When Does a Feasibility Analysis Provide a Sufficient Basis for Investing in an Energy Saving Device (ESD)?
A feasibility analysis provides a sufficient basis for investing in an Energy Saving Device (ESD) when the expected saving, technical feasibility and financial return are sufficiently substantiated to assess the main investment risks. For a Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) or Rudder Bulb, it must be clear which propulsion configuration is being investigated, how much less shaft power it is expected to require and whether the device can be installed within the existing ship structure. The analysis must also reflect the future operational profile and the shipowner’s economic assumptions. A favourable flow calculation or indicative fuel saving is not automatically sufficient. The reliability of the performance prediction, the modifications needed and the consequences of remaining uncertainties are also decisive. Not all detailed engineering needs to have been completed. It must, however, be clear which conditions still have to be met and whether a disappointing outcome could materially change the investment. A good feasibility analysis thus distinguishes between a promising design, a sound investment decision and the technical release that may still be needed for production or installation.
When Is an Initial ESD Screening Still an Insufficient Basis for Investment?
An initial ESD screening helps to determine whether a ship offers enough potential for hydrodynamic improvement to justify further investigation. The existing propulsion configuration, operational profile and possible losses around the propeller are assessed for this purpose. On that basis, it may, for example, become apparent that a Pre-Swirl Stator or Pre-Duct is worth investigating further.
Such an outcome does not yet mean that the proposed design is economically feasible. A device may improve the inflow to the propeller while creating hydrodynamic forces of its own that reduce part of the benefit. The available installation space and structural possibilities may also constrain the feasible geometry.
A screening also often uses data that have not yet been fully checked. The original ship drawings may differ from the current arrangement, and the available operational profile need not match future use. These uncertainties are not always a problem when selecting promising solutions, but may become important once a concrete investment is being considered.
The next phase of investigation must therefore concern a sufficiently defined technical configuration. It makes a difference whether a Pre-Swirl Stator is added to the existing propeller or developed together with a new propeller. The two situations may produce different hydrodynamic improvements and project scopes.
Which alternatives actually compete with one another must also be investigated. A new propeller without an ESD may, for example, already deliver a substantial improvement. If the ESD is considered only as an addition to that new propeller, the additional investment must be compared with the additional saving from the ESD, rather than with the total improvement from the combined package. Maintenance or reoptimization without an ESD may also be a relevant alternative.
The planned implementation must be sufficiently clear. If an ESD is installed during a planned docking, the available engineering time, necessary structural work and expected lead times are important. A technically promising device that cannot be developed or approved in time may not be a feasible solution for the intended project date.
The distinction between screening and feasibility analysis for investment therefore lies in the decision the results can support. A screening may be enough to start further investigation. For an investment decision, it must be more clearly established which arrangement is proposed, what the expected saving rests on and whether the necessary work fits within a realistic project scope.
What Ship Data and Reference Must the Feasibility Analysis Record?
A reliable feasibility analysis starts with a sufficiently accurate description of the ship to which the ESD is applied. The stern shape, propeller geometry, hub structure and rudder arrangement affect the flow in which the device must operate. The available space for attachments and the position of existing appendages may also be decisive.
For a retrofit, original drawings are not always sufficient. Previous propeller replacements, rudder modifications or deviations in the as-built geometry may have changed the current configuration. If such differences matter for ESD matching, they must be established through suitable inspections or geometric measurements.
Not every ship component needs to be measured with the same accuracy. The required level of detail depends on the design. For a PBCF, the boss cap, blade-root geometry and attachment may be important. For a Pre-Duct, the stern shape, position ahead of the propeller and available clearances may instead be decisive.
Alongside geometry, the expected operational profile must be known in sufficient detail. The main speeds, draughts, trim conditions and propeller loadings determine how much of the predicted improvement can be used during operation. Historical operational data provide a starting point, but must be assessed against any changes in routes, cargoes or usual sailing speed.
The condition against which the new ESD is compared must also be clear. For a stand-alone retrofit, the existing propulsion arrangement may provide a suitable reference if the rest of the technical condition remains sufficiently equivalent. If the propeller is replaced or extensive hull maintenance is carried out during the same docking, a different comparison arises.
A measured power reduction between the conditions before and after docking may then have been partly caused by maintenance or propeller optimization. For the individual ESD contribution, it must be known how the intended final configuration would perform without the device. If that condition cannot be measured directly, a substantiated calculated reference may be needed.
For the feasibility analysis, performance must also be assessed under sufficiently comparable ship and environmental conditions. The relevant quantity is the net power demand of the complete propulsion arrangement for the same relevant ship performance, rather than solely a more favourable flow or propeller efficiency.
The analysis must therefore describe both the existing ship configuration and the proposed condition after installation with sufficient accuracy. This includes a clear definition of the performance being investigated: the individual ESD contribution or the improvement from a larger propulsion package.
Missing data are particularly problematic if they may change a material design decision. An uncertain attachment distance that can be established through a targeted inspection need not halt the investigation. If it is still unclear which propeller will be installed, however, it may be too early to define the final ESD geometry and expected saving.
When Is the Predicted Hydrodynamic Improvement Sufficiently Substantiated?
The hydrodynamic assessment must demonstrate that the proposed ESD is expected to deliver a net improvement in propulsion performance under the relevant operating conditions. For this purpose, not only the local flow around the device but also the required shaft power of the complete configuration is investigated.
Computational Fluid Dynamics (CFD) can compare the performance of a design with and without an ESD. The reliability of the result depends on the geometry, propeller modelling, numerical accuracy and suitability of the method for the flow phenomena being investigated.
A numerically well-converged calculation is not yet proof that the predicted improvement will be achieved exactly at full scale. The CFD method used must be sufficiently substantiated for the application concerned. Previous model tests, experimental data or comparisons with sufficiently similar configurations may help to assess prediction quality.
Not every ESD automatically requires new model tests. If the method used has been validated within a sufficiently comparable application range and the remaining uncertainty is acceptable for the investment decision, a ship-specific calculation may provide sufficient substantiation. For a different geometry or flow that is difficult to predict, additional investigation may be needed.
For the feasibility analysis, the reliability of the difference between the configurations with and without an ESD is particularly important. A model may predict total power demand reasonably well while a small additional ESD saving is not distinguished accurately enough. That uncertainty must remain identifiable in the final performance expectation.
The spread between different calculation methods also warrants attention. Original comparative research into the same pre-swirl device showed varying predictions, including both a reduction and an increase in required power demand. This does not imply that every CFD calculation is unreliable. It does show that the choice of propeller modelling and the description of the wake field can affect the predicted ESD effect.
Such a spread also does not automatically constitute a statistical confidence interval. If the investment decision depends on a small predicted benefit, the uncertainties actually investigated and the possible model deviations still outside the assessment must be clear.
The prediction must also reflect future use. A favourable saving at the original design speed has little economic significance if the ship hardly ever operates there any more. The relevant power differences must therefore be linked to the operating hours in the main speed and loading conditions. Operating points with little or no ESD benefit must also count.
For a financial assessment, the predicted power saving must then be translated into fuel consumption. The specific fuel consumption of the engines involved, any drivetrain losses and the energy distribution on board are among the relevant factors. A percentage saving in shaft power is not automatically an equally large fuel saving.
The outcome need not be an exact guaranteed saving percentage. It must, however, be sufficiently plausible that the predicted improvement is technically achievable and that possible deviations do not undermine the economic assessment unnoticed.
If the expected saving is small relative to the uncertainty, or financial feasibility depends strongly on a favourable CFD result, additional validation may be decisive. If the investment remains sufficiently attractive under credible less favourable performance assumptions, the existing technical substantiation may more readily suffice. The required depth of investigation is thus determined not by a fixed number of calculations or model tests, but by the strength of evidence needed for the specific investment decision.
Which Structural and Operational Constraints Must Have Been Investigated in Advance?
A hydrodynamically favourable design does not yet constitute a feasible retrofit project. The feasibility analysis must therefore also investigate whether the device can be attached safely and operate within the available ship geometry.
For a Pre-Swirl Stator, the attachment of the fins and load transfer to the stern are particularly important. A Pre-Duct requires sufficient space and a suitable support structure. For a PBCF, the mechanical connection to the propeller hub and the suitability of the rotating structure must be assessed. For a Rudder Bulb, the connection to the rudder and the available space between the propeller hub and rudder are relevant.
Not every structural detail needs to have been finalized during the feasibility phase. The main technical constraints must, however, be sufficiently known to assess whether a safe and practically feasible solution is possible.
This applies, for example, to the available clearances. If a Pre-Duct is positioned close to the propeller, it must already be plausible before the investment decision that sufficient space will remain for the relevant movements, deformations and installation tolerances. A material problem with the available space may lead to a different ESD geometry or even a different device.
Structural loads also warrant attention. If a stator attachment must withstand large or fluctuating hydrodynamic forces, a strength or fatigue assessment may be needed. A detailed analysis using the Finite Element Method (FEM) does not always need to be ready during the feasibility phase, but the expected load transfer and necessary structural modifications must be sufficiently defined.
The same applies to cavitation, vibration and manoeuvrability. If the proposed ESD introduces a relevant technical risk, it must be clear how that risk is investigated and whether an acceptable solution is plausible. A positive fuel saving must not be used to justify uncertainty about safe operation.
The implementation schedule must also be realistic. Available docking time, possible steelwork, production preparation and required inspections help to determine whether the project can take place during the intended maintenance window. Work affecting the propeller, shaftline or rudder structure in particular may have implications for other engineering and approval activities.
The applicable class and flag-state requirements must be identified in good time. The approvals and tests needed depend on the device and the extent of the modification. ESD installations do not all require the same calculations or formal procedure.
An investment decision may be made on condition that certain detailed calculations or approvals follow later. It must then be clear which outcomes are required, who is responsible for them and what happens if the conditions are not met.
If, however, it is still uncertain whether the proposed device can be installed safely, additional investigation is needed first. The feasibility analysis provides a sufficient basis only when the main structural and practical risks have been defined and a feasible solution is plausible.
How Is the Technical Saving Assessed Financially and Contractually?
A technically feasible ESD becomes an investment opportunity only when the expected saving produces sufficient economic value. The predicted reduction in shaft power must be translated into an annual fuel saving that suits the propulsion plant and operational profile.
This conversion requires attention to the engine’s specific fuel consumption and any coupled energy consumers. A reduction in shaft power by a given percentage does not automatically mean an equally large percentage reduction in fuel demand.
The financial assessment must then reflect the chosen technical configuration. If a Pre-Swirl Stator is developed together with a new propeller, the full saving from the combined design must not be regarded as the stator’s individual return without further substantiation.
The same applies to costs. Only expenditure belonging to the investment being investigated provides an appropriate basis for comparison. For installation during an already planned docking, the additional work and any extra downtime are particularly relevant. If a complete retrofit package is being assessed, the costs and savings must relate to that same package.
For a feasibility analysis supporting an investment decision, the main cost items must have been established in sufficiently concrete terms. This may involve information from suppliers, the shipyard and the engineering parties involved. The analysis must distinguish between confirmed quotations, budgets and items for which only a preliminary estimate is available. Relevant exclusions and work not yet included in the available prices must also be visible.
A saving prediction accompanied by major structural work whose scope is still entirely unknown does not provide a reliable financial basis. Conversely, an amount need not already be contractually fixed if the remaining cost uncertainty is sufficiently defined and the investment remains attractive at a credibly higher figure. Expected maintenance costs, possible additional docking time and the remaining operating period must also be appropriately included.
When a ship is chartered, it also matters who receives the saving. An owner may finance the ESD while a charterer bears the fuel costs during the charter period. The financial return on the investment then depends on the arrangements made or still to be made. The analysis must therefore distinguish between the economic value for the ship and the net cash flows for the investor. A technically attractive measure is not automatically financially attractive to the party paying for the retrofit. Any value from avoided emission costs may also be included only if it is clear for whom that value actually arises.
The simple payback period may be used for an initial assessment, provided the annual net saving is positive. If future cash flows vary substantially or the remaining operating period is important, a net present value calculation may provide more insight. The choice depends on the investment criteria of the party concerned.
A feasibility analysis is sufficiently substantiated financially when the relevant costs and returns have been compared on the same basis and it is clear whether the investment meets the chosen financial objectives. Project-specific data are needed for this; general ESD price indications do not add reliable decision-making information to that assessment.
Which Uncertainties May Remain Open at the Investment Decision?
For an ESD retrofit, certain uncertainties can be fully resolved only after installation. The actual full-scale saving, for example, cannot always be measured directly in advance. Future fuel prices and the operational profile may also change. This does not mean that an investment is justifiable only when every uncertainty has disappeared. What matters more is whether the remaining uncertainties can materially change technical feasibility or the economic outcome.
A distinction must be made between different types of outstanding issues. Uncertainty about the exact annual fuel saving may be financially acceptable if a technically substantiated less favourable scenario still provides a sufficient return. The absence of a final production drawing need not be a problem either, if structural feasibility is already sufficiently plausible and the remaining engineering is clearly defined.
The situation is different for an unknown technical constraint that may lead to a different ESD choice. If, for example, it has not yet been established whether a duct can be installed within the available propeller space, the entire design configuration may change. Such uncertainty warrants investigation before irreversible project commitments are made.
Uncertainty about structural safety also cannot be accepted solely as a financial risk. If the load-bearing capacity of a critical attachment is insufficiently known, it must be established which additional assessment is needed and when safe implementation can be demonstrated.
A conditional investment decision may be appropriate if the remaining issues are defined and the project can still be modified or stopped before irreversible commitments arise. The technical and commercial conditions that must first be met must be clear. Conditional approval must not be interpreted as permission to begin production or installation without the required checks.
Financial uncertainty requires a comparable definition of its limits. If feasibility holds up only with the highest predicted ESD saving and the lowest possible installation costs, the investment basis is vulnerable. A credible less favourable scenario helps to show the margin available for disappointing performance or costs.
Such a scenario is not automatically a statistical lower bound. Without reliable data on the probability of different outcomes, the likelihood of a particular payback period cannot be calculated. The feasibility analysis must keep that limitation clear.
For the investment decision, a distinction is therefore needed between risks that have already been sufficiently investigated, items that can be completed during detailed engineering and conditions that must first be met before further commitments are justified. For a conditional decision, these points must be recorded in concrete terms. It must be clear what investigation is still needed, which outcome is acceptable and who decides whether the next project phase may begin.
Additional investigation is especially worthwhile when its outcome can actually affect the decision. Further CFD refinement may, for example, add little value if the dominant uncertainty lies in the future operational profile. Conversely, a targeted geometric measurement may be decisive if the device’s feasibility has not yet been established.
The feasibility analysis is sufficient in this respect when the main remaining uncertainties are identifiable, their possible consequences have been assessed and it is clear which conditions must be satisfied for the next project step. A conditional decision is then possible, but must not be confused with final technical approval.
What Agreements and Documentation Are Needed for a Feasible Retrofit?
A positive feasibility analysis must be capable of being translated into a concrete engineering and installation project. More is needed than an expected fuel saving and a suitable ESD geometry. Which party is responsible for the data, work and checks still to follow must also be clear.
For a retrofit, the ESD supplier, propeller manufacturer, shipyard and specialist engineering parties may work closely together. The shipowner or technical manager must be able to establish who handles the hydrodynamic matching, who develops the structural integration and who prepares the necessary approvals and inspections.
That allocation of tasks is particularly important if an ESD becomes part of a larger propulsion package. A new propeller may, for example, perform excellently within an individual design study, but must also match the existing or newly designed Pre-Swirl Stator. Without clear responsibility for this mutual matching, a technically promising retrofit may still be developed incorrectly.
The feasibility analysis must therefore contain a sufficiently concrete description of the chosen configuration and the assumptions that must be retained during further engineering. These include relevant ship data, the intended ESD geometry, the operational range and any structural constraints.
The necessary approvals must also have been identified. This does not mean that all certificates must already be available, but that it is known which assessments will be required under the applicable rules and whether the project schedule takes them into account.
Performance assessment also warrants attention. If a saving must be demonstrated after installation, it must be clear in advance which quantity is being compared, how the reference without an ESD is established and under what conditions measurements take place.
This is particularly important if hull cleaning or propeller maintenance is also carried out during the same docking. Without an appropriate reference, the measured difference may have several causes. An agreement on the individual ESD contribution must therefore be consistent with a measurement or calculation method that can distinguish this contribution sufficiently.
Any performance guarantees must be recorded separately. A technical forecast does not automatically constitute a contractual guarantee. The consequences of a different result must also be consistent with the arrangements made between the parties involved.
For a time charter, an agreement on the allocation of costs, downtime and fuel benefits may also be needed. A specific contractual model addendum from 2026 is available for ESD retrofits, through which parties can agree on project implementation and a separate performance protocol. This model does not replace the technical guarantee or supply arrangements with the ESD supplier.
A feasibility report need not develop these agreements in full. It must, however, be apparent which agreements are still needed and whether their absence can block the investment. Responsibility for missing data, any additional engineering and release of the next project phase must also be clear. A transferable basis for the next project phase thus emerges, without material technical or commercial responsibilities remaining unclear.
When Is the Feasibility Analysis Sufficient for a Go/No-Go Decision?
A feasibility analysis provides a sufficient basis for a go/no-go decision when the main technical and financial questions have been answered and the remaining conditions that may still affect further implementation are clear.
The analysis must first describe a concrete ESD configuration. The relevant ship data, future operational profile and reference without an ESD must be sufficiently known to assess the predicted net power improvement. How that improvement is translated into an annual fuel saving must also be clear.
Technical feasibility must then be plausible. There must be no unknown constraints relating to the available space, structural loads, attachment possibilities or main operational risks that could materially change the chosen solution. Where final calculations and approvals are still to follow, the conditions for them must be identifiable.
The financial assessment must reflect the same technical configuration. The additional investment, expected net return and remaining operating period must be sufficiently concrete to determine whether the retrofit meets the chosen investment criteria. When a ship is chartered, this also includes the allocation of financial benefits.
The comparison supporting the decision must be established. Does it concern a stand-alone ESD on an existing propeller, the additional value of an ESD with a new propeller, or a jointly optimized retrofit package? Costs and savings must relate to the same alternative. Only then can the investor assess whether the chosen arrangement adds sufficient value.
The substantiation must also show how sensitive the outcome is to relevant uncertainties. If a small shortfall in ESD performance or a small adverse change in installation costs already makes the investment unattractive, that is a material consideration for the go/no-go decision. A favourable base scenario alone is then insufficient.
If these elements are sufficiently substantiated and no material uncertainties remain that could reverse the decision without further control measures, a positive investment decision can be made. This does not automatically mean that the device may immediately be produced or installed. Any required detailed calculations, design approvals and implementation checks remain part of the technical release.
A conditionally positive decision is appropriate when the project's feasibility has been demonstrated in its main aspects, but certain clearly defined items still need to be completed. Consider a final check of an attachment structure, confirmation of a critical installation dimension or completion of a necessary approval procedure. It must be clear which outcome is required and which financial or technical commitments may be made only after that confirmation.
If an outstanding issue can still determine whether the ESD is technically feasible or the investment produces sufficient economic benefit, an unconditional investment decision is premature. The analysis may then be sufficient to authorize targeted follow-up investigation. That is a different decision from placing a final order for the device.
A negative decision may also be a valuable outcome. If suitable installation proves infeasible, the expected net saving is insufficient or another improvement clearly suits the ship better, deciding against the ESD retrofit may be the most responsible choice.
The quality of a feasibility analysis is therefore not evident from the number of calculations performed or the length of the report. What matters is whether the substantiation makes the important design choices, benefits, constraints and risks sufficiently clear to justify a decision.
A feasibility analysis provides a sufficient basis for investment when a shipowner can establish from the available data that the proposed ESD is technically feasible, can deliver sufficient net benefit under representative conditions and can be implemented through a realistic project approach. Certain details may still remain open, provided their significance is known and the necessary checks are completed before the next irreversible project step. The analysis thus forms a useful transition from a promising hydrodynamic solution to a sound investment and implementation process.
This Article within the Series
The amount of additional substantiation needed depends in part on the effect of uncertain ESD savings on the investment decision. Even when an ESD proves technically and financially feasible, the question remains how the investment compares with propeller or rudder optimization. Both considerations form part of the economic assessment of ESD investments.