When Must an ESD Be Reassessed After a Change to the Propulsion Configuration?
An Energy Saving Device (ESD) must be technically reassessed if a propulsion modification could affect the inflow, propeller loading, geometric matching or structural loads on which the original design is based. A Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) device or Rudder Bulb is developed for a specific interaction with the propeller and surrounding ship structure. A new propeller, modified control of a controllable pitch propeller or changed rudder arrangement may alter that interaction even if the ESD itself is retained. The original energy saving may consequently no longer be representative. Sometimes the device remains technically suitable and only a targeted check is needed. With larger changes, new hydrodynamic calculations, structural assessments or modifications to the ESD may become necessary. The decisive question is whether the modified configuration still falls within the previously substantiated design and operating conditions. It must be established separately whether the device can continue to function safely and whether the expected hydrodynamic benefit remains sufficiently substantiated.
Which Changes Prompt a New ESD Assessment?
An existing ESD is matched to a particular propulsion configuration. Propeller geometry, flow around the stern and expected operating range together determine how the device functions. If one of these assumptions changes, it must be established whether the original technical assessment remains valid.
Propeller replacement is a clear reason for such a check. A different diameter, blade geometry, hub shape or pitch distribution may change both propeller inflow and flow behind the propeller. Even for a propeller with similar principal dimensions, differences in blade loading and hub geometry may be relevant to the performance of an existing ESD.
Other changes mainly influence the propeller operating point. Consider a modified combinator curve for a Controllable Pitch Propeller (CPP), a changed gear ratio or different propulsion control. The geometry may remain unchanged, but the propeller may operate under different conditions at the usual sailing speed.
With modifications to the shaft line, stern or rudder arrangement, the relative geometry may change. A larger propeller diameter may, for example, reduce the space available between the propeller and an existing Pre-Duct. A modified rudder may affect the connection and hydrodynamic performance of a Rudder Bulb.
Not every change justifies a new design study, however. If a damaged component is repaired in accordance with the original specifications, the existing technical substantiation may remain valid. It must, however, be sufficiently certain that the relevant geometry, strength and operating conditions have been preserved.
Nor do routine cleaning and propeller polishing automatically constitute a configuration change. This work may restore current propulsion performance, but, if carried out correctly, generally does not change the intended design geometry. The situation differs if blade geometry is modified during maintenance or components are replaced by a different design.
For a focused reassessment, the old and new configurations are therefore first compared side by side. This concerns not merely the names of the modified components, but their consequences for the propeller operating point, local flow, available clearances and structural loads.
The original design data provide the main reference. A change may remain within the previously investigated operating range even if a component is physically replaced. Conversely, a control modification without any visible structural change may cause the ESD to operate under different loads from then on.
If the new conditions demonstrably remain within the original design assessment, a limited check may be sufficient. Once relevant assumptions are no longer covered, further technical substantiation is needed before the existing ESD can simply be considered suitable.
How Does a Different Propeller Change the Performance of an Existing ESD?
A new propeller may load and utilize an existing ESD differently even if the device itself remains completely unchanged. Propeller geometry helps determine how much thrust is generated, how loading is distributed over the blades and what flow develops around the propeller hub and rudder.
For a Pre-Swirl Stator, the interaction between the pre-swirl generated and the propeller is particularly important. The stator changes the direction of water inflow. The original propeller is designed or assessed for a particular combination of that inflow, rotational speed and blade loading.
If the new propeller has a different pitch or blade-loading distribution, the most favourable pre-swirl may change. The existing stator may consequently no longer be optimal. At the same time, the device may still make a favourable contribution. Whether modified stator geometry actually provides greater savings must become apparent from a new configuration comparison.
The influence also works in both directions. Because of the changed propeller loading and pressure field, a different propeller may also influence inflow and hydrodynamic forces on the existing stator. Investigating only the performance of the new propeller without the existing stator is therefore insufficient.
A Pre-Duct influences flow towards the propeller and generates hydrodynamic forces of its own. For propeller replacement, both changed propeller operation and relative geometry are therefore relevant. A larger propeller diameter may reduce the space available, while different propeller loading may influence pressure distribution and forces on the duct profile.
For a PBCF, the relationship with the propeller is particularly direct. The fins form part of the rotating hub configuration and operate in the flow behind the propeller blade roots. A different hub diameter, changed blade count, different blade foot geometry or loading distribution may therefore affect hydrodynamic performance.
The mechanical connection must also be reassessed. If an existing PBCF is to be used on a new propeller, a suitable attachment alone is insufficient. The boss cap, fin position relative to the propeller blades, available space and mechanical loads must also be suitable. A PBCF is designed for the propeller geometry concerned and therefore cannot be transferred without a technical check. Depending on the design, a modified or entirely new hub device may be needed.
A Rudder Bulb is matched to the flow between propeller hub and rudder. With a different propeller, the hub contour, propeller slipstream and pressure distribution may change. The existing bulb may consequently make a different hydrodynamic contribution even if the rudder is not modified.
The assessment must therefore answer two questions. First, whether the existing ESD remains technically suitable for the new propeller. Second, whether the device still provides sufficient benefit within the modified configuration and whether further optimization is useful.
A more efficient new propeller may produce lower total power demand together with the existing ESD, while the separate contribution from the ESD becomes smaller. This is not a contradiction. Overall propulsion performance and the additional value of the device describe different comparisons.
When Do Changes in Rotational Speed, CPP Control or Engine Power Require Reassessment?
An ESD may begin to function differently without the propeller or device being physically modified. This occurs when the propulsion system is consistently operated at different operating points.
With a CPP, the same ship speed can be achieved with different combinations of blade pitch and rotational speed. If the combinator curve is changed, the loading distribution over the propeller blades may change, and with it the flow with which the ESD interacts.
For a Pre-Swirl Stator or Pre-Duct, it is particularly important whether the modified control causes different inflow and propeller operation. For a PBCF, changes in local hub flow and loads on the rotating structure also matter.
A modified gear ratio may have similar consequences if the propeller is consequently operated at a different rotational speed at the usual ship speed. The gearbox modification is not decisive in itself. What matters is which propeller conditions actually change.
For engine power, a distinction must be made between maximum available power and loading during normal use. Modified engine power limitation need not change ESD performance at an unchanged cruising-speed operating point. If, after the modification, the ship starts operating consistently at a different speed, rotational speed or propeller loading, the original ESD matching may become less representative.
A similar distinction applies to a shaft generator or an installation that extracts power from the shaft (Power Take-Off, PTO) or adds power to it (Power Take-In, PTI). A shaft generator changes the distribution of power supplied by the main engine between the propeller and electrical installation. A PTI may instead add power to the drivetrain. These changes do not automatically mean that hydrodynamic conditions around the propeller change.
If the propeller continues to operate with the same geometry, blade pitch and rotational speed under equivalent ship and environmental conditions, its hydrodynamic operating point may remain essentially unchanged. If the control strategy, rotational speed or usual propeller loading changes because of the new installation, an assessment must be made of whether the ESD needs to be rematched.
Drivetrain modifications may also require separate assessments of mechanical loads, shaft alignment and torsional vibration, even if the ESD remains hydrodynamically suitable. A favourable ESD assessment does not replace those checks.
The most important information is therefore not solely which new system is installed, but how the propeller is subsequently operated. The original ESD matching may remain valid if the relevant operating points are retained. If the installation moves outside the previously assessed operating range, it must be established what that means for hydrodynamic performance and technical load-bearing capacity.
How Do Rudder and Stern Modifications Affect Existing ESD Matching?
An ESD is developed for the flow and geometry around a particular propulsion arrangement. Changes outside the propeller may therefore also affect a device that is not itself modified.
If the stern shape changes, the velocity distribution ahead of the propeller may shift. A Pre-Swirl Stator or Pre-Duct may then receive different inflow from that for which its profile and position were originally optimized. Hydrodynamic forces on the device may also change.
This may occur, for example, if the stern is modified during an extensive retrofit to accommodate a larger propeller. The new geometry changes the available installation space and may also change the flow reaching the propeller and existing ESD.
For rudder modifications, the main influence generally lies behind the propeller. A Rudder Bulb is directly connected to the rudder geometry and is matched to the flow around the propeller hub. When the rudder is replaced, it must therefore be established whether the original bulb geometry still suits the new design.
If the propeller–rudder distance also changes, local flow and available clearances deserve attention. This is relevant not only to a Rudder Bulb, but also to a PBCF located directly behind the propeller hub.
A new rudder need not, however, automatically lead to redesign of a Pre-Swirl Stator. Its direct influence on flow ahead of the propeller may be more limited than that of a modification to the propeller itself. Their hydrodynamic interaction does not therefore disappear. Whether the original stator assessment remains valid depends on the actual consequences of the changed rudder arrangement.
In a configuration with several ESDs, these dependencies become more important. If a Pre-Swirl Stator and Rudder Bulb have been developed together, a new propeller or rudder arrangement may change the performance of both devices. Modifying only one of the two components separately does not then necessarily provide the most favourable overall propulsion performance.
Reassessment must therefore address the location where the modification has an effect. A changed stern requires particular attention to inflow towards the propeller. A new rudder configuration makes flow behind the propeller and geometric connection important. For a larger modification, it may be necessary to investigate the complete hull–propeller–rudder interaction again.
What Consequences Can Configuration Changes Have for Cavitation and Structural Safety?
An existing ESD may still make a favourable hydrodynamic contribution after a propulsion modification while technical loads or safety margins change. Reassessment must therefore not depend solely on the expected saving percentage.
Different propeller geometry or loading distribution may influence cavitation behaviour. For a Pre-Swirl Stator, changed pre-swirl combined with the new propeller may lead to different local blade loads. If the original cavitation assessment does not cover these conditions, further investigation may be needed.
A PBCF and Rudder Bulb may also respond differently to changed hub flow or propeller slipstream. Not every change leads to a cavitation problem. The question is whether the new configuration introduces relevant flow phenomena outside the existing technical substantiation. Fluctuating pressure loads, vibration and manoeuvring performance may also deserve attention.
The structural consequences must be considered separately. If the inflow to a Pre-Swirl Stator changes, the mean and fluctuating forces on the fins may change. This may affect the attachment and fatigue loading of the surrounding structure.
For a Pre-Duct, the forces on the profile, load transfer through the supports and available distance to the propeller deserve attention. A larger propeller diameter requires checks of the clearances. Relevant movements, deformations and installation tolerances must also be taken into account.
For a PBCF, a different rotational speed or modified hub structure may be a reason to reassess rotation-related loads and the attachment. For a Rudder Bulb, a different rudder arrangement may affect the forces transferred through the bulb to the rudder.
A distinction must be made here between two types of outcome. An ESD may continue functioning safely while its expected power saving becomes smaller. This primarily raises a question about the technical and economic value of further optimization.
If, by contrast, there is insufficient certainty about clearances, structural loads, cavitation or safe propeller and rudder operation, that technical suitability must be established before the modified configuration is put into normal service. A positive power prediction is insufficient for this.
An entirely new analysis using the finite element method (FEM) or cavitation test is not automatically needed. If the original calculations demonstrably cover the new load range, a targeted check may be sufficient, insofar as applicable requirements permit this. If that coverage is absent or critical safety margins are affected, the relevant assessment must be extended.
When Is a Limited Check Sufficient, and When Must the ESD Be Re-Optimized?
The depth of investigation is determined by the consequences of the modification and available technical substantiation. A new numerical flow study is not necessary for every change, but an unchanged ESD must not be assumed suitable for every new propulsion configuration without assessment either.
The first step is to establish which original design data remain valid. The existing ESD geometry, propeller data, relevant operating conditions, clearances and structural loads are compared with the new situation. It is also checked which safety and performance margins were originally investigated.
If a component is replaced by a technically equivalent design and relevant conditions are retained, a document check with targeted verification may be sufficient. The conclusion must then rest on demonstrable correspondence, not merely similar principal dimensions. Any formal approval of the replacement component remains a separate question.
A next step is targeted technical analysis. This may be appropriate, for example, when a new propeller has the same diameter but different pitch distribution and blade loading. The existing Pre-Swirl Stator need not then be completely redesigned before it is clear whether the changed propeller actually requires different inflow matching.
With suitable hydrodynamic calculations, an assessment can be made of whether the existing stator performs sufficiently favourably under the new operating conditions. Separate checks of cavitation, loads or clearances may also be needed if the modification affects them.
If this assessment shows that the ESD still functions safely and provides sufficient hydrodynamic benefit, retaining the original device may be a sound technical choice. The fact that further optimization is theoretically possible is not in itself a reason to modify the installation again.
More extensive redesign becomes an option if the original matching clearly no longer suits the new configuration, or if further optimization is expected to provide sufficient benefit to justify the work. This may apply, for example, to a new propeller diameter, substantially changed blade loading or a modified stern and rudder arrangement.
The final choice must not be based solely on the highest calculated saving. The structural modifications required, engineering, downtime and uncertainty of the expected improvement are also relevant.
An important outcome of reassessment may therefore be that the existing ESD is retained. It may also become apparent that a limited modification is useful or that entirely new ESD geometry becomes necessary. If technical risks prove insufficiently manageable, discontinuing use of the existing device may also be a necessary outcome.
For these decisions, there is no universal percentage change in rotational speed, propeller diameter or pitch at which complete re-optimization becomes mandatory. A small change may already be important where clearance is limited, while a larger change in another case still falls within the originally assessed range.
Reassessment must therefore clearly distinguish three outcomes: does the existing technical substantiation remain valid, are additional checks needed to use the modified configuration safely, and is re-optimization expected to provide sufficient additional benefit? Only the last question directly concerns the desirability of a new ESD design.
How Is ESD Performance Revalidated After a Modification?
After a relevant configuration change, it must be clear which performance is being reassessed. This is particularly important if several propulsion components have been modified simultaneously.
Suppose a shortsea vessel receives a new propeller better suited to its usual cruising speed. The existing Pre-Swirl Stator is retained. After the retrofit, the ship proves to require less shaft power under comparable conditions.
That measured improvement describes the change in the complete propulsion system. From this, no conclusion yet follows about how much the existing stator contributes in the new configuration or whether the original fin settings remain optimal.
For those latter questions, three configurations can be compared: the new propeller without a stator, the new propeller with the existing stator, and the new propeller with a potentially re-optimized stator. The difference between the first two provides insight into the additional contribution of the existing stator within the new propeller configuration. The third variant shows whether modifying the stator is expected to provide further benefit.
This comparison uses the same new propeller geometry as its starting point. It is not established in this way how a separately re-optimized propeller without a stator would perform. If that alternative is also investigated, the result describes a comparison between complete propulsion designs, rather than solely the additional contribution of the stator.
The ship conditions investigated and other design assumptions must be equivalent. If the propeller is re-optimized for each variant, a clear distinction must therefore be made as to whether the comparison concerns the stator itself or different integrated propulsion configurations.
Computational Fluid Dynamics (CFD) and, where needed, model tests can investigate these configurations. What additional validation is needed depends on the magnitude of the expected differences and reliability of the methods used.
Full-scale sea trial measurements can then confirm the performance of the modified ship. A comparison of conditions before and after the retrofit does not, however, automatically prove the separate stator contribution, particularly if propeller maintenance or other work has been carried out at the same time. A suitable reference without a stator remains necessary for that purpose.
The required performance assessment must therefore match the original question. A different investigation design may be sufficient to check whether the ship delivers the expected overall performance after retrofit than to establish the separate ESD contribution or benefit of new stator geometry.
Existing performance agreements also deserve attention. A saving expectation substantiated for the original propulsion system cannot be used as a guarantee for a modified configuration without a new assessment. What contractual consequences this has depends on the agreements made.
How Does Reassessment Become Part of the Modification Process and Technical Documentation?
An ESD must already be taken into account when preparing a relevant propulsion modification. If the consequences are investigated only during installation, necessary geometric modifications, calculations or approvals may delay the work.
For a planned propeller replacement, the engineering parties involved must therefore have the data for the existing ESD. These include the actual geometry and position, original design conditions and available hydrodynamic and structural calculations.
The new propeller data must also be available sufficiently early. Diameter, hub shape, blade geometry, relevant rotational speeds and expected loading distribution are particularly important in determining whether the existing device remains well matched.
The original ESD supplier can provide information on design matching and load limits. The propeller manufacturer and any other engineering parties must take the existing ESD into account. In this way, situations are prevented in which individual components are optimized without sufficiently assessing their interaction.
If a PBCF is fitted to a new propeller or the distance between an ESD and another component changes, structural suitability must also be established in good time. This applies, for example, to an existing PBCF that would be fitted to a new boss cap or a Pre-Duct whose distance to the propeller becomes smaller.
For a propeller retrofit, approvals and checks of the new propeller, attachment, shaft alignment and torsional vibration may also be relevant. Depending on the modification, inspections, manoeuvring trials and updated manoeuvring information may also be needed. These assessments concern the complete propulsion installation and are not equivalent to the hydrodynamic reassessment of the ESD.
Applicable class and flag-state requirements must therefore be established separately. A limited change in ESD matching need not require an extensive new structural assessment, while propeller replacement may require additional approvals for the drivetrain.
Statutory energy-efficiency documentation is another consideration. For an extensive alteration classed as a major conversion under applicable rules, recalculation and verification of the Energy Efficiency Existing Ship Index (EEXI) may be mandatory. A change to existing engine or shaft power limitation may also affect system documentation and approval.
For an Engine Power Limitation (EPL) or Shaft Power Limitation (ShaPoLi) system, it must therefore be checked whether the EEXI Technical File and associated documentation, including the Onboard Management Manual where applicable, need to be revised. The required procedure depends on the changed parameters and nature of the installation.
Not every propeller or ESD modification automatically creates a new EEXI certification obligation. A modification that improves energy efficiency without being classified as a major conversion may, under certain conditions, prompt a voluntary update. Conversely, a change in power limitation may have formal consequences while the ESD geometry itself remains suitable. Hydrodynamic, structural and statutory assessments must therefore be carried out separately and coordinated where needed.
After retrofit, the final geometry, modified operating settings and relevant investigation results must be recorded. If new calculations have been carried out, they must correspond to the configuration actually installed. Any revised inspection and maintenance instructions must also remain available. This prevents situations in which, during a subsequent modification, it is unclear which propeller, rudder arrangement or control the existing ESD was originally matched to.
An Energy Saving Device must be reassessed after a change to the propulsion configuration as soon as the validity of the original ESD design assumptions is insufficiently certain. Assessment begins by comparing the old and new geometry, operating conditions and loads. If these demonstrably remain within the existing design range, a limited technical check may be sufficient. With changed inflow, propeller loading or structural boundary conditions, targeted engineering may become necessary. Re-optimization becomes relevant only if the existing ESD is no longer technically suitable or further modification offers a sufficiently substantiated benefit. Assessment thus remains proportionate to the actual modification, and situations are prevented in which a useful device is unnecessarily replaced or outdated design matching is retained without checks.
This Article in the Series
An existing ESD may be readily inspectable and maintainable while propulsion modifications still require a new assessment of hydrodynamic matching. Maintenance and accessibility remain important to technical reliability during operation. For ships to which the relevant rules apply, the possible contribution of an ESD to EEXI and CII also deserves attention, with technical energy efficiency and operational CO2 intensity assessed separately. These issues come together within retrofit, service life and regulations for ESDs.