When Can an ESD Contribute to Better EEXI and CII Performance?

Author: Jeroen Berger • Publication date: 30 October 2026

An Energy Saving Device (ESD) can contribute to improvements in the Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII) when the lower propulsion power demand also produces a more favourable result under the respective assessment method. A Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) or Rudder Bulb can improve hydrodynamic efficiency, so that a ship needs less shaft power at the same speed. For EEXI, the benefit must be substantiated through the prescribed technical calculation and, when the improvement is formally incorporated, included in the approved documentation. CII concerns operational CO₂ intensity over the calendar year, calculated from reported fuel consumption in relation to ship capacity and distance travelled. An ESD that reduces fuel consumption while sailing can therefore improve the annual CII, but does not guarantee a higher CII rating. This distinction is important because EEXI and CII assess different aspects of performance, use different data and do not respond to a hydrodynamic improvement in the same way.

Which Technical Improvement from an ESD Counts towards EEXI?

The Energy Efficiency Existing Ship Index assesses a ship’s technical energy efficiency using a prescribed calculation method. The calculated attained EEXI must meet the required EEXI applicable to the ship concerned. The relevant engine power, certified specific fuel consumption, fuel-specific CO₂ conversion factor, ship capacity and reference speed are among the factors taken into account.

An ESD can improve the attained EEXI because the complete propulsion configuration can achieve a higher speed at a given power. For a conventional hydrodynamic ESD, the benefit is thus linked to the speed–power relationship used in the EEXI calculation.

This differs from the usual commercial assessment of an ESD. In that assessment, it is often established how much less shaft power the ship needs to maintain the same speed. For EEXI, the relevant consideration is how the ESD affects the reference speed determined under the regulations at the prescribed reference power.

That reference power is not necessarily equal to the maximum installed engine power. With an approved engine power limitation or shaft power limitation, the power is determined under the applicable EEXI provisions. The improvement from an ESD must therefore be assessed under the conditions that actually apply to the calculation for the ship concerned.

These differences must not be conflated. A reduction in required shaft power of 5 percent at constant speed, for example, does not automatically mean that the attained EEXI also improves by 5 percent.

A hypothetical example illustrates this. Suppose a seagoing ship without an ESD has a speed of 15,00 knots at the relevant EEXI reference power. With the device, that reference speed increases to 15,15 knots. If all other quantities in the simplified EEXI calculation remain unchanged, this speed improvement of 1 percent results in a reduction in the attained EEXI of approximately 0,99 percent. This result applies only to the chosen calculation assumptions. The example is not a prediction for a specific ESD and does not indicate how much shaft power the ship saves at another sailing speed. If other parameters in the EEXI calculation also change, the resulting difference in the index may be different.

For the technical assessment, it is also important that the ESD actually improves the performance of the complete propulsion arrangement. A favourable change in the inflow to the propeller is insufficient on its own if the device also creates hydrodynamic forces that reduce the net benefit.

This applies to devices ahead of the propeller, such as Pre-Swirl Stators and Pre-Ducts, and equally to PBCFs and Rudder Bulbs behind the propeller. Their local operating principles differ, while for EEXI it is ultimately the effect on the technical propulsion performance determined under the regulations that counts.

Applicability must also be checked. In principle, EEXI applies to the relevant ship categories from 400 GT, while the mandatory CII system applies to the designated ship types from 5.000 GT. A ship may therefore be subject to EEXI requirements without having an annual CII rating.

For ships subject to the Energy Efficiency Design Index (EEDI), there is also a separate technical assessment framework. An approved attained EEDI may, under the applicable conditions, also be used to meet the required EEXI. The formal treatment of an ESD therefore depends in part on the ship’s existing energy-efficiency documentation.

How Is the ESD Contribution Substantiated and Approved for EEXI?

Installing an ESD does not automatically change the already approved attained EEXI. For the hydrodynamic improvement to count formally, it must be demonstrated that the device improves speed–power performance under the relevant reference conditions.

The applicable verification guidelines offer several ways to do this. Depending on the available data, sea-trial measurements, model tests and numerical calculations may be used to establish the relevant speed–power relationship. The chosen method must fall within the scope of the guidelines and be substantiated with sufficient reliability for acceptance by the competent verifier.

For a retrofit, Computational Fluid Dynamics (CFD) may, for example, be used to investigate how the existing propulsion arrangement operates with and without a Pre-Swirl Stator. If the calculations have been validated with sufficient reliability and the input data are suitable, those calculations can help to establish the changed speed–power relationship.

A numerical prediction is not, however, automatically sufficient for a formal EEXI amendment. The geometric input data, calculation method and substantiation of the expected improvement must meet the applicable verification conditions. Additional experimental evidence may be needed.

The same applies to sea trials. A lower power demand after docking may partly result from hull cleaning, propeller polishing or other work. If the ESD contribution is claimed separately, the selected performance comparison must substantiate that contribution with sufficient reliability.

The configurations being compared are therefore important to the assessment. A new propeller with a Pre-Swirl Stator may deliver better propulsion performance as a whole. The difference from the original propeller without a stator then describes the improvement from the complete retrofit, rather than automatically representing the stator’s individual contribution.

The existing EEXI documentation also warrants attention. Many ships already meet the required EEXI, for example through an engine power limitation or shaft power limitation. Installing an ESD does not then automatically mean that the technical documentation must be recertified.

Under certain circumstances, the shipowner may voluntarily request a new EEXI verification to have a demonstrable efficiency improvement incorporated. If the retrofit affects an existing power limitation or forms part of a major conversion, a mandatory reassessment may be required. The procedure needed depends on the modification and the applicable rules.

A distinction must therefore be made in advance between three questions: does the ESD deliver a hydrodynamic improvement, can that improvement be incorporated under the EEXI method, and is an amendment to the existing approval necessary or desired? The answer depends on the original EEXI calculation, the technical modifications made and the available performance substantiation. A reduction in power demand may be commercially valuable without the formally recorded attained EEXI changing.

When Does an ESD Lower the Annual CII?

The Carbon Intensity Indicator assesses a ship’s operational CO₂ intensity over a calendar year. In the usual attained annual operational CII, the calculated CO₂ emissions from reported fuel consumption are related to ship capacity and the total distance travelled.

For bulk carriers, tankers and container ships, among others, capacity is expressed in deadweight tonnage (DWT). Gross tonnage (GT) is used for the relevant ro-ro and passenger ships. The calculation uses this capacity together with distance as a proxy for transport work. It therefore does not directly represent the actual cargo carried on each voyage.

The CII guideline amended in May 2026 confirms that the capacity-based approach is retained for calculating the CII of an individual ship. The total reported distance forms the basis, including distance not classified as underway in the recorded data. Data on actual transport work may, however, be used for further analysis of the system.

An ESD can improve this indicator if the device reduces fuel consumption without changes in distance or other relevant conditions cancelling out the benefit. Suppose a bulk carrier continues to sail the same routes and distances during the calendar year. Because of favourable hydrodynamic performance, the propulsion arrangement requires less power. If the main engine consequently consumes less fuel and the rest of the energy supply remains unchanged, the calculated annual CO₂ emissions also fall.

With unchanged ship capacity and distance travelled, these lower emissions result in a lower attained CII. This is an improvement, because a lower CII value represents a more favourable operational carbon intensity.

A hypothetical example illustrates the relationship. Suppose a bulk carrier with a capacity of 50.000 DWT travels 50.000 nautical miles annually and reports total emissions of 10.000 tonnes of CO₂. Without applicable correction factors or voyage adjustments, the calculated CII is then 4,00 grams of CO₂ per DWT-nautical mile.

If, under otherwise equivalent conditions, an ESD reduces total annual CO₂ emissions by 300 tonnes, emissions fall to 9.700 tonnes. The calculated CII then becomes 3,88 grams of CO₂ per DWT-nautical mile, an improvement of 3 percent.

All values in this example are hypothetical. They concern the ship’s total annual CO₂ emissions, rather than solely the main engine’s emissions. Capacity, distance, fuel-specific conversion factors and other applicable calculation conditions are also assumed to remain unchanged.

In practice, a reduction in propulsion power demand need not produce the same percentage reduction in total annual CO₂ emissions. The main engine may have a different specific fuel consumption at the new load. Energy consumers that are not directly affected by the ESD also remain active during port stays and other operating periods.

The energy balance may also change with a shaft generator drawing power from the shaft (Power Take-Off, PTO), a drive motor adding power to the shaft (Power Take-In, PTI), or diesel-electric propulsion. A lower mechanical power demand at the propeller must therefore be translated into the fuel consumption of the engines and generators involved.

Correction factors or voyage adjustments may also apply to certain ships and operating activities. When these are applied, the official attained CII must be determined under the associated calculation rules. The simple ratio of total CO₂ emissions to capacity and distance is then not necessarily equal to the formally corrected index value.

The ESD can therefore improve CII through lower annual fuel consumption, but the official result follows from the complete applicable CII calculation, rather than directly from the hydrodynamic saving percentage.

Why Does an ESD Not Guarantee a Better CII Rating?

A lower attained CII is favourable, but does not automatically lead to a higher CII rating. The annual rating is determined by comparing the calculated CII with the required value and the associated assessment boundaries for the ship and calendar year concerned.

The ship receives a rating from A to E. An ESD can improve the calculated value while the ship remains within the same rating category. If the original CII is close to an assessment boundary, however, a relatively small improvement may be enough to reach a more favourable category.

For an E rating, or a D rating over three consecutive years, a corrective action plan must be prepared for the ship within the applicable Ship Energy Efficiency Management Plan (SEEMP). An ESD may form part of such an improvement plan, but compliance with the required CII must be shown to be sufficiently plausible for the operation concerned.

The required CII values are tightened over time. For 2026, a reduction factor of 11 percent applies relative to the 2019 reference line. For 2027, 13,625 percent has been set, increasing through 16,25 percent in 2028 and 18,875 percent in 2029 to 21,5 percent in 2030.

These percentages are reduction factors for the required CII relative to the reference line, rather than predicted fuel savings from an ESD. The system is also subject to further evaluation. The rules applicable to the future years concerned must therefore be taken into account in an investment decision.

An ESD may therefore deliver a demonstrable CO₂ reduction without the rating automatically improving in the longer term. If the required CII is tightened more quickly than operational carbon intensity falls, the available margin to an assessment boundary may narrow.

Operation also plays an important role. If the ship uses the hydrodynamic improvement to sail faster, total fuel consumption need not fall to the same extent as in a comparison at constant speed.

The operational profile may also change. A different balance between laden and ballast operation, longer port stays or changed routes affects total fuel consumption and reported distance. The annual CII may consequently differ from a calculation in which only the ESD is added to an otherwise unchanged operating year.

A distinction must be made here between an absolute CO₂ reduction and a lower CO₂ intensity. CII can also change because the distance travelled changes. A lower index value therefore does not, on its own, prove that total annual CO₂ emissions have fallen. Conversely, lower total emissions need not result in the same percentage improvement in CII if the reported distance also changes.

The way in which distance and fuel consumption are reported must comply with the applicable guidelines. Where specific correction factors or voyage adjustments are permitted, they must be incorporated under the applicable conditions. The formal rating is based on the relevant attained CII, rather than on a separate supplier calculation of power saving.

A favourable CII rating therefore cannot be predicted solely from a claim about shaft power savings. A calculation of operational CO₂ intensity over the relevant calendar year is needed, including the expected operating conditions, any permitted corrections and the applicable assessment boundaries.

How Do the Operational Profile and Technical Condition Affect the Contribution to CII?

The hydrodynamic saving from an ESD depends on the conditions in which the ship operates. A device may therefore provide a clear improvement at a particular cruising speed, while its contribution is smaller at other propeller loadings.

For CII, the main consideration is how fuel consumption changes over the entire year. An ESD that shows a favourable result at the nominal design speed therefore need not automatically deliver the greatest annual CO₂ reduction.

For a bulk carrier operating in different loading conditions, draught, trim and inflow to the propeller may, for example, vary considerably. A Pre-Swirl Stator optimized for normal laden operation may make a different hydrodynamic contribution in ballast operation. The annual saving then depends in part on the distribution of operating hours.

For a shortsea ship, regular port calls and periods of low propulsion load may form a significant part of operation. The ESD reduces hydrodynamic power demand while sailing, but does not automatically lower auxiliary-engine fuel consumption during loading, unloading or time at rest.

This distinction is relevant to the CII forecast. If propulsion accounts for only part of the annual fuel demand, a substantial percentage saving in shaft power may yield a smaller relative reduction in total annual CO₂ emissions.

Maintenance also affects the eventual contribution. Fouling and changes in hull and propeller condition may increase power demand. The ESD itself may also become fouled or operate under inflow conditions different from those for which it was originally optimized. In an annual CII forecast, a saving percentage from clean design conditions must therefore be prevented from being applied to all operating hours without further assessment.

The relevant calculation links the expected power saving at each operating point to the fuel consumption of the propulsion plant. Operating hours, distance travelled, other energy consumers and any applicable corrections are then included.

If the operational profile changes, the new starting conditions must also be used. A ship that consistently operates at lower speeds may have different ESD performance from that originally predicted. The potential CII improvement then follows from the combined effects on fuel consumption and distance, rather than solely from the ESD’s operation.

A reliable prediction thus shows not only how much CO₂ the ESD can save under favourable conditions, but above all how much the total annual emissions and the officially calculated CII are expected to change with the ship’s actual use.

What Role Does an ESD Play alongside Engine Power Limitation and Other Measures?

An ESD can be applied both independently and in combination with other efficiency measures. For EEXI in particular, it is important to distinguish the device’s contribution from the effect of an engine power limitation or shaft power limitation.

With Engine Power Limitation (EPL) or Shaft Power Limitation (ShaPoLi), the available propulsion power is limited under prescribed conditions. The calculated attained EEXI may consequently improve.

Such a limitation does not, however, automatically mean that the ship needs less hydrodynamic power at the same sailing speed. The ESD works differently: it is specifically intended to improve propulsion efficiency and reduce power demand for a given ship performance.

That distinction makes a combination technically interesting. If a ship needs less propulsion power to maintain its usual speed, an ESD can help to limit the operational consequences of a power limitation. Whether this actually succeeds depends on the required power reserve, the operational profile and the performance of the modified propulsion arrangement.

The combined effect on EEXI must be calculated using the official method. It is incorrect to add the expected saving percentage from the ESD directly to the improvement from a power limitation. The two measures affect different parts of the calculation and potentially the reference speed as well.

A different assessment applies to CII. An engine power limitation may affect operational consumption if the ship is consequently used differently, but does not automatically produce fuel savings at operating points that remain unchanged. An ESD, by contrast, can directly reduce the required propulsion energy at the same speed.

Combinations with propeller optimization, hull maintenance or other energy-saving measures are also possible. Hydrodynamic and operational interactions must be taken into account. A new propeller design may, for example, change how an existing Pre-Swirl Stator operates, while hull cleaning may improve speed–power performance independently of the ESD.

For a reliable overall result, the measures must therefore be assessed as a combined configuration. The calculated or measured improvement from the complete package must not be attributed entirely to the ESD without separate substantiation.

This is particularly important when an investment is assessed in terms of both fuel costs and potential implications for EEXI and CII. A measure may be attractive for one objective without delivering the same improvement for the other.

What Data Are Needed to Assess an ESD’s Contribution to EEXI and CII?

A reliable assessment starts by establishing which outcome the shipowner wishes to improve. For EEXI, this concerns the technical index and the possible incorporation of the ESD in the approved calculation. For CII, it concerns the expected annual operational CO₂ intensity and the implications for the applicable rating.

For EEXI, the existing Technical File, the relevant reference speed, the prescribed engine power and the substantiation of the speed–power relationship are important. When an ESD is installed, it must be established how the changed performance can be demonstrated using the applicable method and which verification or documentation amendment is needed.

It must also be known whether the ship uses an engine power limitation or shaft power limitation. An ESD can improve propulsion performance without the same formal EEXI implications automatically arising as when the limited power is adjusted.

Different data are needed for CII. Annual fuel consumption by fuel type, the applicable CO₂ conversion factors, ship capacity and reported distance form the basis of the calculation. The split between propulsion and other energy consumers also helps to determine which proportion of emissions can be affected by the ESD.

To predict the future result, it must also be known how the ship is operated. The relevant speeds, loading conditions, operating hours and expected changes in the operational profile determine how much of the hydrodynamic improvement is actually used.

If an ESD forms part of a larger retrofit project, the contribution of other measures must remain separately identifiable. A lower fuel demand after propeller replacement and installation of a Pre-Swirl Stator does not automatically describe only the stator’s effect.

For a CII forecast, the starting position relative to the applicable assessment boundaries is also important. A technical saving may improve operational CO₂ intensity without the rating changing. It must therefore be investigated how much margin there is to the relevant category boundaries, including for years in which the requirements are tightened.

The report must distinguish between the calculated EEXI improvement, the expected annual CO₂ reduction, the change in attained CII and a potential change in the CII rating. These results must not be presented as interchangeable saving percentages.

An ESD can contribute to an EEXI improvement when its hydrodynamic performance produces a more favourable technical index under the prescribed method and that result is verified where necessary. For CII, the benefit arises when the device sufficiently reduces fuel demand during operation and thus the calculated operational CO₂ intensity relative to the applicable capacity and distance basis. A better EEXI does not automatically mean a better CII rating, and a lower annual CII need not lead to a new EEXI calculation. By assessing the two indicators separately, the contribution that the ESD can make to technical energy efficiency, operational CO₂ intensity and compliance with the applicable requirements becomes clear.

This Article within the Series

With changes to the propeller or propulsion control, reassessment of the existing ESD matching may be needed, as may a check of the relevant energy-efficiency documentation. The implications for the annual CII also depend in part on how a lower power demand affects fuel consumption. Both issues form part of the technical integration, service life and regulations for ESDs.