1. How Many Boxes Can One Crane Move in an Hour?
Tall gantry cranes stand along the dock. In interviews with children, a crane operator explains that one crane handles about 35 boxes per hour [1]. This number changes depending on the port and the machine type. The operator sits high up in the cab, so they cannot see the bottom of the ship clearly. They communicate with colleagues in hard-to-see spots to coordinate their work [1].
With boxes moving this fast, there is no time to hesitate. The location for every box must be decided before the work starts. But how do you describe a specific spot on a huge ship? Planners use a coordinate system, like an address, to specify every single location.
2. How Bay, Row, and Tier Give Each Box an Address on the Ship
The position of each box is set by a "stowage plan." News articles explain that specific spots on the ship are identified by coordinates called Bay, Row, and Tier. These positions are chosen through careful calculation [1]. This system is similar to a home address, which uses a town, block number, and room number to find a place. However, this address system only works inside the ship.
If you only know the address, you might think you can just place boxes in order. But there is another condition: the box must be easy to unload at the next port. The address tells you where the box goes, but the unloading order tells you if that spot is actually good.
3. Why Planners Load Boxes Backwards from the Unloading Order
A ship visits several ports on its journey. In the stowage plan, workers make sure that boxes for the next port are not placed on top of boxes for a later port [1]. Moving boxes from the top takes time during cargo handling. This means the ship is loaded by calculating backwards from the unloading order.
Creating this plan takes time. A representative from the shipping company ONE says it takes about 2 hours for a small ship and about 12 hours for a large ship to plan for just one port [1]. But the order of unloading is not the only rule. The ship itself has physical limits that must be respected.
4. How Weight Balance and the Contents of Boxes Affect Loading
If the total weight of the ship leans too far forward or backward, or if it is too concentrated in the center, the ship’s hull can bend. This is called hogging (bending upward) or sagging (bending downward). These conditions put a huge strain on the ship’s structure [1]. Planners calculate the weight to place boxes where they will not cause this strain.
ONE states that when leaving any port, the ship must maintain a balanced state where the load is distributed across the whole ship. This is called "even keel" [1]. The contents of the boxes also matter. For example, flammable cargo is not placed near explosives [1]. You cannot simply say "heavy boxes go at the bottom." Depending on the ship type, this might be different. The goal is always to balance the total weight.
Why did people create such complicated plans? To understand, we look at the very first container ship.
5. How an Old Tanker Carried 58 Boxes in 1956
On April 26, 1956, Malcolm McLean’s Ideal X left Newark, USA, for Houston. It was an old tanker ship. On its deck, 58 aluminum truck trailers were loaded using cranes [1].
According to Transport Geography, McLean estimated that loading a medium-sized ship the traditional way cost $5.83 per ton. With the container method, it cost less than $0.16 per ton [1]. Dividing 0.16 by 5.83 gives approximately 0.027. This means the container method was about 97% cheaper. Note that these are 1956 dollar values. The article also notes that loading took less than 8 hours [1]. After the ship arrives, how do the boxes move through the port?
6. How Imported Boxes Move from the Dock to the Yard and Gate
Explanatory materials from the Ministry of Land, Infrastructure, Transport and Tourism show the flow for imported containers. The gantry crane lifts a box from the ship and places it on a terminal trailer. The trailer carries the box to the yard. A rubber-tired gantry crane (RTG) picks it up and stacks it. Then, an external trailer carries the box out, and it passes through the terminal gate [1].
The port gets busier as ships get bigger. The capacity of the largest container ships in the world increased from 4,600 TEU (Twenty-foot Equivalent Units) in the 1990s to 21,413 TEU in 2018 [1]. TEU is a unit that counts boxes in terms of 20-foot containers. As the number of boxes handled per visit increases, docking times get longer and gates get congested [1].
To help with this, the Ministry studied using AI to plan the yard. The idea is to place boxes that are likely to be picked up soon on top, and boxes for later on the bottom. This study was done in fiscal year 2018, but it is not noted as being fully in practical use yet [1]. Whether on the ship or on land, the same idea is used: "place things by thinking about the order you will take them out."
7. Try It Yourself: Stack Boxes by Unloading Order
You can try this at home. Get about 5 empty snack boxes or building blocks. Try stacking them with the rule: "The box I take out last goes on the bottom," and "The box I take out first goes on top." Change the order in which you take them out and see how the stacking arrangement changes.
If you visit a port or a seaside observation deck, look at the cranes and boxes. You can count the colors of the boxes or the number of stacked layers. Be safe: the area near the dock is dangerous. Watch from a place where visitors are allowed.
For more information, look for children’s materials from the Maritime Bureau of the Ministry of Land, Infrastructure, Transport and Tourism. They include talks from ship captains and crane operators. The flow of the terminal is explained in materials about improving container terminals.