Which Operating Conditions Are Needed to Reliably Assess Energy Saving Device (ESD) Performance?
The performance of an Energy Saving Device (ESD) is reliably assessed by establishing how much shaft power a ship needs with and without the device for equivalent ship performance. Ship speed, draught, trim, water depth, wind, waves and current affect power demand. The condition of the hull and propeller and the propulsion system settings also play a part. When these conditions differ between two measurement campaigns, an apparent saving may arise or a real ESD benefit may remain hidden. A reliable assessment therefore requires a suitable reference, accurate and synchronized measurements, and a method by which relevant influences can be adequately corrected for or bounded. Propeller rotational speed and torque do not have to remain the same: an ESD can change the way the propeller operates. The decisive factors are the power reduction at the same ship performance, the reliability of that difference and the confidence with which the improvement can be attributed to the device.
What Reference Is Needed to Compare ESD Performance?
The reference determines which performance improvement can be demonstrated by the measurements. For an existing ship, the speed–power relationship without the proposed ESD provides a useful starting point, provided the technical condition and measurement conditions are sufficiently well known. After installation, the change in that relationship is investigated. For a newbuild project, a reference without an ESD can be developed using hydrodynamic calculations and model tests. This provides a predicted initial condition, but no directly measured performance of the completed ship without the device.
For a retrofit, it is particularly important to establish which components change between the two measurement campaigns. When only a Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) or Rudder Bulb is added, carefully conducted measurements before and after installation can reveal the separate contribution of the device. The hull and propeller condition must have remained sufficiently comparable. If the hull is cleaned, the propeller polished or the propulsion settings adjusted at the same time, the measured difference may include several improvements.
This distinction must be made when the measurement plan is drawn up. A shipowner may, for example, want to know how much the total power demand has fallen after a retrofit. That is a different research question from how much of that reduction can be attributed solely to the ESD. For the first question, a corrected comparison of the overall condition before and after the work may be sufficient. The second requires a reference in which the influence of other changes has been excluded as far as possible or assessed separately.
The measurement plan therefore specifies which configurations are to be compared, under which conditions the measurements will take place and which results must be reported. The speed range, loading condition, measuring instruments, permissible environmental conditions and method of correction are also determined in advance. Decisions about which measurement points are usable or which conditions are considered comparable are thereby prevented from being made only afterwards.
A reference does not have to have been measured in exactly the same weather conditions. Differences can be corrected for under suitable conditions. The chosen method must, however, suit the ship, the magnitude of the deviations and the expected ESD effect. When the initial technical condition is insufficiently known or important influences cannot be reliably distinguished, an apparently accurate speed–power comparison may still provide little evidence.
In a full-scale comparison, it must also remain clear what has actually been measured and what follows from calculations. A measurement after installation can establish the performance of the ship with the ESD. When the reference without an ESD comes solely from model tests or flow calculations, the calculated initial performance remains one of the factors determining confidence in the attributed saving.
Why Are Ship Speed and Propulsion Settings Decisive?
The propulsion power required is closely related to the speed at which a ship travels through the water. As speed increases, resistance rises for many displacement ships and the propeller must provide more thrust. A relatively small difference in speed between two measurements can therefore cause a power difference of the same order as the expected ESD saving.
Performance is therefore compared at a common speed. During a speed–power trial, several steady measurement runs are carried out to establish the relationship between speed and shaft power. The results for configurations with and without an ESD can then be assessed at corresponding speeds. When the measurement points do not coincide exactly, interpolation within a sufficiently investigated speed range may be useful. Results that depend heavily on extrapolation beyond the measured range warrant considerably greater caution.
Speed over the ground is not the same as speed through the water. A GNSS or GPS measurement describes the movement of the ship relative to the Earth. Current can cause a ship to achieve a different speed over the ground at the same propulsion settings. Speed–power trials are therefore often carried out with successive runs in opposite directions. A speed corrected for current can be derived from the measured speeds using a suitable analysis method.
Simply averaging an outward and return run is not, however, sufficient under all conditions. The current may change during the measurement runs and need not be the same throughout the trial area. The correction method used must therefore suit the way the current varies and the measurement arrangements.
A direct measurement of speed through the water can provide additional information, but is not automatically more accurate. The quality of the sensor used and the influence of the local flow around the hull must also be assessed. Which speed data are most suitable depends on the measurement arrangements and the chosen assessment method.
Propeller rotational speed and torque do not have to remain the same for the ESD comparison. A device can change the propeller inflow, so that a different rotational speed is needed to achieve the same ship speed. Shaft power follows from the torque delivered and the angular velocity of the shaft. A lower rotational speed is therefore not, in itself, evidence of a lower power requirement. Torque may change at the same time.
For a Controllable Pitch Propeller (CPP), the blade pitch actually used must also be known. Depending on the installation, the same speed can be achieved with different combinations of blade pitch and rotational speed. It must be clear in advance whether the trial investigates only the effect of the added ESD or also includes a modified control strategy. An optimized CPP setting may form part of a worthwhile retrofit, but its contribution must not be attributed to the ESD without that distinction being made.
The indicated blade pitch warrants the same attention as other measurement data. When the recorded setting differs from the actual blade pitch, two operating conditions may incorrectly be regarded as comparable. Particularly in a CPP trial, it must therefore be clear how blade pitch is established and checked.
The location of the power measurement must also be specified. A shaft power meter records the power at a particular point in the drivetrain. When a shaft generator or another coupled system draws power, the location of that power take-off relative to the meter must be known. Any power drawn and relevant losses must be accounted for to establish a comparable quantity for propulsion.
The measurement results must ultimately describe the same ship performance. Identical rotational speeds or engine loads do not form the basis of the comparison; reliably established power demand under equivalent conditions does.
How Do Draught, Trim and Water Depth Affect the Comparison?
Draught and trim affect hull resistance and propeller inflow. A seagoing ship with a greater draught during one measurement campaign than another may have a different displacement, submerged hull form and propeller loading. The speed–power relationship may therefore change without the ESD being responsible.
For a reliable comparison, the forward and aft draughts, displacement and relevant loading condition are recorded. A similar mean draught alone is not always sufficient. A different trim can change the position of the stern and the propeller immersion, affecting both resistance and the operation of the device.
These differences are particularly relevant for a coastal tanker that alternates between laden and ballast operation. The propeller may operate under substantially different conditions in the two states. A measurement during laden operation therefore cannot be compared with a reference from ballast operation without further substantiation. Where ESD performance is important in both states, a separate assessment for each representative loading condition is preferable.
Limited differences in draught or trim can sometimes be corrected for using a suitable method. Such corrections are not, however, universally applicable. When the ship conditions differ greatly, the uncertainty of the conversion may exceed the ESD effect that needs to be established. It is then more sensible to take additional measurements under a more comparable loading condition.
Water depth can also affect power demand. In relatively shallow water, the flow between the hull and the seabed changes. Depending on speed and the available water depth, resistance, sinkage and trim may change while the ship is underway. The ship may therefore need more power than at the same speed in deep water.
For seagoing ships, this matters during port approaches, on coastal routes and in certain trial areas, among other situations. During a sea trial, a bulk carrier or tanker may, for example, pass through an area where the available water depth varies considerably from place to place. If this influence is not adequately taken into account, the speed–power relationship may be distorted.
A trial area with sufficient and relatively uniform water depth is therefore preferably chosen. Where shallow-water effects remain relevant, the chosen correction method must be suitable for the hull form, speed and water depth concerned. The availability of a correction does not mean that every measurement in water of limited depth automatically becomes as reliable as a measurement under favourable deep-water conditions.
What ultimately matters to the assessment is whether the influence of draught, trim and water depth is sufficiently well known to distinguish the ESD power reduction from those effects.
How Are Wind, Waves and Current Accounted For?
Wind and waves can significantly affect the propulsion required. Headwinds increase air resistance, while waves cause added resistance and ship motions. The effects depend on the strength and direction of the wind and waves relative to the ship. The changed ship motion can also influence the flow around the propeller and an ESD. Environmental conditions are therefore relevant not only to the measured power demand but, under certain conditions, also to the operation of the device itself.
Calm weather conditions are preferable for accurate speed–power trials. As a result, smaller corrections are needed and the influence of phenomena that are difficult to predict is limited. Where wind and waves are present, the relevant weather data are recorded. It is then assessed which corrections can justifiably be applied.
This warrants particular attention when the expected ESD saving is small. A large correction for wave resistance can strongly influence the final power difference. When the calculated saving depends substantially on such a correction, its uncertainty must also remain identifiable. A correction method may be technically suitable without every corrected measurement becoming sufficiently accurate to demonstrate a small improvement.
Moreover, a correction to a common reference condition cannot by itself demonstrate how the ESD performs in all actual weather conditions. Where the device itself is sensitive to changes in propeller loading or inflow in a seaway, that operational performance requires a separate assessment. A speed–power trial under corrected conditions and a prediction of savings across the full operational profile do not answer the same question.
Current primarily affects the relationship between speed over the ground and speed through the water. Measurements in opposite directions of travel help assess this influence. The method used assumes that the current during the measurement runs can be described appropriately. If the speed or direction of the current changes significantly, the results may still be distorted.
This is a practical consideration for sea trials in coastal and tidal waters. A changing tidal current may mean that conditions during the return run already differ from those during the outward run. The measurement schedule, run duration and analysis must be adapted accordingly. Where the current is insufficiently stable or predictable, additional runs or a different measurement time may be necessary.
The properties of the water also warrant attention. Temperature and salinity affect density and viscosity, among other properties. Differences between measurement campaigns may therefore affect resistance and propulsion performance. Whether a correction is needed depends on the conditions and the accuracy that the assessment requires.
Not every environmental influence has to be eliminated completely. The relevant differences must, however, be small enough or capable of being corrected for with sufficient reliability. Where large corrections for weather or current are necessary, a new sea trial under more favourable conditions may provide a stronger basis than further refinement of the analysis afterwards.
What Measurement Data and Stable Operating Conditions Are Needed?
A reliable speed–power trial requires steady measurement periods. After a change in rotational speed or CPP blade pitch, the ship needs time to reach a new operating condition. During acceleration or deceleration, part of the available propulsive force is used to change speed. Such measurement periods therefore cannot be treated as steady operating points without further assessment.
During a suitable measurement run, the propulsion settings are kept sufficiently constant and the course remains as steady as possible. Propeller rotational speed, torque, shaft power and speed are recorded. For a CPP, this also includes blade pitch. Rudder angle is relevant too, because continuous steering corrections can increase resistance. For a Rudder Bulb, a different rudder position can also change the local hydrodynamic effect.
The measurement run must be long enough to reach the desired operating condition and maintain it for a useful period. Large course changes, noticeable acceleration and disturbances from other ships are excluded from the accepted measurement periods. The required duration of a measurement run depends on the characteristics of the ship, the measurement method and the stability of the conditions.
Instrument accuracy must be appropriate to the expected performance difference. A shaft power measurement may, for example, appear accurate while a small systematic deviation in the torque measurement represents a significant part of the expected saving. Calibration and checking of the measurement chain are therefore important. It must also be clear whether the instruments use the same calibration basis for the measurements before and after installation, or what differences have arisen.
The data must also be recorded on a common time basis. When a power value is paired with a speed measurement taken a few moments earlier, an incorrect operating point may result while the loading is changing. Synchronization is therefore not merely an administrative requirement, but a prerequisite for correctly linking the measured speed and power demand.
For a RoRo ship or coastal vessel with frequent port calls, steady operating periods are not always easy to find during normal operation. Course changes, traffic situations, speed restrictions and manoeuvres regularly interrupt the passage. Where performance monitoring is used, such periods must be identifiable and excluded where necessary. The remaining data must contain enough comparable operating points to allow a reliable analysis.
Repeated measurements also provide insight into reproducibility. When different runs under comparable conditions produce widely differing results, the cause must be investigated. It may lie in measurement errors, changing environmental conditions or insufficiently steady propulsion. Selecting only the most favourable measurement run would distort the picture.
A reliable measurement campaign therefore provides more than a collection of speeds and power values. The calibration, measurement conditions, stability and criteria by which the data have been selected must also be known.
When Can the ESD Effect Not Be Separated from Other Work?
In an ESD retrofit, the condition of the hull and propeller is an important prerequisite for a performance comparison free from confounding effects. Fouling, coating roughness and damage to the propeller blades can increase power demand. If maintenance takes place between the measurements, the initial condition against which the ESD is assessed changes.
This is particularly relevant when a device is installed during a scheduled dry-docking. Before the work, the hull may be fouled and the propeller may have a rougher surface condition. Afterwards, the hull and propeller may have been cleaned or repaired, with the ESD installed at the same time. A lower power demand after dry-docking then describes the combined result of these changes.
A stronger measurement arrangement seeks to keep the relevant technical condition as consistent as possible between the two configurations. This may prompt sea trials immediately before and after a dedicated ESD installation, with as few other changes as possible. Even then, it must be checked whether the hull and propeller condition has actually remained comparable.
Research on a bulk carrier with a Pre-Swirl Stator shows that such a focused approach is practicable. Performance was investigated through separate speed–power trials shortly before and after installation, with the conditions carefully controlled. This research example illustrates the importance of the measurement arrangements, but does not mean that every retrofit can be assessed with the same accuracy or under the same conditions.
Where a separate measurement is not possible, hydrodynamic calculations, model tests and previous performance data can be used to estimate the contributions of different measures. For example, the improvement that may be expected with a clean hull and propeller without an ESD can be investigated. The difference from the measured performance after installation then provides additional information. The outcome of such an analysis is not, however, equivalent to a directly measured ESD effect. The estimated contribution remains dependent on the models, assumptions and uncertainties involved. That distinction must remain clear in the reporting.
The same applies when a new propeller is installed, the CPP control is modified or other changes are made to the propulsion system at the same time. Measurements before and after the work can then provide valuable information about the complete retrofit, but the separate contribution of each measure requires further substantiation.
When Are the Measurement Results Reliable Enough to Establish a Saving?
A measured reduction in shaft power is convincing only when the reference is suitable, the operating conditions are sufficiently comparable and the uncertainty of the performance difference is known. This concerns more than the accuracy of individual instruments. Corrections for speed, wind, waves, current and loading also affect the result.
The uncertainty of the difference between the configurations is particularly important. When two measurement campaigns have been carried out using the same instruments and analysis method, certain measurement deviations may partly coincide. Other deviations, such as changed weather conditions or a different calibration, may instead have separate effects. The uncertainty of the saving must therefore be assessed from the complete comparison, rather than solely from the error margin of each individual power measurement.
The analysis must then make clear how the speed–power relationship was established. Which measurement runs were used? Which corrections were applied? How great is their influence on the outcome? And do the measurement data cover the speed range in which ESD performance must be assessed? Such questions determine whether an identified improvement is sufficiently reliable to support a technical conclusion.
Several outcomes are possible. When the corrected measurements show a power difference established with sufficient reliability and other technical changes have been excluded or accounted for, the saving can be attributed to the ESD within the limits of the research design and measurement uncertainty. When the calculated improvement is small relative to that uncertainty, its magnitude, or even the existence of a positive improvement, remains insufficiently established. And when other relevant work has been carried out at the same time, an overall performance improvement may be demonstrated without sufficient certainty about the separate ESD contribution.
Those last two situations must not be confused. A small improvement that has not been measured accurately enough may call for better measurements. An improvement that cannot be separated from hull cleaning or propeller repair primarily calls for a better reference or an additional method for distinguishing the measures.
Controlled sea trials and performance monitoring during operation each have their own function. A sea trial under favourable conditions can make a relatively small change in the speed–power relationship more apparent. Longer-term monitoring can show how performance develops during operation, but requires sufficiently comparable operating conditions and reliable information about the technical condition.
Recognized methods exist for the preparation, execution and correction of measurements in speed–power trials. Their scope may be limited by factors including ship type and length. Separate methods also exist for tracking changes in hull and propeller performance over longer periods. These serve a different purpose and do not automatically apply to every propulsion configuration. Particularly for CPP installations, the suitability of the chosen method must be checked.
Even a correctly conducted speed–power trial using a recognized method does not automatically prove that a measured improvement was caused solely by the ESD. The technical reference must also be sufficiently free from confounding effects. Measurement accuracy and the ability to attribute the effect to the device are two separate requirements.
Where the available data provide insufficient certainty, an additional sea trial or targeted calculation may be needed. Sometimes the main requirement is to improve the measurement conditions. In other cases, the separate ESD contribution can no longer be established with sufficient certainty, for example because several changes were made at once and a usable reference is missing. That limitation then explicitly forms part of the reported result.
A reliable sea trial can substantiate the power improvement under the investigated and corrected reference conditions. It is therefore not yet established how much fuel the ship saves over the course of its operation. The operating time at different loads and the characteristics of the engine installation must also be taken into account for that purpose. A percentage reduction in shaft power must therefore not be presented as the same percentage fuel saving without further analysis.
A reliable assessment of an Energy Saving Device begins with a suitable reference and measurement conditions that allow an equivalent comparison. Ship speed, loading, water depth, weather conditions, propulsion settings and the technical condition must be sufficiently well known. Only when the measurements, applied corrections and uncertainties support a convincing conclusion can it be established how much shaft power the ESD saves under the assessed conditions and with what confidence that benefit can be attributed to the device.
This Article in the Series
A reliable performance measurement can test the expectations from the CFD optimization of an ESD, provided the reference and operating conditions are sufficiently similar. Equivalent assumptions are also essential when comparing different ESD designs under representative operating conditions, where the power saving across the operational profile can influence the design choice. Both topics form part of hydrodynamic design and performance validation of ESDs.