Container ships built to cruise at high speed spend most of their working lives moving considerably slower. The practice is called slow steaming, and it follows from the physics of pushing a hull through water.
Fuel burn rises far faster than speed
The resistance a hull encounters grows sharply with velocity, and the power required to overcome it grows faster still.
The practical consequence is that a modest reduction in speed produces a disproportionately large reduction in fuel consumed per day.
Because fuel is the largest variable cost of operating a ship, that relationship dominates almost every other consideration.
Slower voyages absorb surplus capacity
A service between two ports needs a certain number of ships to maintain a weekly departure, and that number depends on round-trip duration.
Slowing the ships lengthens the rotation and requires more vessels to hold the same schedule.
When the fleet has more capacity than the market needs, this is an advantage. Idle ships are absorbed into active services instead of sitting unused.
Emissions rules made slowness a compliance tool
International regulations now set efficiency requirements for existing ships, measured through indicators of carbon intensity relative to work performed.
Reducing speed is the fastest and cheapest way for an older vessel to improve its rating, since it requires no modification to the ship.
Fuel saving and regulatory compliance therefore point in the same direction, which is why the practice has persisted.
The costs fall on cargo owners and schedules
Slower transit means goods spend longer in transit, which ties up working capital and forces importers to order earlier.
It also reduces the buffer available to recover from a delay, so a ship already running slowly has less ability to make up lost time before a berth window.
Carriers manage this by building slack into published schedules rather than by speeding up.
Speed returns when freight rates are high
The calculation reverses when the revenue from carrying cargo rises sharply, because an extra voyage per year then outweighs the additional fuel.
Ships also speed up to catch a missed berth slot, since waiting offshore for a new window can cost more than the fuel to arrive on time.
Fuel type matters as well, since a vessel burning a more expensive low-sulphur fuel has a stronger reason to hold its speed down than one running on cheaper grades.
Sailing speed is therefore a continuously adjusted commercial decision rather than a property of the vessel.