How Weather Impacts Fuel Consumption and Emissions
In the drive towards maritime decarbonisation, weather is often treated as an uncontrollable background factor, something to endure rather than quantify. Yet for shipowners, operators and charterers, weather is one of the most significant variables influencing fuel consumption, voyage performance and emissions. Two voyages by the same vessel on the same route and at the same power setting can produce very different outcomes simply because the environmental conditions were not the same.
Understanding how weather affects a vessel’s energy requirements is essential not only for operational efficiency but also for managing exposure to regulations such as CII, the EU ETS and FuelEU Maritime. Weather is not only a safety or routing concern. It is a major economic and environmental one.
The physics: why weather changes a vessel’s energy demand
Ships expend energy to overcome two main types of resistance:
- Hydrodynamic resistance, related to the interaction between the hull and the water
- Aerodynamic resistance, created by the wind acting on the hull and superstructure
Weather influences both of these. Waves, swell, currents and wind dramatically affect the total resistance a ship must overcome. Even at a constant power setting, the vessel’s speed may fluctuate significantly depending on the conditions.
This is why two otherwise identical voyages can produce very different fuel consumption and emissions figures.
Waves and sea state: the most complex source of added resistance
Sea state has the strongest influence on fuel consumption. Waves introduce several forms of added resistance, including:
- Added resistance in waves, due to the vessel’s vertical motions
- Slamming, the impact of the bow or bottom with the water surface
- Pitching and heaving, which impact the propeller’s effective immersion and reduce propulsive efficiency
The degree of impact depends not only on wave height but also on wave period, steepness and direction. Long-period swell can be highly disruptive, while short, steep wind waves generate more violent motions.
In adverse sea states, propulsion power requirements can increase by 30 to 50 per cent or more. For container vessels and RoRo ships, the penalty can be higher still.
Currents and swell misalignment
Currents have a decisive impact on real-world vessel performance:
- Favourable currents increase speed over ground and reduce fuel consumption
- Adverse currents slow the vessel and increase power demand
Major currents such as the Gulf Stream, the Agulhas Current and the Kuroshio can significantly influence voyage efficiency depending on their strength and position. Tidal currents, if not correctly leveraged, also have a significant impact on performance.
A more subtle factor is swell and wind misalignment. When swell arrives from one direction and wind-generated waves from another, the vessel may experience destabilising motions. This forces the autopilot to make frequent adjustments, leading to increased rudder use, higher drag and additional fuel consumption.
Wind: a major aerodynamic driver of extra fuel burn
Wind can impose a substantial fuel penalty, especially for vessels with significant windage (containerships, car carriers, passenger vessels, etc.).
- Headwinds: sailing into a headwind increases aerodynamic drag. Even modest winds can have a noticeable effect, while strong headwinds may increase fuel consumption by 10 to 25 per cent.
- Crosswinds: they require constant rudder corrections to maintain course. Rudder activity increases hydrodynamic drag, which raises fuel consumption.
Whether directly or indirectly, wind-driven forces translate into higher emissions.
Weather avoidance, slow steaming and operational decisions
Weather affects not only resistance but also decision-making on the bridge and in fleet operation centres.
Masters often reduce speed in heavy seas to protect the vessel and crew. Slow steaming reduces fuel consumption per hour but increases total voyage time. Depending on conditions, overall fuel consumption for a weather-affected voyage may rise.
Weather-optimised routing can produce notable benefits. By adjusting route and speed based on forecasts and vessel-specific performance models, operators achieve fuel savings while avoiding unsafe conditions.
Weather is therefore both a constraint and an opportunity. With accurate modelling and informed decision-making, it becomes a tool for efficiency rather than simply a hazard.
Regulatory implications: CII, EU ETS, FuelEU Maritime (and more to come)
Weather’s impact on fuel consumption has direct consequences for regulatory compliance.
- CII (Carbon Intensity Indicator): because the CII of a vessel reflects (in a nutshell) her CO2 emissions per nautical mile, adverse weather that reduces speed over ground and increases fuel burn can negatively affect annual ratings.
- EU ETS & FuelEU Maritime: fuel consumption directly influences carbon costs, voyages that encounter poor weather incur higher EU ETS exposure and decreased FuelEU compliance due to additional emissions.
Accurate quantification of weather impacts is increasingly important for compliance planning, budgeting and fleet performance management.
Conclusion: weather as a strategic variable in maritime decarbonisation
Weather profoundly affects vessel performance, fuel consumption and emissions. From wave-induced resistance to wind-driven drag and current variability, environmental conditions shape the economic and environmental profile of every voyage.
As the decarbonisation agenda advances and regulatory requirements tighten, weather can no longer be considered an external, unpredictable influence. It must be integrated into performance analysis, voyage planning and optimisation processes.
By leveraging accurate weather data, vessel-specific performance models and modern routing intelligence, operators can reduce fuel consumption, control emissions and improve regulatory compliance. In this way, weather becomes not only a challenge but a strategic advantage.