawrence Seaway inbound or outbound from the Great Lakes system.
The locks of the seaway are the limiting factor and can receive ships no larger than 225.5 meters or 740 feet of overall length, about 24 meters or 78 feet of beam, about 8 meters or 26 feet of draft, and an air draft of 35.5 meters or 116 feet above the water.
Larger vessels operate on the lakes but they are unable to reach the sea because of the bottleneck at the locks.
Supermax, Handymax – Once again this is a class of ships that is not restricted by a specific set of locks or bridges but instead it refers to cargo capacity and the ability to use ports. Ports are often designated to be Supermax or Handymax compatiable.
Supermax as you probably guessed is the largest of the vessels with a size of around 50,000 to 60,000 DWT and can be as long as 200 meters or 656 feet.
Handymax vessels are slightly smaller and have a displacement of 40,000 to 50,000 DWT. These ships are usually at least 150 meters or 492 feet.
Suezmax – The Suez Canal’s dimensions are the limiting factor for ship size in this case. Since there are no locks along the one hundred plus miles of the canal the only limitations are draft and air draft.
The canal has a useful draft of 19 meters or 62 feet and vessels are limited by the height of the Suez Canal Bridge which has a clearance of 68 meters or 223 feet.