A container ship transports thousands of containers across the world's oceans. The journey is long, the schedule is tight, and the next transport legs are already waiting at the destination port. You would think the motto should be: get to the destination as quickly as possible.

However, in international shipping, it has been common practice for years for ships to intentionally travel at reduced speeds. This process is called slow steaming. What sounds contradictory at first has a simple background: even relatively small changes in speed can significantly influence a large ship's fuel consumption.

However, it is by no means just about consuming as little fuel as possible. The planning of schedules, ship capacities, and port calls also plays a decisive role.

What does slow steaming mean?

Slow steaming refers to the intentional operation of a ship at a speed lower than its technically or operationally possible cruising speed. The ship is not traveling slower because of a defect or because weather conditions force it to. The speed is deliberately chosen to make ship operations more economical, reduce emissions, or support schedule planning.

There is no uniform speed limit at which a ship is officially considered to be in slow steaming mode. Which speed makes sense depends on factors such as the ship type, cargo, route, weather conditions, and the propulsion system used.

How did slow steaming originate?

The principle of intentionally operating ships at slower speeds is not new. However, slow steaming gained particular importance in the wake of the global economic and shipping crisis starting in 2008. Demand for transport capacity declined, while at the same time, numerous new ships entered the market. A significant imbalance between available capacity and actual demand emerged for shipping companies.

By reducing speeds, they were able to cut fuel costs while simultaneously integrating more ships into existing liner services to compensate for longer turnaround times. What began primarily as an economic measure evolved into an important tool for more energy-efficient ship operations. Today, in addition to costs, environmental goals and the requirements for the most efficient transport possible play a major role.

Why does speed make such a big difference?

A large container ship must displace enormous masses of water and overcome water resistance. The faster it travels, the more propulsion power is required. Under comparable operating conditions, the required propulsion power increases approximately with the cube of the speed.

This means that if the speed is reduced, the required power drops disproportionately. A simplified example illustrates the relationship.

Speed decreases by 20 percent. In this simplified model, the required propulsion power is reduced by around 49 percent. However, the ship now requires 25 percent more time for the same distance. When accounting for this longer transit time, the calculated reduction in required propulsion energy is approximately 36 percent.

Important: These values are based on a physical model calculation and are not guaranteed fuel savings. Actual consumption depends on factors such as ship design, engine efficiency, cargo load, weather, and other operating conditions. Nevertheless, this example illustrates why speed is such a significant economic factor.

Less fuel, fewer emissions

Fuel is one of the primary operating costs in maritime shipping. Under the right conditions, a lower cruising speed can significantly reduce consumption. Consequently, this also lowers the CO₂ emissions directly associated with fuel combustion.

For shipping companies, slow steaming is therefore not just a matter of cost, but also a tool for reducing emissions. However, the environmental impact must be considered across the entire transport chain. If ships are at sea for longer, it may become necessary, for example, to deploy additional vessels to maintain the same departure frequency. Their operation also generates energy consumption and emissions.

Furthermore, ship engines do not operate with the same efficiency under all load conditions. Depending on the machinery, sustained low-load operation may require specific technical measures and can impact maintenance and wear. Slow steaming is therefore not automatically the optimal solution at any given speed. The overall balance is what matters.

So why don't all ships travel as slowly as possible?

Because a ship cannot be viewed in isolation. It is part of a transport network in which ships, terminals, schedules, containers, and connecting services must be coordinated. A lower speed initially means a longer transit time.

If a ship travels slower, it needs more hours or days to cover the same distance. To still offer weekly departures on a route, it may be necessary to add extra ships to the rotation.

These ships also incur costs. Slow steaming is therefore always a trade-off between fuel savings, transit time, capacity, and reliability.

Why a ship sometimes shouldn't arrive earlier

Another interesting aspect is arrival planning. Not every ship can be processed in a port immediately upon arrival. Perhaps the assigned berth is still occupied, or a terminal window may only become available at a specific time.

In such situations, it can make sense to adjust speed while still at sea rather than racing to the port only to wait at anchor. This principle is often referred to as Just-in-Time Arrival. The goal is to align the arrival as closely as possible with the actual availability of a berth, the fairway, and the necessary nautical services.

The difference from classic slow steaming is important: while slow steaming describes an operating strategy of intentionally reduced speed, Just-in-Time Arrival is about adjusting speed so that the ship arrives at the most appropriate time.

This can reduce unnecessary waiting times outside a port and, under the right conditions, also save fuel. However, this requires that the shipping company, port, terminal, and other stakeholders exchange reliable information in a timely manner.

What does slow steaming mean for the supply chain?

For shippers, the actual transit time is what matters most. A slower sea voyage can mean longer lead times. This must be taken into account when planning delivery dates, inventory levels, and connecting transport. However, one cannot determine whether a shipment is delayed based solely on a ship's speed.

If a lower speed is already factored into the regular schedule, the ship can arrive perfectly on time. Conversely, high speed does not guarantee reliable delivery. Port congestion, weather conditions, skipped port calls, or missed transshipment connections can also affect transit time.

Therefore, for planning purposes, the decisive factor is not how fast a ship could theoretically travel, but rather the transit time and reliability offered by the specific connection.

Can container ships simply make up for lost time?

Partially, yes. Shipping companies can increase a vessel's speed if necessary to reduce delays or make a scheduled port call. In practice, however, this is not possible without limits.

Available engine power, additional fuel consumption, weather conditions, and the overall schedule all set boundaries. Furthermore, it is not necessarily worth burning extra fuel to make up time if the ship would only have to wait for an available berth at the next port anyway.

That is why speed is planned not just with the next port in mind, but as part of the entire rotation whenever possible.

How fast do container ships actually travel?

In maritime shipping, speed is usually measured in knots. One knot is equal to one nautical mile per hour, or 1.852 kilometers per hour. Large container ships can be designed for significantly higher speeds than they actually maintain during regular liner service.

The speed chosen for a specific voyage depends on the ship and the operational circumstances. A blanket statement like "container ships always travel at 18 knots" would therefore be misleading.

Slow steaming is more than just sailing slowly

A container ship's speed affects much more than just travel time. It impacts energy requirements, operating costs, emissions, and the planning of entire liner services. It also illustrates why modern logistics does not always operate on the principle that "faster is better."

Sometimes, a slightly slower voyage is the more economical solution. At other times, an adjusted speed even helps to avoid unnecessary waiting times outside a port. For shippers, the key remains ensuring that speed, schedule, and the actual delivery date are in sync.