How Do You Compare Different ESD Designs Under Representative Operating Conditions?
Different designs for Energy Saving Devices (ESDs) are compared by establishing how much shaft power is saved with each design at the same ship performance and what this delivers across the expected operational profile. A Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) or Rudder Bulb may perform favourably at a particular speed and loading condition, while another device offers a greater benefit under different conditions. Geometric variants of the same ESD type may therefore also rank differently. A reliable comparison begins with a common reference and investigates the designs at representative operating points using hydrodynamic methods of sufficient accuracy. The power differences are then linked to the corresponding operating hours. This reveals which version is expected to save the most propulsion energy, how sensitive the ranking is to changes in the operational profile and whether the performance difference is sufficiently reliable to justify a technical preference.
Which Operating Points Are Representative for a Fair Design Comparison?
A design comparison begins by establishing the conditions in which the ship actually uses propulsion energy. The principal ship speeds, loading conditions and propeller loads are identified for this purpose. For a Controllable Pitch Propeller (CPP), the combinations of blade pitch and rotational speed used are also relevant. The aim is to select a set of operating points that represents operation with sufficient accuracy, without having to perform calculations for every individual voyage.
The distribution of energy use guides this selection. A ship may operate for many hours with relatively low power demand, while shorter periods at higher load also account for a significant share of propulsion energy. Both operating time and the shaft power required are therefore important. An operating point that occurs infrequently and uses little energy need not receive the same attention in assessing annual savings as a condition in which the ship operates for long periods or with high power demand.
Operating points may also be important because they reveal differences between designs. Two ESD geometries may perform almost equally at the nominal cruising speed but produce different results at lower propeller loading. In that case, an additional calculation may be needed to understand the behaviour of both geometries better, even when the operating point concerned represents only a limited part of the operational profile.
This distinction matters, for example, for a coastal tanker that alternates between laden and ballast operation. Draught and propeller loading change between the two states, so the flow around the stern and the operation of an ESD may also differ. If the ship also operates at varying speeds, a separate assessment of several operating points for each loading condition may be needed. This reveals whether a design provides a benefit mainly during laden voyages or also performs favourably at lower load.
The selection must continue to distinguish between representative operating conditions and additional points investigated. An extra calculation may be valuable for establishing a design difference, but does not automatically carry more weight in the expected annual energy saving as a result. That contribution is determined by the actual or expected operating hours in the condition concerned.
The quality of the operational profile itself also warrants attention. Historical operating data provide a useful starting point where future deployment remains sufficiently comparable. Changes in operating area, ship speed or the ratio of laden to ballast voyages may make other operating points important. It must also be clear whether the performance investigated relates to calm-water conditions or whether relevant wind and wave conditions have been included. A saving predicted for calm water need not be maintained to the same extent in a seaway.
Operating conditions with similar hydrodynamic behaviour can be grouped together under suitable conditions. Where performance changes substantially within a particular speed or load range, a finer subdivision may instead be needed. The number of calculations therefore follows from the operational profile and the sensitivity of the designs, rather than from a predetermined number of operating points.
Alongside this comparison of energy performance, checks are needed for conditions in which an ESD must remain technically functional. During manoeuvring or at different propeller loads, a device need not provide the same efficiency improvement as during normal ahead operation. The structure and propulsion system must, however, continue to function safely and reliably under those conditions. Such checks can determine the suitability of a design without carrying substantial weight in the annual energy calculation.
What Must Remain the Same for All ESD Designs?
A fair comparison begins with a common reference without the ESD now being investigated. The shaft power required by this initial configuration is recorded for each representative operating point. The same ship speed through the water, draught, trim and relevant environmental conditions are maintained. The assumptions concerning hull condition and the existing propulsion arrangement must also match.
This does not mean that the propeller must operate at the same rotational speed or deliver the same torque or thrust for every design. An ESD can change propeller inflow and generate hydrodynamic forces of its own. The forces on the hull and rudder may also change as a result. A different balance between propeller thrust and the other forces can therefore arise at the same ship speed. The comparison must establish how much shaft power the complete configuration needs to deliver the required ship performance.
For a CPP, the control strategy warrants particular attention. If the combination of blade pitch and rotational speed is reoptimized for one design while the original settings are retained for another, the comparison is not equivalent. The permissible settings and freedom to optimize must be specified in advance. All candidates are then assessed on the same basis. If the reference configuration may also be reoptimized, the same scope for assessment must apply to it.
The scope of the modifications being investigated must also be clear. When two separate ESDs are investigated on the same existing propeller, the hydrodynamic contributions of both ESDs can be compared under corresponding conditions. A Pre-Swirl Stator optimized together with a new propeller, by contrast, constitutes a different propulsion configuration. The overall improvement may be technically attractive, but must not be attributed entirely to the stator without further analysis.
Both approaches are possible, provided what is being compared is specified in advance. For separate devices, the comparison concerns the contribution of each device to the same existing propulsion arrangement. For complete propulsion configurations, it concerns the performance of the full design variants, including any changes to the propeller or control.
The investigation methods used must also be appropriate to the differences being examined. A simplified propeller representation may, for example, be useful for an initial assessment of a Pre-Duct, whereas a PBCF may require a more detailed description of the flow around the blade roots and hub. Results from different models do not automatically have the same accuracy.
For the final ranking, it must therefore be clear whether the chosen methods predict the relevant performance with sufficiently comparable reliability. Particularly where two designs differ only slightly, additional assessment using more accurate calculations or model tests may be needed. Without checks, a design analysed with a detailed propeller model cannot be ranked against another candidate for which a simpler model was used. Otherwise, there is a risk that the investigation method influences the ranking more strongly than the hydrodynamic properties of the designs.
What Performance Is Compared at Each Operating Point?
For each ESD design, the shaft power required by the ship under the chosen conditions is established. The difference from the common reference constitutes the power saving at that operating point. This can be expressed in kilowatts and as a percentage of the original shaft power.
The two values have different meanings. The percentage describes the relative improvement, while the saving in kilowatts indicates the actual reduction in power. A high percentage saving at low power demand may therefore contribute less to the total energy saving than a smaller percentage improvement at a more heavily loaded operating point.
Adverse performance must also remain visible. Under certain conditions, an ESD may require more shaft power than the reference without the device. Such a loss must not be omitted because the same design performs favourably at other operating points. It reduces the total energy saving and may affect the final ranking.
Alongside shaft power, propeller rotational speed and torque are recorded, supplemented by blade pitch where a CPP is used. Hydrodynamic calculations can also provide insight into the forces on the ESD, hull and rudder. These data help explain why two designs produce different power demands at the same ship speed.
A Pre-Duct can, for example, improve propeller inflow while causing hydrodynamic losses of its own. A Pre-Swirl Stator can allow the propeller to operate more efficiently while the stator fins generate their own forces. Behind the propeller, a PBCF and Rudder Bulb can change the hub flow and the forces on the rudder. A favourable local change in flow therefore need not improve the complete propulsion system.
The ranking depends on the shaft power required by the complete configuration under investigation. The flow and force data explain the differences and can guide further optimization. The absolute power difference then forms the basis for calculating the energy saving across the operational profile.
How Does the Energy Saving Across the Operational Profile Determine the Preferred Option?
The power saving at each operating point becomes meaningful for operation when it is linked to the corresponding operating hours. The reduction in shaft power during those hours represents the propulsion energy saved. Adding the contributions from all representative operating points provides a comparison of the expected energy saving over the period investigated.
Take, as a hypothetical example, a coastal tanker that operates laden for 3.000 hours annually with an original shaft power demand of 2.500 kW. The ship also operates in ballast for 1.000 hours, requiring 1.200 kW. Two ESD designs are investigated for both conditions, with the same initial conditions.
Design A reduces power demand by 4% during laden operation and by 1% during ballast operation. This corresponds to 100 and 12 kW respectively. Over the stated operating hours, the annual saving amounts to 312.000 kWh of propulsion energy.
Design B provides an improvement of 2% during laden operation and 6% during ballast operation. This represents 50 and 72 kW respectively. Over the same 4.000 sailing hours, the saving amounts to 222.000 kWh.
Although design B achieves the highest individual percentage saving during ballast operation, design A delivers an additional energy saving of 90.000 kWh across this operational profile. The greater power reduction from design A coincides with the long periods of laden operation, during which the ship also needs more power.
The original propulsion energy use in this example is 8.700.000 kWh per year. Design A saves approximately 3,59% of this and design B approximately 2,55%. These percentages describe the energy saving across the full assumed operational profile and therefore differ from the individual percentages for laden and ballast operation.
All values are purely illustrative. They are not research results or measured performance figures for existing ESDs. The example also assumes that power demand and ESD savings within each loading condition remain representative of the corresponding operating hours. It includes no additional differences in speed, weather conditions or other operating conditions.
For an actual ship, a finer subdivision may be needed. Where speed and propeller loading vary considerably during laden voyages, the relevant power savings are determined separately. Any performance losses must also count. The total saving follows from the absolute power differences over the corresponding operating hours, rather than from simply averaging the individual percentage savings.
Further conversion into fuel savings requires information about the propulsion engine and any coupled systems. The change in engine load may affect specific fuel consumption. Where a shaft generator is fitted, the electrical energy balance may also change. The reduction in shaft energy therefore remains a separate technical result alongside any assessment of fuel consumption and emissions reduction.
When Does a Different Operational Profile Change the Ranking?
A design that saves the most energy under the current operational profile need not remain the preferred option when the ship's deployment changes. In particular, a different distribution of operating hours can reverse the ranking when two ESDs provide their greatest improvements under different conditions.
In the hypothetical example, design A saves 100 kW during laden operation, compared with 50 kW for design B. In ballast, design B is more favourable, at 72 kW compared with 12 kW for design A.
Suppose the coastal tanker still operates for 4.000 hours annually, but the distribution changes to 2.000 hours of laden operation and 2.000 hours of ballast operation. If the original power demands and ESD performance in each condition remain the same, design A then saves 224.000 kWh per year. Design B reaches 244.000 kWh.
The preference is therefore reversed. With 3.000 laden hours and 1.000 ballast hours, design A provides the greatest saving. With an equal distribution, design B instead saves 20.000 kWh more.
The reversal occurs because design A saves 50 kW more than design B during laden operation, but 60 kW less during ballast operation. Within this example, both designs provide the same annual saving when the number of laden sailing hours is 1,2 times the number of ballast hours. For a total of 4.000 sailing hours, this threshold is approximately 2.182 laden hours and 1.818 ballast hours, or around 54,5% laden operating hours.
Above this ratio, design A provides a greater energy saving; below it, design B does. The ratio of 1,2 applies to the assumed power savings in each condition; only the stated numbers of hours are based on a total of 4.000 sailing hours. If speed, propeller loading or other conditions change, the performance difference between the designs may also shift. The calculated threshold for the change in preference is therefore neither a universal decision point nor an exact prediction of future operation.
A sensitivity analysis shows how stable the ranking remains under plausible changes in the operational profile. For a coastal tanker, a different ratio of laden to ballast voyages may be relevant. For a container ship, a lasting change in the speed range or route may be more important. Extrapolation of performance data to operating points that have not been investigated must be avoided unless adequately substantiated.
When the same design remains the preferred option across different realistic operational profiles, the outcome is less sensitive to changes in operation. If the expected deployment is close to the threshold for a change in preference, the choice is more vulnerable to a shift in operating hours. The comparison must therefore indicate not only which design performs better under the current assumptions, but also under which conditions that preference changes.
When Is the Performance Difference Large Enough to Justify a Technical Preference?
A calculated ranking is not automatically a reliable ranking. Two ESD designs may differ only slightly in predicted energy saving, while uncertainty in the calculations or model tests used is substantial. It must then first be established whether the difference has been determined accurately enough to form the basis of a technical preference.
In hydrodynamic calculations, uncertainties arise from factors including the numerical method, propeller modelling and the description of relevant flow phenomena. Model tests also have measurement and extrapolation uncertainties. Particularly where different ESD types affect different parts of the flow, the same method need not provide an equally accurate performance prediction for every design.
The uncertainty of the difference between the designs is particularly important for the ranking. With comparable configurations, certain model deviations may partly coincide and therefore have less influence on the difference between them. Other deviations may have different effects because the geometry or flow behaviour changes. The reliability of the ranking therefore cannot be derived solely from the individual uncertainties of the two power calculations.
Comparative research into ESD performance calculations shows that different hydrodynamic methods can produce different predictions for the same device. Prediction of the wake field and modelling of the working propeller can be particularly decisive. Where two designs have been investigated using different levels of modelling, the reliability of the comparison between them therefore warrants separate attention.
There is also uncertainty about the future operational profile. Even when the power saving at each operating point is accurately known, a different distribution of speed and loading can change the expected annual saving. This profile uncertainty must be distinguished from uncertainty in the hydrodynamic performance prediction.
The hypothetical worked example shows why this matters. When the expected distribution of operating hours is close to the threshold for a change in preference, the preferred option may change without the designs themselves being modified. If the predicted power savings are also uncertain, the position of that threshold is less certain too. A single calculated threshold value must then not be treated as an exact technical decision point.
Where the predicted advantage cannot be distinguished from the relevant uncertainties with sufficient reliability, no clear hydrodynamic preference has been demonstrated. This does not mean that both designs are exactly equally efficient. It means that the available information provides insufficient certainty for the calculated difference to be decisive.
Additional calculations or model tests are particularly worthwhile when an uncertainty that actually determines the design choice can be reduced in this way. If two designs remain comparable after further assessment, structural simplicity, maintainability and less sensitive operation under changing conditions may become more important than a minimal calculated advantage.
Which Technical Limitations Determine the Final ESD Choice?
A high calculated energy saving is relevant only when the ESD design is technically feasible. The available installation space, structural loads and effects on propulsion are therefore considered during the initial selection. A device that cannot function safely or reliably should not remain an equivalent candidate in the final ranking.
For a Pre-Swirl Stator or Pre-Duct, important factors include the available space ahead of the propeller and the attachment options at the stern. A PBCF must suit the hub construction and the surrounding propeller geometry. For a Rudder Bulb, the rudder shape, propeller–rudder distance and relative alignment play a part. Particularly on existing ships, these conditions may limit the hydrodynamically desirable geometry.
Cavitation, vibration and manoeuvrability can also be decisive. An ESD that provides a favourable power saving at normal cruising speed may cause undesirable loads in other necessary operating conditions. If these effects cannot be adequately controlled, different geometry may be needed or the device no longer qualifies. Technical suitability is therefore a prerequisite for the final choice, rather than a correction applied to the percentage saving afterwards.
For the remaining designs, the expected energy saving, reliability of performance and sensitivity to the operational profile are assessed together. A convincingly demonstrated advantage can determine the hydrodynamic preference. Where results are comparable or cannot be distinguished sufficiently, simpler construction, better access for maintenance and less extensive modifications may be decisive.
The final assessment must also make clear which configuration has actually been investigated. A separate ESD on the existing propeller is not the same as an integrated solution in which the propeller and device have been optimized together. The reported power saving must therefore relate to the complete version being considered for the ship.
A sound design comparison thus provides more than a ranking based on individual percentage savings. The results must show how much energy each technically suitable configuration can save across the expected operational profile, how reliable the differences are and when a change in the ship's deployment may alter the preference. The best choice is therefore not necessarily the design with the greatest improvement at an individual operating point, but a feasible configuration whose expected energy saving is substantiated with sufficient reliability and suits the operating conditions that actually matter to the ship.
This Article in the Series
A fair design comparison requires suitable operating conditions and a reliable measurement basis. When interpreting the results, the effect of an ESD on overall propulsion performance is also decisive, because more favourable propeller operation may be partly offset by additional losses elsewhere. This relationship is central to hydrodynamic design and performance validation of ESDs.