How Does a More Efficient Ship Propeller Contribute to MARPOL Annex VI, EEXI/CII, and NOx Reduction?
Author: Jeroen Berger • Publication date:
International shipping faces the task of structurally reducing harmful emissions. With the entry into force of MARPOL Annex VI, global limits have been established that set thresholds for emissions of nitrogen oxides (NOx), sulfur oxides (SOx) and greenhouse gases. For shipping companies and shipowners, this means that investments in more efficient propulsion not only cut fuel costs, but are increasingly necessary to remain compliant with tightened international and regional regulations.
Within this framework, the ship propeller plays a central yet often underestimated role. Although emissions are primarily linked to engine type and fuel choice, the hydrodynamic efficiency of the propeller largely determines how much power is actually required to propel a vessel. A more efficient propeller design or targeted optimization can therefore directly lower fuel consumption and structurally reduce carbon dioxide (CO2) emissions, while NOx formation is indirectly influenced through more favorable engine loading.
This article explains how a more efficient propeller contributes to compliance with MARPOL Annex VI and how these optimizations carry through within the Energy Efficiency Existing Ship Index (EEXI) and the Carbon Intensity Indicator (CII). It examines the relationship between propeller efficiency, required propulsive power and emissions performance, and discusses the extent to which propeller optimization can contribute to NOx reduction in conjunction with engine tuning and aftertreatment systems such as Selective Catalytic Reduction (SCR) catalysts and Exhaust Gas Recirculation (EGR) systems. Finally, these technical improvements are placed in an economic and strategic context, in which instruments such as the European Union Emissions Trading System (EU ETS) and FuelEU Maritime play an increasing role in investment decisions for existing vessels.
MARPOL Annex VI: Emission Limits and Technical Requirements
MARPOL Annex VI has been in force since 2005 and forms the international framework for limiting air emissions from shipping. The regulation sets global limits on emissions of nitrogen oxides (NOx) and sulfur oxides (SOx), and over the years has been expanded with measures aimed at limiting greenhouse gas emissions. In addition, Emission Control Areas (ECAs) have been designated, where stricter emission requirements apply to both fuel quality and engine technology.
Compliance with MARPOL Annex VI is determined primarily by engine type, fuel choice and any aftertreatment systems. At the same time, the framework obliges shipping companies and shipowners to take measures, where possible, that reduce energy use per unit of transport work. In this respect, fuel efficiency plays a supporting role in reducing absolute emissions, particularly for CO2, but indirectly also for NOx and SOx.
A more efficient ship propeller does not, in itself, deliver emission reduction within this framework, but it does influence the conditions under which the propulsion system operates. By increasing hydrodynamic efficiency, less propulsive power is required to achieve a given speed. This results in lower fuel demand across the vessel’s operational profile, thereby reducing total emissions, all other factors being equal.
The contribution of propeller optimization within MARPOL Annex VI should therefore be seen as conditional and supportive. This optimization is not a substitute for engine-related measures, but it can be part of a coherent strategy to lower energy use and thereby structurally improve the vessel’s emissions performance. Especially for existing vessels, where major changes to the engine or fuel system are not always feasible or economical, optimizing propulsion can be a relevant additional measure within the limits of the regulatory framework.
EEXI and CII: Efficiency and Operational Performance
Since 2023, the Energy Efficiency Existing Ship Index (EEXI) and the Carbon Intensity Indicator (CII) have formed two central pillars within the IMO framework for managing energy and emissions from existing vessels. EEXI focuses primarily on the vessel’s technical efficiency and sets requirements for the ratio between installed propulsion power and transport capacity. CII, by contrast, assesses actual operational behavior by relating annual CO2 emissions to distance sailed and cargo carried.
Within this distinction, the propeller plays a dual role. A well-aligned propeller design can reduce required power at the design point, which directly supports achieving the EEXI target. At the same time, the same optimization lowers specific fuel consumption across the operational profile, reducing operational CO2 intensity and enabling a more favorable CII score.
Effectiveness depends explicitly on the extent to which the vessel operates within the intended design envelope. A propeller tuned to hull form, loading level and speed distribution delivers the most value when the dominant operating point corresponds to the design assumptions. In that case, propeller optimization can contribute to both certainty of certification and operational stability within the CII framework.
For shipping companies and shipowners, this means that a more efficient propeller is not an administrative solution, but a technical measure with direct impact on both the design and the operational framework. Propeller optimization is therefore a relevant instrument within a broader strategy to balance performance, compliance and economic predictability.
NOx Reduction and Engine Loading
Although the propeller does not directly influence combustion inside the engine, it largely determines how the engine is loaded. Hydrodynamic efficiency affects required propulsive power, the engine operating point and rpm stability. A more efficient propeller configuration reduces hydrodynamic losses and thus lowers power demand at a constant vessel speed.
Lower and, above all, more even loading directly affects thermodynamic conditions in the engine. Fluctuations in load and rpm generally lead to peaks in combustion temperature and cylinder pressure, conditions that promote the formation of nitrogen oxides (NOx). By allowing the engine to operate more consistently and closer to a favorable design point, an optimized propeller can help limit such peaks and thereby lower specific NOx formation.
The contribution of propeller optimization to NOx reduction remains explicitly indirect and conditional. Propeller optimization does not replace engine-focused measures, but it supports the emissions concept. In combination with aftertreatment systems such as Selective Catalytic Reduction (SCR) or internal measures such as Exhaust Gas Recirculation (EGR), more stable engine loading can increase the effectiveness and reliability of these systems. SCR installations, for example, perform optimally within a defined temperature and loading range, while EGR systems are sensitive to highly variable operating conditions.
Within this interplay, propeller optimization functions as an enabling measure that lowers energy demand, stabilizes engine operation and increases the window within which engine and aftertreatment systems can operate effectively. Especially on existing vessels, where major engine modifications are not always feasible or economically desirable, optimizing propulsion can play a relevant additional role in managing NOx emissions within the limits of the applicable regulatory framework.
Economic and Strategic Significance
For shipping companies and shipowners, the impact extends beyond emissions reduction alone. A more efficient ship propeller lowers required propulsive power across the relevant operational profile and translates that efficiency gain directly into lower fuel consumption. Operating costs therefore decrease structurally, while the vessel simultaneously gains a more robust energy profile under varying deployment conditions.
Propeller optimization also has strategic significance, because lower fuel consumption directly reduces CO2 emissions. Within the European Union Emissions Trading System (EU ETS), lower CO2 emissions reduce exposure to emissions costs. Within FuelEU Maritime, where the greenhouse gas intensity of onboard energy is progressively tightened, lower energy demand increases the scope to remain within target using available fuels and a package of measures, or to limit the required use of more expensive compliance options.
Investment in propulsion optimization requires project-specific substantiation. The business case depends on operational profile, speed distribution, loading level, hull effects, maintenance condition and the extent to which the propeller actually represents the dominant loss mechanism. When that substantiation is sound, propeller optimization strengthens competitiveness through lower energy costs, better predictability of performance and reduced exposure to regulatory and cost risks in the years ahead.
Conclusion
The role of the propeller in the decarbonization of shipping deserves a sober but serious place in the overall measures package. By increasing the hydrodynamic efficiency of propulsion, a vessel can deliver the same transport performance with less fuel. That directly reduces CO2 emissions. For nitrogen oxides (NOx), the effect generally remains indirect and conditional. Lower and more stable power demand can make engine operation more favorable and support the effectiveness of engine and aftertreatment measures, but it does not determine NOx performance on its own.
Within regulation, this efficiency gain has effects at multiple levels. Under MARPOL Annex VI, propeller optimization contributes via lower energy use and thus better emissions performance per unit of transport work, without replacing engine or fuel measures. Within the Energy Efficiency Existing Ship Index (EEXI), lower power demand or more favorable use of available power can support achieving the required index. Within the Carbon Intensity Indicator (CII), structurally lower fuel consumption can contribute to a more favorable annual CO2 intensity, provided vessel deployment aligns with the intended design and operating point.
For shipping companies and shipowners, propeller optimization is therefore not a marginal adjustment, but a technical measure with strategic value, lower operating costs, a more manageable emissions position and greater predictability within a regulatory environment in which energy performance carries increasing weight.
About This Article
This article forms part of the background information on the propeller in relation to emissions and regulation and falls within the cluster Ship Propeller Life Cycle, Retrofit and Regulatory Framework. Its core premise is that propeller efficiency conditionally carries through into fuel consumption, power demand and emissions indicators, and therefore influences performance assessment within frameworks such as MARPOL Annex VI, EEXI, CII and NOx-related requirements. This interaction only gains significance when hydrodynamic efficiency demonstrably aligns with the actual operating profile and when performance over time remains reproducible. For a project-specific elaboration, the page Custom Ship Propeller logically builds on this topic.
For the design basis of this relationship, What Are Important Design Principles for an Efficient Ship Propeller connects directly, because that article describes how blade geometry, operational profile and material selection determine efficiency and power demand.
The influence of flow-conditioning measures on efficiency and emissions is set out further in Can Devices Such as Propeller Nozzles, Fins, or PBCFs Improve Ship Propeller Efficiency, which explains the conditional contribution of Energy Saving Devices within the correct operational context.
For practical substantiation of efficiency gains and emissions effects, How Is Ship Propeller Performance Measured and Validated is relevant, because that article explains how improvements are demonstrated and reproduced in relation to the vessel’s actual operating point.