How Does the Design of Energy Saving Devices (ESDs) Differ Between Retrofit and Newbuild Projects?
The design of Energy Saving Devices (ESDs) differs between retrofit and newbuild projects mainly because of the available design freedom and the initial technical conditions. In a retrofit, a Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) device or Rudder Bulb is usually matched to an existing ship. Stern shape, propeller geometry, rudder arrangement and attachment options determine which design is feasible. In a newbuild, these components can be developed together from an early design stage, so that the ESD becomes part of the complete propulsion configuration. This offers opportunities for further hydrodynamic optimization, but does not guarantee greater energy savings. The benefits depend on the design choices, expected operational profile and technical constraints. Data on actual operation are often available for retrofits, while newbuilds may offer more scope for matching geometry and structure. In both cases, the chosen design must deliver a reduction in required shaft power that is substantiated with sufficient reliability and must be capable of being applied in a technically sound manner.
What Design Freedom Determines the Difference Between Retrofit and Newbuild Projects?
In a retrofit, much of the ship configuration is usually fixed. Stern shape, shaft line, propeller diameter and rudder position usually cannot be changed without major work. An ESD must therefore be developed within the available space and hydrodynamic conditions of the existing propulsion arrangement. This does not necessarily limit the number of suitable ESD types, but it does limit the geometries and positions that are actually feasible.
A Pre-Swirl Stator may, for example, benefit hydrodynamically from a particular fin position ahead of the propeller. If there is insufficient space or an unsuitable attachment structure at that location, the design must be modified. That change may affect the pre-swirl generated and the eventual power saving. Technical design freedom is therefore determined not only by which device fits, but also by whether it can operate in a hydrodynamically favourable position.
In a newbuild, the components involved can be investigated together at an earlier stage. Stern shape, propeller geometry and rudder arrangement need not yet be fully fixed. An ESD can therefore be included in choices that are difficult to change later, such as the available propeller space, distance from the rudder and position of structural attachments. The propeller can also be matched from the outset to the flow that results from the device.
This freedom decreases, however, as the ship design is developed in more detail. Once the stern structure, shaft line and propeller order have been finalized, modifications may have substantial consequences for engineering, production and the build schedule. An ESD added only at this stage may have to be designed within almost the same constraints as a retrofit.
There is also a difference between improving an existing flow and modifying the hull shape that causes it. In an early newbuild design, an unfavourable wake field can sometimes already be improved by optimizing the stern and propeller together. The remaining potential for improvement from a separate ESD may therefore be smaller. Greater design freedom need not therefore lead to a larger saving from the device itself.
The distinction between retrofit and newbuild projects is therefore also determined by when the ESD is introduced into the project. With an existing ship configuration, the design must adapt to fixed components. Where those components can still be changed, there is scope to improve the complete propulsion arrangement jointly. That freedom is valuable particularly when the additional possibilities actually lead to a better technical solution.
Which Existing Ship Data Determine the Retrofit Design?
For a retrofit, the ship’s actual configuration must first be established. Original drawings are an important starting point, but do not always show the current geometry. During earlier dockings, propeller blades may have been replaced, a different boss cap fitted or modifications made to the rudder and stern. Construction and installation tolerances may also affect the available space.
Reliable data on the underwater hull form, propeller diameter, blade geometry, hub structure and rudder position are therefore needed for ESD design. The required level of detail depends on the device. For a Pre-Swirl Stator or Pre-Duct, the local stern shape and available space ahead of the propeller plane are particularly important. For a PBCF or Rudder Bulb, exact contours and distances around the propeller hub may be decisive.
If the documentation is insufficiently accurate, inspections, geometric measurements or a 3D scan may become necessary. Whether the device fits is not the only aspect checked. The actual geometry also determines the flow on which the hydrodynamic design is based. A deviation in a critical distance may therefore affect both structural feasibility and ESD performance.
Alongside geometry, current operating data are important. For an existing coastal vessel, it may become apparent from the available performance data that the ship now operates at different speeds or with a different loading distribution from that originally envisaged. That information makes it possible to match the ESD to the conditions in which the propulsion arrangement is actually used. It must, however, be established whether the historical data remain representative of future operation.
Planned work must also be known early. If the hull is cleaned, the propeller polished or a different propeller installed during the same docking, the subsequent flow and propeller loading may differ from the condition at the start of the work. The ESD must therefore be assessed on the basis of the configuration in which it will ultimately function. A design matched solely to the existing, possibly deteriorated ship condition may therefore have an unsuitable starting point.
This also affects the performance comparison. If maintenance and ESD installation take place together, before-and-after measurements may describe the combined improvement. For the separate contribution of the ESD, a suitable reference is then needed in which the influence of the other work is distinguished.
The first technical decision in a retrofit is which components will be retained and which may actually change. If the existing propeller is retained, its geometry forms a fixed constraint. If the propeller is replaced, the ESD can be developed together with the new design. This boundary determines the available design freedom and prevents a device from being based on outdated data.
How Are Hull, Propeller, Rudder and ESD Designed Together in a Newbuild?
In a newbuild, an ESD can form part of the development of the complete propulsion configuration. Rather than adding a device to an already established design, stern shape, propeller geometry and rudder arrangement can be matched to the flow that arises with the ESD. As a result, design variants emerge that would be difficult to implement on an existing ship structure.
A Pre-Swirl Stator provides a clear example. The stator fins change the direction of inflow to the propeller. If propeller geometry has not yet been fixed, blade geometry and loading distribution can be developed for that changed inflow. In a retrofit retaining the existing propeller, by contrast, the stator must be matched to the way in which the existing blades can use the pre-swirl.
A similar opportunity exists with a Pre-Duct. The position and profile shape of the duct influence the flow towards the propeller and the hydrodynamic forces on the device. If the stern can still be modified, stern shape, duct position and propeller inflow can be investigated together. The duct therefore need not be fitted solely within an existing geometry.
This design freedom may extend beyond modifying the duct itself. If the original stern produces an unfavourable flow distribution, a change in hull shape may improve the inflow. Whether adding the duct still offers sufficient additional benefit must then be investigated. The best overall propulsion configuration is not automatically the design in which the individual ESD produces the greatest local flow change.
The interaction between propeller and rudder can also be included from the outset. If a Rudder Bulb forms part of the rudder design, the hub contour, propeller–rudder distance and bulb shape can be matched together. This offers more possibilities than a bulb that must be fitted to an existing rudder of fixed dimensions.
Joint optimization must address the performance of the complete configuration. A modified propeller may operate more favourably because of the new inflow, while the stator or duct itself also generates hydrodynamic forces. The most favourable design combination is therefore not necessarily the arrangement in which each component individually achieves its highest efficiency.
The reference deserves particular attention in the performance assessment. If a new propeller and Pre-Swirl Stator have been optimized together, the difference from the original design configuration describes the improvement of the complete alternative. To establish the stator’s additional contribution to that new propeller, a suitable configuration without the stator is also needed. This differs from a comparison between two alternatives in which both the design with a stator and the design without one have been optimized independently.
The benefit of early integration therefore lies in the opportunity to develop propulsion as a whole. The additional power saving this delivers depends on the ship’s hydrodynamic characteristics and on the extent to which the combined design choices actually outperform a suitable configuration without an ESD.
What Does Retrofit or Newbuild Mean for the Different ESD Types?
The differences between retrofit and newbuild projects become concrete when the available space, attachment options and geometric matching are examined for each ESD type. Devices do not all require the same modifications, and some constraints may be decisive for one design while hardly affecting another.
For a Pre-Swirl Stator, the position and orientation of the fins, local propeller inflow and attachment to the stern are particularly important. In a retrofit, the fins must be matched to the existing propeller and structure. If the desired position offers insufficient space for a suitable attachment, the geometry may have to be modified. In an early newbuild design, attachment points and propeller geometry can be included in development from the outset.
A Pre-Duct requires space for the duct profile and support structure. The distance from the propeller plane and the shape of the stern in particular may limit the feasible diameter, position and orientation. In a retrofit, that space is usually fixed. In a newbuild, the stern structure and duct position can be matched at an earlier stage, potentially reducing the compromises needed in the desired geometry.
For a Propeller Boss Cap Fins device, the main constraint concerns the rotating hub structure. In a retrofit, the PBCF must suit the existing boss cap, blade geometry and available clearance from the rudder. The attachment and mechanical loading of the rotating device must also be suitable. With a Controllable Pitch Propeller (CPP), additional constraints may arise from the hub structure and the required movement of the adjustable blades. CPP designs do not all offer the same possibilities for a PBCF.
In a newbuild, the boss cap and any PBCF can be developed together at an earlier stage. Freedom nevertheless remains limited by requirements for the propeller structure, attachment and, where applicable, pitch adjustment mechanism. A PBCF in a newbuild need not necessarily lead to a completely new propeller design either. If the existing or planned hub configuration is suitable, the device can be developed as a targeted addition.
A Rudder Bulb is matched to the geometry and flow between propeller hub and rudder. In a retrofit, the existing rudder shape, relative alignment and propeller–rudder distance determine the possible bulb contour. If the available space is insufficient, modification of the existing rudder may be needed. In a newbuild, the bulb can become part of the rudder design earlier, and the connection with the propeller hub can be developed with greater freedom.
These differences explain why the scale of a modification does not follow directly from the ESD type. With a suitable existing hub structure, a PBCF may be a relatively limited retrofit, whereas a large Pre-Duct or rudder modification may require more structural work. On another ship, the circumstances may be different. The available geometry and necessary modifications determine the actual complexity.
All four ESD types can be applied in retrofit and newbuild projects under suitable conditions. The main technical consideration is which design geometry can be achieved within the available space and how much hydrodynamic benefit remains after all necessary modifications.
Which Structural and Practical Requirements Differ Between the Two Project Types?
A hydrodynamically promising ESD design must be capable of being built, attached and maintained. Similar principles of strength, reliability and safe operation apply to retrofit and newbuild projects. The difference lies mainly in how these requirements are incorporated into the design and implementation.
In a retrofit, the attachment structure must suit the existing stern. A Pre-Swirl Stator or Pre-Duct transfers hydrodynamic forces through its supports to the hull. It must therefore be established whether the existing steel structure has sufficient strength and stiffness and where reinforcement may be needed. Fluctuating loads and fatigue may also determine the design.
Installation imposes additional constraints. The available space in the final operating condition matters, as does access during installation. The device must be capable of being fitted and aligned in the correct position. Actual dimensions, installation tolerances and necessary clearances are important. Sufficient space must remain, particularly around the rotating propeller hub and moving rudder, including when components deform under load.
In a newbuild, supports, attachment points and installation provisions can be incorporated into the steel structure at an earlier stage. The installation sequence can also be coordinated with construction of the stern and fitting of the propeller and rudder. Later modifications can thereby be limited. That benefit partly disappears if the ESD design is completed only after the relevant structure has already been produced.
The time available for implementation may also be important in a retrofit. An ESD is often fitted during a planned docking. If extensive steelwork, additional inspections or a longer docking period are needed for this, it affects the technical and practical appeal of the design. A design with a slightly smaller calculated power saving may therefore be more suitable if it can be installed more easily and entails fewer implementation risks.
Not every retrofit requires a dry dock, however. Installation without dry-docking has been documented for certain PBCF installations, with the ship trimmed so that the propeller becomes accessible above the waterline. This is not a universally applicable installation method. The possibilities depend on factors including the ship’s floating condition, stability, available space and safe access to the propeller. The hub design and installation requirements must also be suitable.
For both project types, the consequences for cavitation, vibration and manoeuvrability are relevant. The necessary structural calculations, drawings, inspections and approvals must also be coordinated in good time. The applicable class and flag-state requirements depend on the nature and extent of the modifications. A relatively simple boss cap replacement need not require the same assessment as fitting a large structure attached to the stern. If the propeller, shaft line or other propulsion components are also modified, additional assessments may be needed.
The practical difference is therefore that a retrofit requires a suitable solution within an existing structure and implementation schedule, whereas a newbuild offers more opportunities to incorporate those conditions into the design in advance. In both cases, a hydrodynamically favourable geometry may still cease to be an option if the structural design proves insufficiently reliable or impractical.
How Does the Operational Profile Affect Design in Retrofit and Newbuild Projects?
In both retrofit and newbuild projects, an ESD is matched to the speeds, draughts and propeller loading conditions that matter to the ship. The difference lies mainly in the information on which that matching is based.
For a retrofit, operating data can show how the ship is actually used. Particularly where operating patterns have changed over the years, these data may differ from the original design assumptions. An existing coastal vessel that consistently sails more slowly than at delivery may, for example, have different propeller loading from that for which the original propulsion arrangement was designed.
The available data must, however, be interpreted carefully. Higher power demand due to fouling or propeller contamination does not automatically indicate the hydrodynamic operating point for which a new ESD must be optimally designed. Actual operating hours may guide the selection of representative operating points, while the performance assessment must reflect the intended technical condition after the retrofit.
It must also be established whether the historical operational profile is representative of future operation. A change in trading area, cargo flows or schedule may mean that future use differs from the recorded operational profile.
For a newbuild, operating data from the future ship are unavailable. Design matching is therefore based on assumptions about expected operation. Data from comparable ships, planned routes, speed ranges and loading conditions may help. Particularly where different operating conditions account for a significant share of future energy use, investigating only the nominal cruising speed is insufficient.
The main difference is therefore not that a retrofit automatically has a more accurate operational profile. Current use may be better known for existing ships, but future changes remain uncertain. For newbuilds, the assumptions are usually more extensive, while a range of operating conditions can be taken into account earlier during design development.
The available design freedom then determines how that information can be acted on. In a retrofit retaining the existing propeller, the ESD is matched to the existing geometry and expected use. In an early newbuild design, propeller and device can be developed together for the main operating points. In both cases, the choice must suit the operational profile for which the investment is actually intended.
How Are Retrofit and Newbuild Performance Compared Reliably?
Expected performance in retrofit and newbuild projects is assessed by establishing how much shaft power the complete propulsion configuration requires under equivalent conditions. The main differences lie in the available reference and the extent of the design modifications being investigated.
For a retrofit, the existing propulsion arrangement can be described using technical data, hydrodynamic calculations and possibly performance measurements. A reference without the proposed ESD thus emerges. If the device is installed separately and the rest of the ship’s condition remains sufficiently similar, controlled before-and-after measurements may provide additional information on the actual saving.
If the hull and propeller are also maintained during the same docking, the initial technical conditions change. Lower subsequent power demand may represent the combined result of maintenance and ESD installation. To establish the separate ESD contribution, it must be known how the ship would have performed after the same maintenance work without an ESD.
For a newbuild, design variants are usually compared using hydrodynamic calculations and model tests. If an ESD is added separately to an existing design configuration, the power difference from that modification can be investigated. With integrated optimization, however, the propeller, rudder or stern shape may also change.
In that case, the performance being compared must be clear in advance. A jointly optimized design with a Pre-Swirl Stator may perform more favourably than an original configuration without a stator. That difference describes the improvement of the overall design. A suitable reference without a stator is needed for the stator’s additional contribution to the selected propeller geometry. The difference between two separately optimized alternatives, by contrast, describes the performance of the complete design concepts.
The certainty of the prediction also matters. Model tests can help substantiate calculated performance, while additional analyses may be needed where cavitation, vibration or scale effects are a major source of uncertainty. After installation, sea trials can assess the performance of the actual ship.
The strength of the evidence varies by project. In a retrofit, suitable before-and-after measurements may provide a strong basis for establishing the separate ESD effect. In a newbuild, the sea trial may confirm the performance of the complete ship, but without a directly measured reference without an ESD, the separate saving remains partly dependent on the calculated or tested design comparison.
For the technical choice, it is therefore insufficient for one project type to have more data available than the other. The reference used must suit the actual design question, and the uncertainty in the performance difference must be sufficiently well known.
When Does a Major Retrofit Offer More Scope for Joint ESD Optimization?
A retrofit need not be limited to fitting an additional device to the existing propulsion arrangement. When a ship undergoes extensive modernization, the propeller, rudder and certain parts of the stern can also be modified. Scope for an integrated ESD design thus arises.
This may be attractive, for example, for a bulk carrier whose operational profile has changed substantially and for which a new propeller is being considered. If a Pre-Swirl Stator is investigated at the same time, the propeller geometry can be developed for the changed inflow. The stator then need not be matched solely to the existing blade geometry.
A planned rudder replacement may also offer new possibilities. A Rudder Bulb can form part of the new rudder design, so that the bulb contour and the flow between propeller hub and rudder are investigated together. The existing shaft line and stern may continue to impose constraints, but some of the original design freedom is regained.
In a major modernization, even the propeller position, shaft line and stern structure can be changed. This may provide additional design freedom, but usually requires much more structural engineering than fitting a separate ESD. Modifications to the rudder supports, drivetrain and ship structure may also become necessary.
That additional freedom therefore brings more engineering and implementation risks. A new propeller or rudder structure may require additional calculations, approvals and work. It must also be established whether the potential additional hydrodynamic benefit justifies the more extensive modification and the available downtime. An integrated retrofit is therefore not automatically the better choice.
The decisive factor is therefore not solely whether a ship is being newly built or is already in service. What matters more is which design choices can still be changed, how reliable the initial data are and what technical consequences the desired modifications have.
A limited retrofit can be an excellent solution if the existing propulsion arrangement is already well matched and an ESD can specifically reduce the remaining losses. An integrated newbuild design or major retrofit may become more attractive where joint modifications demonstrably offer greater benefit. In both cases, the power improvement that is technically feasible and substantiated with sufficient reliability determines the preferred design approach.
This Article in the Series
While additional measurements alongside CFD can substantiate predicted hydrodynamic performance, further development also requires confidence in structural suitability. Where load transfer is complex or stresses are critical, FEM analysis for the structural integration of an ESD may be needed for that purpose. These technical considerations form part of the integration, service life and regulations for ESDs.