When Is an ESD Not the Best First Step for Improving Propulsion Efficiency?
An Energy Saving Device (ESD) is not always the best first step for improving a ship’s propulsion efficiency. If power demand has increased mainly because of fouling, propeller damage or poor matching of the propulsion plant, addressing that cause may deliver more than adding a hydrodynamic device. It matters whether the ship’s previous performance has deteriorated, the existing propulsion system no longer suits its current use or a ship that functions well technically still has potential for hydrodynamic improvement. Each situation calls for a different assessment. Hull and propeller maintenance, propeller reoptimization or changes to operating practices may therefore take priority over a Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) or Rudder Bulb. The appropriate measure follows from the cause of the performance loss or inefficiency, not solely from the potential saving of an individual ESD.
When Has Propulsion Performance Actually Deteriorated?
Rising fuel consumption does not automatically mean that hydrodynamic propulsion efficiency has deteriorated. Fuel demand also depends on the efficiency and loading of the main engine, the use of auxiliary generating sets and operating conditions. It must therefore first be clear whether the ship actually needs more shaft power to maintain the same speed under comparable conditions.
If the main engine consumes more fuel at the same power output, the cause may lie in engine performance or other factors affecting specific fuel consumption. An ESD can reduce hydrodynamic power demand, but does not restore deteriorated combustion efficiency. Nor does the device directly remedy additional fuel consumption by auxiliary generating sets. Where a shaft generator is coupled to the shaft, it must also be clear which part of the power is delivered to the propeller and which part is used to generate electricity. The location at which shaft power is measured matters here.
An increase in required shaft power at the same speed through the water is more indicative of changed resistance or propulsion performance. The cause has not yet been established from this. A different draught, altered trim, fouling or greater resistance from wind and waves can cause the same phenomenon. The measurement conditions must also be sufficiently similar. A comparison based solely on GPS speed and engine load, for example, is insufficient if current, water depth or other external influences differ between the two passages.
For seagoing ships that regularly change operating area, the representativeness of the performance data warrants particular attention. A coastal vessel may encounter different resistance conditions during a voyage in open water from those when approaching a port with limited water depth. A tanker in a laden condition compared with previous ballast voyages may also show higher power demand without its technical condition having deteriorated. Such differences must be distinguished from an actual change in hull and propeller performance.
The initial assessment therefore focuses on the relationship between ship speed, shaft power, propulsion settings and fuel consumption. Historical performance data can be used for this if differences in loading and environmental conditions are adequately corrected for. The reliability of the power measurements must also be known. A deviation in the recorded engine load is not, on its own, evidence of a change in the shaft power actually delivered.
If the data show that the engine uses more fuel without a corresponding increase in hydrodynamic power demand, the initial investigation lies outside ESD selection. If the data do indicate higher power demand, it must be established whether this stems from deteriorated technical condition, changed operating conditions or less suitable propulsion matching.
When Does the Ship Need Its Performance Restored First?
If a ship has gradually become less efficient, the condition of the hull and propeller warrants attention. Fouling, damaged coatings and deteriorated propeller surfaces can increase power demand without the original propulsion design being fundamentally inadequate. In such a situation, the first opportunity for improvement may lie in restoring the technical condition.
Fouling increases hull surface roughness and therefore hydrodynamic resistance. The propeller must then deliver more thrust to maintain the same speed. An ESD can still make a favourable contribution under those conditions, but does not remove the fouling. If the additional power demand arises mainly from fouling, an additional device therefore does not address the principal cause.
The maintenance measure depends on the actual condition of the underwater hull. If the coating condition is suitable, cleaning may suffice to remove fouling. Damage or severe wear may warrant coating repairs. The properties of the coating system and the possible consequences of cleaning must be taken into account. Underwater cleaning may, for example, damage the coating or spread unwanted substances and organisms in the water. The choice of cleaning method must therefore suit the coating, the nature of the fouling and the locally applicable requirements.
A similar assessment applies to the ship’s propeller. Fouling, deposits, damaged blade edges and increased surface roughness disturb the flow along the blades. The propeller may consequently perform less favourably and require more power for the necessary propulsion. If inspection reveals such defects, cleaning, polishing or repair may offer a more direct improvement than installing a PBCF or another ESD. The appropriate treatment depends on the nature and extent of the deviation. Damaged blade geometry is not automatically restored by polishing the surface alone.
For a coastal vessel that regularly spends time in ports and shows higher power demand over time, this is a logical starting point for investigation. The combination of fouling, coating condition and propeller surface condition may explain the deterioration. If the previous speed–power performance largely returns after appropriate maintenance, this supports the diagnosis, provided the performance has been assessed under sufficiently comparable conditions. If a deviation remains, further technical assessment is needed.
The assessment of any subsequent ESD saving is also affected by this distinction. If the hull is cleaned, the propeller polished and an ESD installed during the same docking, the subsequent performance improvement represents the combined result of that work. Without further information, it cannot be established which part was caused solely by the ESD.
For a reliable technical assessment, the additional ESD improvement is therefore related to a representative hull and propeller condition. If the original condition cannot be restored and measured separately, calculations and historical data can help distinguish the expected contributions of maintenance and the ESD. That separate attribution then remains dependent on the assumptions used and the associated uncertainty.
Restoring performance and optimizing an ESD are not mutually exclusive. A clean hull and well-maintained propeller may still have hydrodynamic losses that are reduced by an additional device. The difference is that it must first be clear which part of the power demand stems from a deteriorated condition and which part remains even on a well-maintained ship.
When Is Reoptimizing the Existing Propeller More Sensible?
A ship can be well maintained technically and still sail with a less suitable propulsion configuration. This happens, for example, if the current operational profile differs substantially from the speed and propeller loading for which the plant was originally designed. The propeller may then function in accordance with the original design assumptions, but those assumptions no longer match its current use optimally.
On a Fixed Pitch Propeller (FPP), the geometric blade pitch remains unchanged during operation. The propeller is designed for a particular range of inflow speeds, rotational speeds and loads. If a container ship or bulk carrier starts routinely sailing at a lower speed, the actual operating point may shift to a range in which the original blade geometry performs less favourably. This need not lead to a significant deterioration in efficiency with every reduction in speed, but may warrant reassessing the propeller matching.
In that situation, a modified propeller may offer a more direct improvement than a device that only influences the flow ahead of or behind the existing propeller. A different pitch distribution, blade geometry or propeller diameter may better match the propulsion system to the changed loading. The available space, engine load, cavitation characteristics and manoeuvring performance must also be assessed. A new propeller is not automatically necessary, but warrants investigation if the existing geometry limits the main potential for improvement.
For a Controllable Pitch Propeller (CPP), the initial optimization may instead lie in the control system. A CPP can deliver the required thrust with different combinations of blade pitch and rotational speed. These combinations differ in propeller efficiency and main-engine loading. If the existing combinator curve no longer suits the operational profile, adjusting the control system may improve efficiency without immediately installing an additional ESD.
That freedom is limited by the propulsion plant. With a coupled shaft generator, a nearly constant rotational speed may be necessary if the electrical configuration requires it. With a suitable frequency converter, more rotational speed settings may become available, depending on the installation. Engine limits, cavitation, vibration and manoeuvring requirements also determine which combinations of blade pitch and rotational speed can actually be used.
The control strategy must therefore be assessed together with the propeller and the energy supply. Favourable propeller efficiency at a particular setting does not automatically yield the lowest total fuel demand if, as a result, the main engine operates less efficiently or electricity generation has to be arranged differently.
If the existing control system is already well matched, the geometry of the CPP blades may still offer scope for improvement. Redesign may be worthwhile, particularly where the operational profile has changed substantially. An ESD remains a possible additional measure, but the interaction must be investigated early. A Pre-Swirl Stator developed for the existing propeller, for example, need not retain the same performance if the blade geometry is subsequently changed.
Reoptimization and the use of an ESD therefore address different design questions. Reoptimization improves the matching of the existing propulsion system to the required performance. An ESD seeks to reduce hydrodynamic losses around that propulsion system further. If both options are promising, a joint design study may have more value than two separate modifications that later have to be matched to each other again.
When Can Operating Practices Deliver More Than an ESD?
Not every reduction in propulsion energy use calls for a structural modification. The chosen ship speed, trim, route and distribution of operating hours affect how much energy is needed during a ship’s operation. If clear opportunities for improvement exist in these areas, changing operating practices may be a logical first measure.
On many displacement ships, speed has a major influence on power demand. A modest speed reduction can therefore deliver a substantial reduction in required shaft power. The energy saving over a complete voyage, however, is not equal to the percentage reduction in that power. At a lower speed, the sailing time over the same distance generally increases. Engine load, auxiliary systems and the available voyage time also determine the eventual outcome.
For a short-sea vessel with regular port calls, for example, matching sailing speed to the available arrival time may be important. If the ship sails to a port at high speed and then waits for a prolonged period, better arrival planning may allow a lower speed en route. Energy use may thereby decrease without an ESD being needed immediately. The options depend on the schedule, contractual arrangements, weather conditions and the availability of the port and nautical services.
Route selection may also influence the required power. Wind and waves can change resistance and propulsion loading. Current mainly affects speed over the ground and voyage time; a uniform current does not, in itself, increase ship resistance at the same speed through the water. Weather routing may therefore produce a different distribution of ship speeds and engine loads. A technically favourable route must meet the requirements for safety, voyage time and operation.
Trim offers another possible improvement. At the same displacement, a different attitude of the ship may change resistance. More favourable trim may therefore reduce the required propeller thrust. The options are limited by cargo distribution, stability, sightlines, propeller immersion and the other requirements for safe navigation. Particularly where there is sufficient freedom to distribute ballast and cargo, trim optimization may be a worthwhile measure.
These operational changes are also relevant to any subsequent ESD. If a ship starts routinely sailing more slowly or comes to have a different distribution of laden and ballast passages, the conditions for which the device is designed shift. An ESD matched to the original speed range need not then deliver the greatest energy saving over the future operational profile.
The aim is therefore not to implement every operational improvement as a mandatory step before an ESD is investigated. It is more important that foreseeable changes in the operational profile are taken into account in good time. In this way, situations are prevented in which a new device is developed for operating conditions that will hardly occur during future operation.
When Is the Remaining Hydrodynamic Gain Insufficient?
Hydrodynamic losses remain even on a well-maintained, suitably matched ship. Not every loss can, however, be reduced sufficiently by an additional device. The existing hull form, propeller geometry and rudder arrangement determine which options remain available.
If propeller inflow is already favourable and the propeller–rudder arrangement works together efficiently, the remaining potential for improvement from a Pre-Swirl Stator or Pre-Duct may be limited. Both devices themselves generate hydrodynamic forces and possible flow losses. A favourable change in propeller inflow therefore yields a net benefit only if the complete propulsion system needs less power.
The same assessment applies to devices behind the propeller. A PBCF can influence the hub flow, but the extent of the improvement depends on the existing propeller and hub geometry and the rudder’s response. A Rudder Bulb can improve the transition between propeller hub and rudder, while the existing rudder shape and propeller–rudder distance determine which geometries are possible. There is therefore an important difference between identifying a hydrodynamic loss and demonstrating a suitable ESD solution. A strong hub vortex, for example, is an indication to investigate the flow around the propeller hub further. It does not yet mean that a PBCF or Rudder Bulb reduces the power demand of the complete ship sufficiently. The forces on the rudder and the device itself must also be taken into account.
Sometimes the structure limits the improvement that can be achieved. A stator cannot, for example, be attached at the hydrodynamically desired position, or the available propeller–rudder distance leaves insufficient room for the desired hub device. A design that appears attractive in an initial calculation may therefore become less suitable technically when the actual structure is taken into account.
The reliability of a small predicted saving also warrants attention. If the difference in shaft power between the existing configuration and the proposed ESD cannot reliably be distinguished from the uncertainty of the assessment method, it has not yet been sufficiently demonstrated how much benefit the device actually delivers. A limited improvement can be valuable over the course of operation, but the substantiation must be sufficiently accurate to support a technical decision.
Alongside average power demand, cavitation, vibration and manoeuvrability may limit the choice. A device that performs favourably at normal cruising speed must also continue to function acceptably from a technical standpoint under other relevant operating conditions. If the expected gain is small and the necessary structural modifications or operational risks are substantial, it may be more sensible not to fit an ESD.
The best technical solution is therefore not necessarily the configuration with the most additional devices. If the existing propulsion system already functions well and the remaining improvement is limited, uncertain or difficult to achieve, retaining the current configuration may be a defensible technical choice.
How Do You Determine Which Improvement Should Take Priority?
The choice between maintenance, reoptimization and an additional ESD becomes clearer when it is first established what type of performance problem is present. The distinction between deteriorated technical condition, changed design assumptions and remaining potential for hydrodynamic improvement is decisive.
Restoring performance is relevant when the ship’s efficiency has declined. An increasing power demand at a given speed may then be associated with fouling, coating damage or deteriorated propeller condition. The initial assessment focuses on the technical condition and the possibility of restoring the original performance. Inspection, maintenance records and comparison with earlier performance measurements provide guidance for this.
In the case of reoptimization, the ship functions well technically, but the propulsion configuration does not adequately suit its current deployment. A consistently lower speed range, different propeller loading or unfavourable CPP control may then warrant modifying the existing propeller or operating settings. The first technical question is what improvement is achievable with better-matched propulsion.
For additional hydrodynamic improvement, the technical condition and existing matching are adequate, but losses remain that can be reduced by a targeted device. Investigation of a Pre-Swirl Stator, Pre-Duct, PBCF or Rudder Bulb then becomes more relevant. The assessment must show how the device influences the flow around the existing propulsion system and how much shaft power can thereby be saved at the same ship performance.
These distinctions also matter when several causes are present simultaneously. A bulk carrier may, for example, have a fouled hull and also routinely sail more slowly than originally envisaged. Hull maintenance may then be needed to reverse the deterioration in resistance, while propeller optimization can better match the propulsion system to the current operational profile. An ESD may subsequently offer further improvement, but need not be the first or most important measure.
The order in which possible improvements are identified need not be the same as the order in which they are implemented. During a planned docking, it may be practically attractive to combine maintenance and hydrodynamic modifications. For a newbuild or an extensive retrofit, it may also be sensible to investigate propeller optimization and ESD design together from the outset.
However, it must remain clear what contribution is expected from each measure. If the propeller is repaired and an ESD fitted during the same docking, the total subsequent performance improvement must not be attributed entirely to the ESD without further substantiation. For a technical assessment of the individual device, a representative reference is needed in which the effect of the repair is adequately accounted for.
The appropriate first step is therefore the measure that most directly addresses the identified performance problem. Where the technical condition has deteriorated, restoration takes priority. If the existing propulsion system no longer suits its current use, reoptimization is a logical choice. An ESD becomes particularly relevant if hydrodynamic losses also remain that can be reduced sufficiently by an additional device.
An Energy Saving Device is therefore not the best first step if maintenance, propeller optimization or changed operating practices can better address the main cause of high power or fuel demand. This does not rule out a subsequent ESD application or one designed concurrently. A suitable improvement starts with a reliable diagnosis of the existing propulsion system and a representative assessment of future operating conditions. On that basis, it can be established which technical measure, alone or combined with other improvements, makes the greatest reliably substantiated contribution to propulsion efficiency.
This Article in the Series
Even when multiple ESDs can complement each other hydrodynamically, their additional benefit must be weighed against maintenance and reoptimization of the existing propulsion system. This assessment forms part of the selection and configuration of ESDs within ship propulsion. If an additional device remains promising, CFD can be used for targeted optimization of the ESD geometry.