Retrofit, Service Life and Regulations for Energy Saving Devices (ESDs) on Ships
An Energy Saving Device (ESD) can reduce the propulsion power required. To realize that benefit in service, the structure, mounting and maintenance options must suit the ship. In a retrofit, the device is integrated into an existing propulsion system; in a newbuild, the components involved can be developed together at an earlier stage. After installation, the surface condition and structural connections need attention. Later changes to the propeller, control or rudder can also alter the original matching. Where the relevant regulations apply, an ESD may also contribute to EEXI and CII. Technical preparation brings these questions together: which design is feasible, how does it remain serviceable and what assessment is needed when changes occur?
Retrofit and Newbuild Offer Different Design Opportunities
On an existing ship, the stern shape, propeller position and rudder arrangement are usually largely fixed. A Pre-Swirl Stator (PSS) or Pre-Duct requires installation space ahead of the propeller plane and suitable mounting to the stern. For Propeller Boss Cap Fins (PBCF) or a Rudder Bulb, the hub geometry and distance to the rudder are important. The available structure thus limits the feasible shapes and positions.
Which components can still be modified partly determines the design of ESDs for retrofit and newbuild. Early in a newbuild project, the stern, propeller, rudder and ESD can be developed together. This provides greater design freedom, but no guarantee of a larger saving from the ESD alone: an improved hull and propeller may already reduce some of the losses. An extensive retrofit also offers scope for joint development where propeller replacement or rudder modification is planned.
For a retrofit, current performance data help match the device to future service, provided the recorded operational profile remains representative of that service. The drawings must correspond to the actual configuration. Where critical dimensions are uncertain, additional measurements or a 3D scan may be needed. The final geometry is matched to the condition after the planned work, within the available installation space and time window.
Structural Integration and Loads on ESDs
The results of hydrodynamic design and performance validation of ESDs provide insight into the flow and hydrodynamic forces on the device. The structural assessment examines how the ESD, its mountings and the adjoining ship structure carry these loads. A strong fin alone, for example, is not sufficient if the fin root or underlying hull structure is not adequately suited to the task.
For a Pre-Duct, the forces are transferred through the profile and supports to the stern. For a PBCF, rotation-related loads and attachment to the propeller hub matter alongside the hydrodynamic forces. A Rudder Bulb transfers its forces to the rudder and can also change the loading on other rudder components.
Where simple calculations provide insufficient confidence about local stresses, deformations or fluctuating loads, FEM analysis for the structural integration of an ESD can help. This finite element method examines how the structure responds to the specified loads. Depending on the design, fatigue, stability and vibration behaviour also require assessment. The analysis required follows from the structural risk and applicable requirements, not solely from the dimensions of the device.
The loads investigated must also encompass relevant manoeuvres, propeller loads and ship motions. Normal cruising speed is not necessarily the governing condition. Deformations must not compromise the required clearance to the propeller and rudder. Where a stronger fin or additional support is needed, the effect of that modification on the flow is also assessed. In this way, the structural design and hydrodynamic shape remain matched to each other.
Fouling, Corrosion and Damage During the Service Life
The profile shape, position and surface condition may change in service. The effect of fouling, corrosion and damage on ESD performance extends beyond additional drag. Fouling can affect the pre-swirl from stator fins or the way a Pre-Duct guides the flow. Even a clean device may perform differently when hull fouling changes the inflow.
Corrosion and mechanical damage can also damage the structure. For a PBCF, a damaged fin may affect the loading and balance of the rotating assembly. For a Rudder Bulb, the contour, connection to the rudder and available clearances are among the important considerations. An ESD may still offer a hydrodynamic benefit while a mounting is already insufficiently reliable. Conversely, visible fouling does not mean that the saving has been lost entirely.
Inspection helps determine whether cleaning is sufficient or whether restoration of the geometry, surface protection or structure is needed. Where safe operation is in doubt, structural assessment takes priority. An increase in the required shaft power may prompt investigation, but does not on its own identify whether the cause lies with the hull, propeller or ESD.
Maintainability and Accessibility Determine Availability
A technical deviation must be capable of being investigated and, where necessary, rectified. This is why maintenance and accessibility form part of the design considerations. A stator mounting may be visible without sufficient space for closer investigation or repair. On a Pre-Duct, the inner profile and supports may be difficult to reach. The design of a PBCF partly determines what dismantling and repair are possible.
An underwater camera or Remotely Operated Vehicle (ROV) can record visible fouling, deformation and damage. This does not rule out hidden cracks or local material loss. For a critical connection, a different inspection method or a docking may be needed. Access must therefore suit the inspection required for the component concerned, not merely allow it to be viewed.
Cleaning also requires a suitable approach. Thin fin profiles, coatings and precise contours may require different means from those used on large hull surfaces. Underwater cleaning is also subject to the applicable environmental requirements. Good accessibility does not automatically prevent damage, but can make investigation and repair more practicable. Coordination with suitable maintenance opportunities then helps limit unnecessary uncertainty and downtime.
Reassessing an Existing ESD After Propulsion Modifications
A different propeller, a change in the control of a Controllable Pitch Propeller (CPP) or a modified rudder arrangement can alter the performance of an unchanged ESD. The effects on inflow, loads and clearances determine when an ESD must be reassessed after a change to the propulsion configuration. A new propeller may respond differently to the pre-swirl from a stator; for a PBCF, hub geometry and blade loading may also influence the mechanical connection.
The old and new configurations are therefore compared in terms of the relevant geometry, operating settings and loads. Where the new conditions demonstrably remain within the range previously investigated, a targeted check may be sufficient. A technically equivalent replacement need not require a new design. Conversely, a change in control without a visible structural modification may fall outside the scope of the original substantiation.
Technical suitability and the remaining hydrodynamic benefit are assessed separately. An ESD may continue to function safely while the saving decreases. Where the previous substantiation does not cover the new conditions, additional calculations may be needed. Redesign becomes an option when the design is no longer suitable or a modification can offer sufficient additional benefit. Retaining a serviceable device therefore remains an equally valid outcome.
The Contribution of ESDs to EEXI and CII
Lower propulsion power demand can contribute to reduced fuel consumption and CO2 emissions. The Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII) assess different aspects of energy efficiency. For ships to which the relevant rules apply, the potential contribution of an ESD to improving EEXI and CII therefore requires separate calculations.
EEXI describes technical energy efficiency according to a prescribed method. The reference speed at the specified power is one of the factors that matters. Lower shaft power at a constant speed therefore does not automatically produce the same percentage improvement in attained EEXI. An ESD may also be of interest alongside an engine or shaft power limitation where it enables the ship to reach its usual speed with less power. The combined effect on EEXI is assessed according to the applicable method.
CII concerns annual operational CO2 intensity. Lower fuel consumption while underway can reduce attained CII, but consumption by other installations, ship capacity, distance travelled and applicable corrections also matter. A lower CII value does not automatically mean that the ship moves into a more favourable rating category.
Installing an ESD does not in itself make renewed EEXI verification mandatory. Additional requirements may, however, apply to a conversion classed as major under the applicable rules or to a relevant change in an existing power limitation. The nature of the changes determines what assessment and amendments to the technical documentation are needed.
From Design Selection to Implementation and Operation
In preparation for implementation, the selected geometry and mountings are linked to the current structural drawings, the work required and the available time in the ship's schedule. We support this coordination with a specialist international partner, with attention to installation and subsequent access for inspection and maintenance. For a retrofit, the planned docking date may guide implementation; for a newbuild, mountings and maintenance access can be incorporated into the structure at an earlier stage.
The engineering and installation required, maintenance needs and any additional downtime then form the basis for the investment and economic assessment of ESDs. That assessment links the technically feasible solution to the expected saving over the intended period of use.