What Comparison Basis Is Needed to Link a Measured Performance Difference to an ESD?
To be able to link a measured performance difference to an Energy Saving Device (ESD), it must be known how much shaft power the same ship would need without the device under equivalent conditions. Lower power demand after installation does not automatically prove that the entire improvement was caused by the ESD. When the hull is also cleaned, the propeller repaired or the propulsion control changes during dry-docking, several measures may contribute to the result. The comparison basis must therefore specify both the ship conditions and the initial technical configuration. Preferably, the situations with and without a Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) or Rudder Bulb differ only in the device being investigated. Where such a comparison cannot be measured directly, the missing reference must be substantiated with additional data or calculations. The quality of that substantiation determines whether the ESD saving can be established directly, can only be estimated or cannot be distinguished separately from other improvements.
Which Initial Configuration Provides a Reliable ESD Reference?
A reliable ESD reference describes the propulsion arrangement without the device being investigated, but with an otherwise equivalent technical configuration. This concerns more than the hull and propeller. The rudder arrangement, propulsion control and relevant coupled systems must also match the situation against which ESD performance is compared.
For an existing bulk carrier, a speed–power trial before installation may provide a suitable reference. The measured relationship between ship speed and shaft power then describes how the ship performs without an ESD. To use that relationship later, it must be clear what technical condition existed during the trial and whether it has remained sufficiently similar between the two measurement campaigns.
An earlier delivery trial is not automatically suitable for this purpose. The hull coating, propeller condition and operating settings may have changed since the ship was built. A modification to the propulsion system may also have taken place. A comparison with the original performance at delivery can provide valuable information about how the ship has developed, but does not necessarily describe the separate contribution of an ESD installed later.
The appropriate reference therefore depends on the question to be answered. Where only a PBCF is fitted to an existing propeller, a comparison with the same propeller without a PBCF is the obvious choice. If a new propeller and Pre-Swirl Stator are installed together, a comparison with the original propulsion arrangement can establish the improvement of the complete package. For the additional contribution of the stator to the new propeller, a comparison is needed with that same new propeller without the stator.
The latter point is an important limitation of the evidence. Initial performance measured before the retrofit does not automatically describe how the ship would have performed after all the other work, but without the ESD. It is precisely this missing condition that is needed to establish the separate effect of the device.
A similar situation arises in a newbuild project when the ship is delivered only with an ESD. The performance of the version without the device can then be derived from calculations or model tests, but has not been measured directly at full scale. The comparison remains dependent on the reliability with which that missing configuration has been predicted.
Surface condition also forms part of the definition of the reference. A fouled hull without an ESD does not provide an equivalent initial condition for the same hull after cleaning with an ESD if the aim is to assess the device alone. For that purpose, it must be established how the clean configuration would have performed without the ESD.
A technically useful reference therefore comprises more than a previously measured power value. The configuration, maintenance condition and operating settings must have been recorded with sufficient accuracy to determine which change is actually being investigated. Particularly where the expected ESD saving is small, this definition may be more decisive than the accuracy of an individual measuring instrument.
Why Must the Technical Condition Remain Comparable Between Measurements?
The hydrodynamic performance of a ship changes when the condition of the hull or propeller changes. Fouling and increased coating roughness can increase resistance. Damaged blade edges, deposits and a rougher propeller surface can also impair propeller operation. When such conditions change between two measurement campaigns, power demand may also change without the ESD being responsible.
This makes a planned dry-docking a particular case for performance validation. Dry-docking provides a practical opportunity to install an ESD, but often coincides with work that also improves propulsion performance. A tanker may, for example, enter dry dock with fouling and leave after coating maintenance, propeller polishing and installation of a Pre-Duct.
If the ship subsequently needs less power at the same speed, the overall improvement is relevant. For the separate ESD contribution, however, an important comparison is still missing: how would the ship have performed after the same maintenance, but without the Pre-Duct?
The influence of maintenance cannot always be determined using a general percentage saving. The original fouling, coating condition and propeller roughness differ from ship to ship. The hydrodynamic contribution of an ESD may also change when hull and propeller condition change. A device matched to a particular inflow need not retain exactly the same performance in a substantially different wake field. Simply subtracting an assumed maintenance saving from the measured overall improvement is therefore not a sufficiently substantiated method. Such a calculation assumes that the separate effects are known and can be added on the comparison basis concerned. Where maintenance also affects ESD operation, that assumption may be inadequate.
Targeted measurements before and after a separate ESD installation provide a stronger starting point. Where the same measuring equipment is used, the interval between trials is short and the other aspects of the technical condition demonstrably change little, the ability to attribute the power difference to the device increases.
This approach has been applied in published research into a Pre-Swirl Stator on a seagoing bulk carrier. Separate speed–power trials were carried out before and after installation, supplemented by hydrodynamic calculations. The short interval between measurements and the favourable sea trial conditions helped limit the influence of other changes. The result remained specific to the ship configuration and measurement arrangements investigated.
Checks are still needed even with a short interval. A ship may nevertheless have been cleaned, experienced a change in propeller condition or undergone changes to its propulsion control during the work. Inspection data, maintenance records and an overview of the work carried out therefore form part of the technical substantiation.
If the condition cannot be kept equivalent, the performance assessment need not be abandoned. Its evidential strength does, however, change. The overall improvement can still be measured, while the separate ESD contribution requires further substantiation. This distinction must be clear before the work begins.
How Are the Measurements Put on a Common Performance Basis?
An equivalent technical configuration provides a useful comparison basis only when the measured ship performance also relates to the same conditions. The reference and the version with an ESD must therefore be compared at corresponding speed through the water, draught, trim and relevant environmental conditions.
The speed–power relationship provides the starting point. Even a small speed difference can cause a noticeable difference in the power required by a displacement seagoing ship. By comparing suitable measurement points within a common speed range, it can be established which power demand corresponds to the same ship speed.
Differences in wind, waves, current and water depth must also be sufficiently well known. Their influence on the measurements can be corrected for insofar as the chosen method applies to those conditions. Large differences warrant caution because the necessary corrections themselves add uncertainty. For a coastal tanker, a different loading condition or trim may also change the result, even when the measured ship speed is the same.
The measuring instruments and correction methods used must be sufficiently comparable for the two situations. Particularly where the expected ESD saving is small, calibration differences or systematic measurement deviations may become important. It must also be clear which measurement runs were used and whether operating conditions during those runs were sufficiently stable.
The comparison basis must also specify which power is being assessed. Where a shaft generator is coupled to the system, the power at a measurement point in the drivetrain may differ from the power actually delivered to the propeller. The position of the power measurement and any coupled power take-offs must therefore be known.
Propeller rotational speed and torque need not remain identical with and without an ESD. A device can indeed change propeller operation, so that a different propulsion operating point arises at the same ship performance. The assessment must be able to include that difference without basing the comparison on unequal speeds or ship conditions.
The common conditions and the method for any corrections are preferably defined before the measurements. It thus remains clear what both speed–power relationships represent.
However, an accurate correction for weather conditions or vessel loading cannot replace a missing technical reference. If the propeller has also been changed or the hull cleaned alongside the ESD, separate substantiation remains necessary to determine which part of the improvement can be attributed to the device.
When Can Other Changes Distort the Measured ESD Contribution?
Not every change to the propulsion system is immediately visible during inspection. Particularly with a Controllable Pitch Propeller (CPP), changed settings can affect the measured shaft power.
Depending on the installation, a CPP can deliver the required propulsion with different combinations of blade pitch and rotational speed. If the combinator curve is adjusted at the same time as an ESD is installed, the efficiency of the existing propulsion system may also change. A before-and-after measurement then describes the combined result of both changes.
To determine the additional ESD contribution under the new control settings, it must be known how the propulsion system would have performed without the ESD but with those adjusted settings. Simply comparing the original settings without an ESD with the new settings including an ESD does not allow that distinction to be made.
A similar situation arises in the joint optimization of a propeller and Pre-Swirl Stator. The original propeller can be replaced by a geometry that better suits the current operational profile. The stator is then matched to the inflow of that new propeller. Both measures interact hydrodynamically, so the individual savings cannot simply be added together.
In such a case, three configurations are particularly informative: the original propulsion system, the new propeller without a stator and the new propeller with a stator. The difference between the latter two describes the stator's additional contribution to the new propeller configuration, provided the other underlying conditions are equivalent.
That is not necessarily the same improvement the stator would have delivered with the original propeller. After all, the geometry and inflow have changed. The contribution of an ESD is therefore tied to the configuration in which the device is used.
With a jointly optimized design, another distinction deserves attention. If the new propeller has been developed specifically to work with the stator, removing the stator from that same geometry does describe a technically comparable intermediate configuration. However, it does not prove how much improvement the stator would have delivered as a separate measure with the original propeller. Nor does it show, without further design study, which new propeller would have been optimal without a stator.
The choice of reference therefore determines not only the outcome but also the meaning of the reported effect. The comparison may concern the added device with a fixed propeller geometry, a jointly optimized propulsion system or the difference between two independently optimized alternatives. Those outcomes answer different technical questions.
Changes to the engine control or electrical power supply can also affect the assessment when fuel consumption is used as the performance metric. Lower fuel demand need not arise solely from lower hydrodynamic power demand. The change in shaft power must therefore remain identifiable alongside any improvements in engine operation and onboard power generation.
A combined retrofit need not be avoided to achieve a good performance improvement. However, the chosen comparison of configurations determines which contribution can be established separately. If a suitable intermediate configuration is missing, the total improvement may be demonstrable without the separate ESD saving having been measured directly.
When Can a Calculated Reference Supplement the Missing Measurement?
In many retrofit projects, it is not practically possible to test all relevant configurations separately at full scale. An ESD is installed during dry-docking and the ship then returns to service. The condition with the same state of maintenance but without the ESD may then never be measured directly.
Hydrodynamic calculations can help approximate this missing reference. With Computational Fluid Dynamics (CFD) or model tests, it can, for example, be investigated how much shaft power a ship would need after hull and propeller maintenance without the proposed ESD. That calculated result can be used to assess the performance measured after installation.
A hypothetical example illustrates the difference in strength of evidence. Suppose that, before dry-docking, a bulk carrier needs 4.000 kW of shaft power at a particular speed. After hull cleaning, propeller polishing and installation of a Pre-Swirl Stator, 3.850 kW is recorded under comparable conditions. The measured total improvement is then 150 kW.
To assess the stator's separate contribution, it is then calculated how much power the cleaned and restored configuration would require without the stator. Suppose that a hydrodynamic model developed for this purpose predicts 3.900 kW. Comparison with the measured 3.850 kW then indicates an additional improvement of approximately 50 kW.
However, that 50 kW has not been measured directly as a separate stator saving. The result is a model-assisted estimate. What matters here is not only the accuracy of the calculated reference but also the comparability of the calculation model and the actual measurement. After all, a systematic deviation in the model can explain part of the difference found.
The substantiation becomes stronger when the calculation model can be checked against reliable measurement data and the configurations with and without a stator are calculated using consistent underlying conditions. Even then, it must be investigated how reliable the predicted difference between the two configurations in particular is. Good agreement with the measured power in the original or final condition does not automatically prove that the separate stator contribution has been determined with the same accuracy.
All power values in this example are hypothetical. They are not research results from an existing ship. The example shows why a measured total improvement of 150 kW and an estimated ESD contribution of approximately 50 kW are different conclusions. The reliability of the estimated 50 kW cannot be established without an uncertainty assessment.
For a useful calculated reference, the geometry of the ship configuration under investigation must be known sufficiently accurately. The condition of the hull and propeller, relevant operating settings and the correct propulsion operating point must also be taken into account. A model matched to the situation before maintenance cannot be used to predict the clean condition after dry-docking without further assessment.
The suitability of the calculation method also depends on the device under investigation. A Pre-Duct influences, among other things, the inflow to the propeller, whereas a PBCF interacts directly with the flow around the blade roots and hub. A model that describes the original speed–power performance well does not necessarily predict all the effects of these different ESD types with the same accuracy.
Comparison with a sister ship can also provide supporting information. However, differences in construction, propeller geometry, maintenance condition and operating settings must then be sufficiently well known. Even among ships from the same series, that variation can be significant relative to a small expected ESD saving.
A calculated or alternative reference can substantially strengthen the technical assessment when no directly comparable measurement is available. However, the outcome remains dependent on assumptions, model quality and uncertainty. That distinction must remain visible in the reporting.
When May a Measured Performance Improvement Be Attributed to the ESD?
A performance improvement can be attributed to an ESD with sufficient confidence when the reference is suitable, the measurements describe equivalent ship performance and other relevant changes have been sufficiently excluded or accounted for separately. The accuracy of the established power difference must also be appropriate to the size of the expected saving.
Two kinds of certainty matter here. The first concerns the performance difference itself: how much less shaft power does the ship require under the chosen conditions? The second concerns attribution: which part of that difference has actually been caused by the ESD?
A measurement campaign can be convincing on the first point and fall short on the second. If a clear power reduction is measured after dry-docking, it may thereby be established that the ship has become more efficient. If several technical measures have been carried out at the same time, the separate ESD contribution may nevertheless remain unknown.
The reverse is also possible. A technical condition that is readily comparable before and after installation provides a strong basis for attribution. But if the predicted ESD saving is small relative to the measurement uncertainty, the difference may still not be established with sufficient reliability.
The uncertainty must therefore be assessed for the complete comparison. Certain measurement deviations may partly coincide when the same instruments are used, whereas other uncertainties have separate effects. The influence of applied corrections and any calculated references must also be taken into account. The accuracy of an individual power measurement does not automatically indicate how accurately the saving has been determined.
The technical reporting must reflect what the results actually demonstrate. If the ESD is the only relevant change and the power difference has been established with sufficient reliability, a separate ESD saving can be reported. If several measures have been carried out at the same time, possibly only the total performance improvement can be demonstrated directly. An additional calculation can estimate the ESD contribution, but that outcome must remain identifiable as a model-assisted result.
International measurement guidelines provide methods for conducting speed–power trials and performance monitoring consistently. However, the purpose and scope differ between guidelines. A method for correcting sea trial data does not automatically prove which technical change caused the measured performance difference. Nor are methods for long-term hull and propeller performance monitoring applicable to every propulsion configuration without further substantiation.
For a shipowner or technical manager, the result must therefore make clear which power has been compared, which reference has been used and which measures are included in the reported improvement. It must also be clear whether the separate ESD contribution has been measured directly, estimated using additional calculations or cannot reliably be distinguished from other changes.
The basis of comparison thus determines which conclusion may be drawn from a measured performance difference. A reliable speed–power comparison shows how propulsion performance has changed. To link that change to an Energy Saving Device, it must also be sufficiently clear how the same ship configuration would have performed without the device. If that reference is missing or influenced by other changes, a total improvement can still be valuable, but the separate ESD saving remains limited to what the available measurements and calculations actually substantiate.
This Article Within the Series
The basis of comparison must align with the assessment of overall propulsion performance, where the changed forces on the hull and rudder also count. If the available calculations or measurements provide insufficient certainty about ESD operation, the next question is which additional measurements are needed alongside CFD. This relationship between the reference, performance difference and technical substantiation is central to hydrodynamic design and performance validation of ESDs.