How Do Fouling, Corrosion and Damage Affect ESD Performance?
Fouling, corrosion and mechanical damage can change the performance of an Energy Saving Device (ESD) because its surface condition, geometry or structural integrity is degraded. A Pre-Swirl Stator (PSS), Pre-Duct, Propeller Boss Cap Fins (PBCF) device or Rudder Bulb is designed to interact favourably with the propeller and rudder under particular flow conditions. Fouling can increase surface roughness and change the flow around a profile. Corrosion can affect both the surface and the strength of components and connections, while mechanical damage can alter the shape, position or attachment of the device. The consequences range from a limited change in hydrodynamic performance to a structural risk requiring immediate technical assessment. An ESD may also function differently when the hull or propeller becomes fouled, even if the device itself is unaffected. A reliable assessment therefore relates the condition of the ESD to the surrounding propulsion components and changes in ship performance. A distinction must be made between loss of hydrodynamic benefit and degradation that threatens the safe operation of the structure.
How Does Fouling Change the Hydrodynamic Performance of an ESD?
An ESD is designed for a particular profile geometry and surface condition. Flow along these surfaces helps determine the hydrodynamic forces that arise and how the device interacts with the propeller. When organisms attach to the surface, both the resistance of the device and the flow it influences may change.
Extensive fouling is not always needed for this to happen. Even a thin biological layer, often called microfouling, can change surface roughness and influence the boundary layer. With larger organisms and thicker deposits, the effective profile shape may also change. The distinction matters: a rougher surface can cause additional frictional losses, while local fouling around a profile edge may also influence pressure distribution and flow separation.
The hydrodynamic influence of fouling and surface roughness has been investigated for ship hulls, propellers and various coating surfaces. This does not, however, directly establish how much performance loss a thin biological layer causes on a particular ESD. That depends on factors including roughness, fouling distribution, profile geometry and local flow conditions.
For a Pre-Swirl Stator, this affects the way the fins generate pre-swirl. The original profile shape and setting angle are matched to the propeller inflow. Fouling can change the hydrodynamic forces on the fins, so that the pre-swirl generated may be less well suited to propeller operation. The disadvantage may then extend beyond additional resistance from the stator itself: the relative inflow and loading of the propeller blades may also change.
For a Pre-Duct, the emphasis is on guiding flow towards the propeller plane. The pressure distribution on the duct profile helps determine the forces the device itself generates. If fouling changes this distribution, the combined result of duct and propeller may become less favourable. Deposits that disturb the flow around profile edges therefore deserve particular attention.
The consequences cannot be inferred directly from the amount of fouling. Two surfaces with similar fouling coverage may respond differently hydrodynamically if the fouling occurs in different locations. The shape, thickness and roughness of the deposits also matter. Limited fouling on a profile edge sensitive to flow may therefore be more important than a larger deposit on a less critical surface.
Nor does every change in surface condition lead to a proportional change in energy savings. An ESD may still provide a benefit over a configuration without the device despite some fouling. The changed flow may also influence several propulsion components. The magnitude and direction of the net effect must therefore be assessed for the configuration concerned.
Why Does Susceptibility to Fouling Differ by ESD Type?
The four ESD types operate in different flow regions and have different geometries. The hydrodynamically important surfaces, locations where fouling may develop and possibilities for inspecting or cleaning the device therefore differ.
For a Pre-Swirl Stator, the fin profiles and their connection to the stern deserve particular attention. The leading edges and profile shape influence how the stator fins deflect the flow. Around the fin root, local flow may differ from that along the free profile. Fouling in this area may therefore have different hydrodynamic significance from fouling on the middle part of the fin. During inspection, it matters whether the original contour and position of the fins have been preserved.
A Pre-Duct has both internal and external surfaces in contact with the water. The internal profile influences flow towards the propeller, while the pressure distribution over the entire duct contributes to hydrodynamic forces. The supports and connections to the stern may also be locations where fouling accumulates. An inspection covering only the clearly visible exterior may therefore give an incomplete picture of the condition.
A Propeller Boss Cap Fins device is located on the rotating propeller hub. Its fins operate in the flow behind the propeller blades and influence, among other things, the development of the hub vortex. Fouling or deposits may change the local profile shape and hydrodynamic loading. Rotation and local flow speed make the exposure different from that of fixed devices. This does not, however, mean that a PBCF automatically stays clean during operation. Biological fouling can also develop on moving submerged surfaces. Prolonged inactivity may encourage its development. Which deposits remain after the propeller is used again depends on factors including their adhesion, surface condition and operating conditions. Without ship-specific data, a general ranking of fouling susceptibility between PBCFs and fixed ESDs is not justified.
For a Rudder Bulb, the streamlined contour and transition to the rudder are particularly important. The bulb operates in the flow between propeller hub and rudder. Fouling on the bulb may influence the local pressure distribution, while fouling on the adjoining rudder blade may also affect their combined performance. Both surfaces must therefore be assessed together.
The operational profile also matters. A shortsea vessel regularly spending extended periods in port may experience different fouling conditions from a seagoing ship operating at sea for long periods. Water temperature, salinity, trading area, coating condition and periods of inactivity influence which organisms can develop. Local flow around appendages and their attachments may also differ from that along the large hull surfaces.
ESD type is therefore not the only important factor for inspection. The actual geometry, local flow conditions, surface protection and maintenance history determine which components deserve particular attention. This justifies targeted inspection of the device, but not an assumption that a particular ESD type becomes fouled more quickly or slowly under all conditions.
How Do Corrosion and Coating Damage Affect Performance?
Corrosion may impair the hydrodynamic performance of an ESD, but is above all a separate concern for structural reliability. Surface corrosion, pitting and local material loss may occur on metal components. The consequences depend on the material used, conditions and corrosion protection present.
If corrosion roughens the surface or degrades the original profile contour, flow along the device may change. Material loss may influence hydrodynamic properties particularly on relatively thin fin profiles and carefully shaped edges. The extent of any deterioration in power performance does not, however, follow directly from visible degradation. For this, an assessment must be made of how the modified geometry interacts with the complete propulsion system.
Nor is every pit or instance of surface degradation caused by corrosion. Cavitation erosion, abrasive particles and mechanical damage may also cause material loss. These degradation mechanisms may occur together under some conditions. If the cause is unclear, it must be investigated before a repair method is chosen. A surface treatment that counteracts corrosion does not, for example, automatically resolve an ongoing cavitation problem.
Structurally, the situation may be more serious than the visible change in profile shape suggests. For a Pre-Swirl Stator, hydrodynamic forces are transferred through the fins and fin attachments to the stern. Corrosion around a welded connection or fin root may reduce effective material thickness and further weaken a detail susceptible to fatigue. Local degradation may also be important at Pre-Duct supports, even when the duct still appears to perform almost normally hydrodynamically.
A PBCF requires attention to the boss cap, fins and the way the complete device is connected to the rotating propeller structure. The material and surface treatment may differ from those of the hull. Some PBCFs, for example, are made from a copper alloy and have no conventional protective coating. The assessment must therefore start from the actual material combination and structure, rather than a general assumption about corrosion protection.
When different metals are electrically connected and exposed to seawater, galvanic corrosion may be a concern. Whether that risk actually exists depends on factors including the material combination, electrical connection, ratio of exposed surface areas and protection present. During retrofit, it must therefore be checked whether the new device is compatible with the existing corrosion protection strategy.
For a Rudder Bulb, the connection to the rudder and condition of the adjoining structure are particularly important. Damaged coating may expose metal locally and allow further degradation. During repair, the coating system and surface finish must suit the material, flow loading and requirements for the design concerned.
Coating damage does not automatically mean that the underlying material is already seriously degraded. Conversely, local material loss may be present without being immediately apparent from the outside. Further inspections are needed if remaining thickness or strength is uncertain.
For a technical manager, the distinction between hydrodynamic and structural consequences is particularly important. An ESD may still provide a favourable power saving even when its attachment or material condition is no longer sufficiently reliable. Restoring structural safety then takes priority over assessing a possible loss of efficiency.
What Are the Consequences of Mechanical Damage and Deformation?
Mechanical damage may change the hydrodynamic performance of an ESD through loss of its original profile shape, position or alignment. Damage may result from contact with objects in the water, work on the underwater hull or loads exceeding those for which the structure was designed.
A bent stator fin may, for example, generate different pre-swirl from the original geometry. The fin need not be completely damaged for this to happen. A changed setting angle or deformed profile edge may already influence the distribution of inflow towards the propeller. Hydrodynamic loading on the fin and its attachment may also change as a result.
For a Pre-Duct, deformations of the duct profile and support structure may influence flow guidance. A dented profile or displaced support may also change the distance to the propeller. If the available clearance becomes smaller as a result, a structural and operational concern also arises alongside a possible loss of efficiency.
Damage to a PBCF deserves particular care because of its rotating design. A deformed or cracked fin, or one that has partly broken off, may change flow around the hub. Depending on the damage, the mass distribution, balance and fluctuating mechanical loads of the rotating configuration may also change. The consequences for the boss cap, fasteners and other parts of the propeller structure must then be assessed separately.
For a Rudder Bulb, damage to the contour or connection may change the flow between propeller hub and rudder. The required clearance for rudder movements must also be maintained. If the bulb or rudder is deformed, not only the hydrodynamic shape but also loading on the rudder structure and the possible influence on manoeuvring behaviour deserve attention.
Not all damage is immediately visible. Cracks around welded connections, loosening attachments or local weakening of material may be present even when the external contour still appears largely intact. If such damage is indicated, targeted inspections and suitable non-destructive testing methods may be needed.
The severity of mechanical damage must therefore not be inferred solely from changes in fuel consumption or shaft power. A structural risk may already be present before a reliably measurable performance loss occurs.
If a damaged ESD could lose components, strike the propeller or rudder, or be insufficiently strong for expected loads, its technical suitability must be assessed before continued normal use can be considered acceptable. The measures immediately needed depend on the nature, location and severity of the damage.
For less severe deviations, a targeted assessment of remaining geometric and structural suitability may be sufficient. It must then be established whether the original function is retained and whether repair is necessary. No general limit for permissible deformation or damage can be given for all ESD types.
How Do You Distinguish ESD Degradation from Deteriorating Hull and Propeller Performance?
Higher power demand during operation may indicate deteriorating hydrodynamic performance, but does not identify the component responsible. Hull fouling may increase resistance, a rough or damaged propeller may operate less efficiently, and the original contribution from a degraded ESD may partly disappear.
These changes also influence each other. If the hull becomes fouled, resistance may increase and flow around the stern may change. A Pre-Swirl Stator or Pre-Duct may consequently operate under different inflow conditions. The device itself need not be fouled or damaged to function differently.
The reverse is also possible. An ESD may show local fouling or damage while the rest of the hull is relatively clean. An inspection that focuses only on large hull surfaces may therefore miss a relevant problem around the propeller plane.
Targeted diagnosis therefore relates changes in performance to the condition of hull, propeller, ESD and rudder. For the ESD, the location and severity of fouling, surface condition, any profile deformations and condition of the attachments are recorded. Photographs, previous inspection reports and, where needed, geometric measurements help identify changes from the original configuration.
The available speed–power data must also be assessed on a comparable basis. Differences in draught, trim, sailing speed, wind and waves may cause higher power demand without a change in technical condition. For a Controllable Pitch Propeller (CPP), the blade pitch settings and rotational speeds used are also relevant.
If only an ESD with fouling is cleaned and the remaining condition stays sufficiently similar, a suitable before-and-after measurement may indicate the effect of the fouling removed. The reliability of that conclusion depends on measurement accuracy and any other changes during the same period.
If the hull and propeller are also cleaned during a dry-docking, the measured improvement describes the combined result of that work. The separate recovery of ESD performance cannot be inferred from it without further substantiation.
There is an important distinction here. A comparison before and after ESD cleaning can help establish how much power demand has changed through restoration. This is not automatically the same as the total energy savings the ESD provides compared with a ship without the device. For the latter question, a suitable reference without an ESD is needed.
Nor does an observed performance improvement after cleaning automatically prove which hydrodynamic mechanism has changed. A cleaned stator may, for example, cause less resistance of its own while also changing propeller inflow. Additional calculations or measurements may be needed to explain these contributions separately.
The technical conclusion must therefore reflect what has actually been established. Inspection can reveal degradation and support a hydrodynamic explanation. A reliable power comparison can establish a change in performance. To attribute its magnitude exclusively to the ESD, the other influences must be sufficiently well known.
How Is an ESD Maintained Without Damaging Its Hydrodynamic Performance?
Maintenance of an ESD must preserve the original profile shape, surface protection and structural reliability as far as possible. The choice of cleaning or repair method therefore depends on the material, geometry and degradation present.
For a Pre-Swirl Stator or Pre-Duct, profile edges, fin roots, supports and transitions that are difficult to reach are particularly important. Fouling must be removed using a method suitable for the surfaces concerned. A cleaning agent or mechanical treatment that works well on a large hull surface may unintentionally damage a thin profile or vulnerable coating.
Local accessibility also deserves attention. The interior of a duct and its connections to the stern may offer different cleaning possibilities from its exterior. For a stator, the fins and attachments must be able to be properly inspected without unnecessary damage to surfaces or adjoining propeller components.
For a PBCF, the maintenance method must suit the boss cap, material and fin design. Some designs can be cleaned or polished as part of propeller maintenance. This does not mean that every PBCF requires the same surface treatment. The manufacturer’s instructions and design data remain decisive. It must also be ensured that cleaning does not adversely affect the original profile shape or the condition of the attachment.
A Rudder Bulb is maintained together with the adjoining rudder. The contour, connection, any coating and condition of the surrounding rudder structure are assessed. Particularly where damage occurs around the transition between bulb and rudder, it must be established whether surface cleaning alone is sufficient.
For in-water cleaning, environmental requirements also play a part. Organisms, coating particles and other substances may enter the water during fouling removal. The cleaning method, any capture and treatment of removed material, and applicable local regulations must therefore be considered during the work.
International guidelines for biofouling management and in-water cleaning emphasize, among other things, coating protection, management of underwater areas that are difficult to reach, and limiting the spread of organisms and harmful substances. Cleaning without capture is not appropriate in all circumstances. The method that is permissible and technically suitable depends on factors including fouling, the surface and local requirements.
The cleaning method must also suit the surface protection actually present. An ESD may have a different material or coating system from the hull. A method assessed for the hull is therefore not automatically suitable for fins, the boss cap or rudder bulb. Further assessment is needed if coating condition or compatibility with the cleaning system is uncertain.
Maintenance planning must also suit the ship’s use. Prolonged inactivity, frequent port calls and trading area may influence the development of fouling. A fixed cleaning interval is therefore not automatically the best choice for every ship configuration.
Targeted inspections make it possible to match maintenance needs more closely to actual condition. By recording where fouling develops, how coating condition changes and which surfaces prove particularly susceptible, it can be determined when cleaning is needed and when a more extensive technical assessment must take place.
When Is Cleaning Sufficient, and When Is Repair or Replacement Needed?
Cleaning is particularly useful when removable fouling is present and the underlying profile geometry, surface protection and structure are sufficiently intact. Suitable treatment can then improve surface condition without the ESD itself having to be modified.
The original hydrodynamic performance does not automatically return in full as a result. Corrosion pits, permanent deformations and damaged coatings do not disappear when fouling is removed. Inflow may also have changed because hull or propeller condition has deteriorated.
If the coating is damaged, local or more extensive repair may be needed. It must then be established whether the substrate remains suitable and whether the new finish meets the original hydrodynamic and corrosion protection requirements. In cases of demonstrable material loss, an assessment of the remaining strength is also necessary.
If pitting or other degradation has occurred, the mechanism responsible must first be clear. Corrosion, cavitation erosion and mechanical wear may require different repairs. Simply smoothing a damaged surface is insufficient if the cause persists or remaining material thickness is inadequate.
For mechanical damage, repair must also address the original geometry. A bent stator fin or deformed duct profile may still cause unfavourable flow after structural repair if the design contour has not been adequately restored. The same applies to a Rudder Bulb whose position or connection to the rudder has changed.
For a damaged PBCF, fin geometry and the mechanical condition of the rotating structure deserve particular attention. Depending on the design, the boss cap, attachment and mass balance may form part of the assessment. A repair is acceptable only when the repaired configuration corresponds sufficiently to the required geometric and structural condition.
In cases of cracks, substantial corrosion or uncertainty about the attachment, further investigation may be needed. This may comprise non-destructive inspection, thickness measurements or a structural assessment. If local stresses, fatigue or load transfer cannot be adequately assessed using simple methods, an analysis using the finite element method (FEM) may be an appropriate addition.
Replacement becomes an option when the original geometry or structural reliability cannot be responsibly restored. It may also be more attractive if the repair needed becomes extensive or the existing design no longer adequately meets the technical requirements. The final decision depends on the ESD type, damage and possibilities for achieving a safe design.
After repair, the relevant geometry, alignment, attachments and clearances must be checked. If the work is also expected to have a measurable influence on propulsion performance, suitable performance data may help assess the result. A distinction must be made between which work has been carried out and which improvement can be attributed to it.
Fouling, corrosion and mechanical damage affect an Energy Saving Device in different ways. Fouling primarily changes flow along the surfaces, corrosion may affect both hydrodynamic properties and material strength, and mechanical damage may alter geometry and structural reliability. The appropriate measure therefore follows from the nature and severity of degradation, not solely from higher power demand. It can be determined when cleaning is sufficient and when repair or replacement becomes necessary by inspecting ESD condition specifically and relating it to hull, propeller and rudder performance.
This Article in the Series
If corrosion or mechanical damage affects critical connections, FEM analysis of the ESD structure may be needed to assess remaining strength and load-bearing capacity. Whether such defects can be identified and remedied in time depends partly on the accessibility and maintainability of the device. These aspects form part of the technical integration and preservation of ESD performance throughout its service life.