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Pre-Swirl Stator (PSS)
A Pre-Swirl Stator (PSS) is an Energy Saving Device (ESD) with fixed stator fins at the stern, ahead of the ship propeller. The fins impart a directed rotational component to the inflow, known as pre-swirl, through which rotational losses in the propeller slipstream can be reduced. A suitable design can lower the required propeller power at the same ship speed and thereby reduce fuel consumption and the associated CO2 emissions. In suitable applications, a fuel saving of approximately 3–5% is a realistic indication; the actual benefit is determined by factors including the wake field, propeller configuration, operating conditions and operational profile. The shape and position of the stator fins are therefore matched to the flow behind the hull and the interaction between hull, stator, propeller and rudder. In cooperation with a specialist international partner, we support shipping companies, technical managers and shipyards in selecting a PSS and coordinating the project, from the initial feasibility assessment and hydrodynamic analysis through design and engineering, production and delivery to preparation for implementation on board.
Available worldwide
How Pre-Swirl Influences Propeller Operation
The propeller accelerates the water and also sets it in rotation. Part of the supplied energy therefore remains in the propeller slipstream as rotational energy. A PSS imparts a rotational component to the water ahead of the propeller that counteracts the rotation generated by the propeller. The propeller blades are consequently subjected to flow at a different angle, changing their loading and the relationship between thrust and required power.
The stator fins can also influence axial velocity: the velocity of the water in the ship’s longitudinal direction. They also cause flow losses of their own and change the pressure distribution around the stern. During design, the use of pre-swirl and these additional effects are assessed together. The selected flow modification must thus contribute to lower required power for the complete propulsion configuration.
The Wake Field Determines the Position and Shape of the Stator Fins
The stator fins are designed for the local flow at their mounting position. The stern shape and existing appendages influence how the water flows there. Velocity and flow direction may also vary along an individual fin. The number, length, profile shape, setting angle and position are therefore selected together. The geometry may differ between fins to suit the local inflow.
The propeller geometry and direction of rotation determine how the changed inflow is used. If the existing propeller is retained, the PSS is matched to it. If a new propeller is also being considered, the two are developed together. For use with a controllable pitch propeller, the pitch settings and combinations of rotational speed and load used are also considered in the matching.
Operation in laden and ballast conditions, as well as differences in speed and trim, may result in different inflow conditions. The design choice is therefore based on operating conditions that account for a significant part of actual service. Data from a comparable ship may support the initial investigation. The final stator geometry and expected performance are developed for the ship’s own wake field and propeller.
When Does a PSS Offer Sufficient Hydrodynamic Potential?
A PSS may be particularly attractive if the inflow to the propeller offers sufficient potential for hydrodynamic improvement. This may be the case, for example, if the stern causes an uneven distribution of water velocity and flow direction and directed pre-swirl can improve propeller operation. Sufficient space must also be available around the stern to place the stator fins in technically suitable positions.
Suitability is therefore determined more by the wake field, propeller loading and available installation space than by ship type alone. Two ships in the same market segment may have very different potential for improvement because of differences in stern, propeller, draught and operational profile.
An initial hydrodynamic assessment therefore maps how the existing inflow is distributed across the propeller plane, which loss mechanisms are relevant and whether stator fins can address them specifically. The operating points at which the ship spends most hours also matter. It can thus be established early whether further development of a PSS offers sufficient potential or whether another propulsion measure better addresses the losses identified.
Assessing PSS, Pre-Duct and Optimization Around the Propeller Hub
In a PSS, the stator fins mainly influence the inflow direction. A Pre-Duct uses duct geometry to change the inflow to the propeller and can combine that effect with fins that generate pre-swirl. The operating principles may therefore overlap. A PSS can also influence the axial velocity distribution. The comparison focuses on the required flow modification, propeller loading and the effect of the complete design on power.
A Propeller Boss Cap Fins (PBCF) device targets the flow around the propeller hub and the vortex at the hub, also known as the hub vortex. A Rudder Bulb influences the interaction between that hub flow and the rudder. The PSS changes the flow reaching these components and may thereby affect their operation. For a combination, the combined effect on power is therefore calculated, taking the interaction into account.
The condition of the existing plant is the starting point. If the propeller is damaged, fouling is severe or the propeller is less well matched to the operational profile, the necessary repair or modification is established first. That choice provides the basis for assessing the additional value of a PSS.
Assessing the PSS Design with the Hull, Propeller and Rudder
Computational Fluid Dynamics (CFD) can show the flow around the stern and the interaction between the stator and rotating propeller. Different fin shapes, setting angles and positions are compared for their influence on the local flow and required propulsion power. The forces on the stator and the changed inflow to the rudder also count. The improvement at the propeller and any additional resistance elsewhere in the same configuration are thus assessed together.
For the power comparison, the self-propulsion point is established, internationally referred to as self-propulsion. The propulsive and opposing forces are then in equilibrium at the ship speed being investigated. The designs with and without a PSS are compared at the same speed and corresponding draught, trim and ship condition. The required rotational speed may differ. Torque and rotational speed together determine how much power is delivered to the propeller. For a controllable pitch propeller, the pitch setting used is also recorded.
A calculation of the propeller in uniform inflow can provide insight into its individual characteristics. For the ship’s performance, that information is linked to the interaction between hull, stator, propeller and rudder. The selected calculation or test method must represent this interaction sufficiently to substantiate the power difference at the same ship speed.
Including Cavitation and Fluctuating Loads in the Design Choice
The changed inflow influences how the propeller blades are loaded during each revolution. Alongside average power, relevant changes in cavitation, pressure pulses and vibration excitation are therefore investigated. Where conditions warrant it, cavitation and fluctuating loads around the stator fins are also assessed.
Propeller loading, immersion and operating conditions determine which situations require attention. Ballast operation may, for example, call for a different cavitation assessment from laden operation. The design choice therefore combines the expected power benefit with acceptable cavitation behaviour and suitable hydrodynamic and structural loads. The necessary calculations and tests are tailored to these considerations and the performance that must be substantiated.
From Calculation to Full-Scale Performance
The model-scale wake field differs from that of the actual ship. Among other factors, the thickness of the boundary layer along the hull and the local velocity distribution change with scale. Because each stator fin is matched to the local inflow, these differences may affect the predicted benefit. During extrapolation, the axial flow and generated pre-swirl are therefore assessed with attention to their different scaling behaviour.
Full-scale calculations can reduce some of the need for scale extrapolation. Their outcome depends on factors including the turbulence model, computational mesh resolution, propeller modelling and assumed surface roughness. Comparison with suitable model tests or other measurement data helps to assess the calculation. The uncertainty is compared with the predicted power difference and taken into account in the design decision.
After installation, the achieved effect can be assessed using sea-trial or operational data. Measurements of speed, power and rotational speed are linked to loading, draught, trim and relevant weather and sailing conditions. For a retrofit, work undertaken during the same docking is also recorded. Hull cleaning, a new coating or propeller maintenance may also lower the required power. The reference and measurement setup are adjusted accordingly so that the PSS contribution can be assessed separately as far as possible. Calculated expectations and measured results remain clearly linked to their respective conditions and assessment methods.
Structural Integration of Stator Fins in Retrofit and Newbuild
The stator fins transfer their hydrodynamic forces and moments through the attachments to the stern. Their shape and mounting position are therefore developed together with the adjoining ship structure. The assessment covers strength, stiffness, fatigue and possible resonance under the relevant operational loads.
Where necessary, stresses and deformations are investigated using the method known internationally as the Finite Element Method (FEM). Suitable load cases are used for the fins, connections and surrounding structure. Clearance from the propeller and accessibility for inspection and maintenance also form part of the integration.
For a retrofit, the propeller position, stern structure and existing appendages are largely fixed. The required reinforcements, installation work and access are matched to the available docking period. For a newbuild, the hull, stator, propeller and rudder can be designed together at an earlier stage. In both cases, the technical documentation and structural design are tailored to the ship-specific assessment and applicable classification requirements.
From Propeller Power to Annual Fuel Savings
The annual contribution of a PSS follows from the power difference at the speeds and loading conditions in which the ship actually operates. The calculated or measured differences are weighted by the corresponding operating hours. Both favourable and less favourable operating points contribute. The proportion of the design benefit that can be used in actual service thus becomes apparent.
To translate this into fuel consumption, losses in the drivetrain and the engine’s specific fuel consumption at the changed load are taken into account. The difference in propeller power is thus linked to the fuel consumption of the propulsion plant. The expected CO2 reduction follows from the calculated difference in fuel consumption and the type of fuel used.
The economic assessment weighs the expected annual saving against design, engineering, production, ship modifications, installation and any additional docking time. Maintenance and the ship’s remaining period of operation also count. Different scenarios for the operational profile and achieved saving show under which conditions the investment offers sufficient potential.
If the propeller, rudder or operational profile is subsequently changed, how the stator matches the changed inflow and propeller loading is reassessed. It is also established in this way which previous performance expectations remain usable and what further development is needed.
Pre-Swirl Stator (PSS)
In cooperation with a specialist international partner, we support shipping companies, technical managers and shipyards in selecting a PSS and coordinating the project, from the initial feasibility assessment and hydrodynamic analysis through design and engineering, production and delivery to preparation for implementation on board.
What Is a Pre-Swirl Stator (PSS)?
A Pre-Swirl Stator (PSS) is an Energy Saving Device (ESD) with fixed stator fins at the stern, ahead of the ship propeller. The fins impart a directed rotational component to the inflow, known as pre-swirl, through which rotational losses in the propeller slipstream can be reduced.
A suitable design can lower the required propeller power at the same ship speed and thereby reduce fuel consumption and the associated CO2 emissions.
In suitable applications, a fuel saving of approximately 3–5% is a realistic indication; the actual benefit is determined by factors including the wake field, propeller configuration, operating conditions and operational profile.
The shape and position of the stator fins are therefore matched to the flow behind the hull and the interaction between hull, stator, propeller and rudder.
How Pre-Swirl Influences Propeller Operation
The propeller accelerates the water and also sets it in rotation. Part of the supplied energy therefore remains in the propeller slipstream as rotational energy. A PSS imparts a rotational component to the water ahead of the propeller that counteracts the rotation generated by the propeller. The propeller blades are consequently subjected to flow at a different angle, changing their loading and the relationship between thrust and required power.
The stator fins can also influence axial velocity: the velocity of the water in the ship’s longitudinal direction. They also cause flow losses of their own and change the pressure distribution around the stern. During design, the use of pre-swirl and these additional effects are assessed together. The selected flow modification must thus contribute to lower required power for the complete propulsion configuration.
The Wake Field Determines the Position and Shape of the Stator Fins
The stator fins are designed for the local flow at their mounting position. The stern shape and existing appendages influence how the water flows there. Velocity and flow direction may also vary along an individual fin. The number, length, profile shape, setting angle and position are therefore selected together. The geometry may differ between fins to suit the local inflow.
The propeller geometry and direction of rotation determine how the changed inflow is used. If the existing propeller is retained, the PSS is matched to it. If a new propeller is also being considered, the two are developed together. For use with a controllable pitch propeller, the pitch settings and combinations of rotational speed and load used are also considered in the matching.
Operation in laden and ballast conditions, as well as differences in speed and trim, may result in different inflow conditions. The design choice is therefore based on operating conditions that account for a significant part of actual service. Data from a comparable ship may support the initial investigation. The final stator geometry and expected performance are developed for the ship’s own wake field and propeller.
When Does a PSS Offer Sufficient Hydrodynamic Potential?
A PSS may be particularly attractive if the inflow to the propeller offers sufficient potential for hydrodynamic improvement. This may be the case, for example, if the stern causes an uneven distribution of water velocity and flow direction and directed pre-swirl can improve propeller operation. Sufficient space must also be available around the stern to place the stator fins in technically suitable positions.
Suitability is therefore determined more by the wake field, propeller loading and available installation space than by ship type alone. Two ships in the same market segment may have very different potential for improvement because of differences in stern, propeller, draught and operational profile.
An initial hydrodynamic assessment therefore maps how the existing inflow is distributed across the propeller plane, which loss mechanisms are relevant and whether stator fins can address them specifically. The operating points at which the ship spends most hours also matter. It can thus be established early whether further development of a PSS offers sufficient potential or whether another propulsion measure better addresses the losses identified.
Assessing PSS, Pre-Duct and Optimization Around the Propeller Hub
In a PSS, the stator fins mainly influence the inflow direction. A Pre-Duct uses duct geometry to change the inflow to the propeller and can combine that effect with fins that generate pre-swirl. The operating principles may therefore overlap. A PSS can also influence the axial velocity distribution. The comparison focuses on the required flow modification, propeller loading and the effect of the complete design on power.
A Propeller Boss Cap Fins (PBCF) device targets the flow around the propeller hub and the vortex at the hub, also known as the hub vortex. A Rudder Bulb influences the interaction between that hub flow and the rudder. The PSS changes the flow reaching these components and may thereby affect their operation. For a combination, the combined effect on power is therefore calculated, taking the interaction into account.
The condition of the existing plant is the starting point. If the propeller is damaged, fouling is severe or the propeller is less well matched to the operational profile, the necessary repair or modification is established first. That choice provides the basis for assessing the additional value of a PSS.
Assessing the PSS Design with the Hull, Propeller and Rudder
Computational Fluid Dynamics (CFD) can show the flow around the stern and the interaction between the stator and rotating propeller. Different fin shapes, setting angles and positions are compared for their influence on the local flow and required propulsion power. The forces on the stator and the changed inflow to the rudder also count. The improvement at the propeller and any additional resistance elsewhere in the same configuration are thus assessed together.
For the power comparison, the self-propulsion point is established, internationally referred to as self-propulsion. The propulsive and opposing forces are then in equilibrium at the ship speed being investigated. The designs with and without a PSS are compared at the same speed and corresponding draught, trim and ship condition. The required rotational speed may differ. Torque and rotational speed together determine how much power is delivered to the propeller. For a controllable pitch propeller, the pitch setting used is also recorded.
A calculation of the propeller in uniform inflow can provide insight into its individual characteristics. For the ship’s performance, that information is linked to the interaction between hull, stator, propeller and rudder. The selected calculation or test method must represent this interaction sufficiently to substantiate the power difference at the same ship speed.
Including Cavitation and Fluctuating Loads in the Design Choice
The changed inflow influences how the propeller blades are loaded during each revolution. Alongside average power, relevant changes in cavitation, pressure pulses and vibration excitation are therefore investigated. Where conditions warrant it, cavitation and fluctuating loads around the stator fins are also assessed.
Propeller loading, immersion and operating conditions determine which situations require attention. Ballast operation may, for example, call for a different cavitation assessment from laden operation. The design choice therefore combines the expected power benefit with acceptable cavitation behaviour and suitable hydrodynamic and structural loads. The necessary calculations and tests are tailored to these considerations and the performance that must be substantiated.
From Calculation to Full-Scale Performance
The model-scale wake field differs from that of the actual ship. Among other factors, the thickness of the boundary layer along the hull and the local velocity distribution change with scale. Because each stator fin is matched to the local inflow, these differences may affect the predicted benefit. During extrapolation, the axial flow and generated pre-swirl are therefore assessed with attention to their different scaling behaviour.
Full-scale calculations can reduce some of the need for scale extrapolation. Their outcome depends on factors including the turbulence model, computational mesh resolution, propeller modelling and assumed surface roughness. Comparison with suitable model tests or other measurement data helps to assess the calculation. The uncertainty is compared with the predicted power difference and taken into account in the design decision.
After installation, the achieved effect can be assessed using sea-trial or operational data. Measurements of speed, power and rotational speed are linked to loading, draught, trim and relevant weather and sailing conditions. For a retrofit, work undertaken during the same docking is also recorded. Hull cleaning, a new coating or propeller maintenance may also lower the required power. The reference and measurement setup are adjusted accordingly so that the PSS contribution can be assessed separately as far as possible. Calculated expectations and measured results remain clearly linked to their respective conditions and assessment methods.
Structural Integration of Stator Fins in Retrofit and Newbuild
The stator fins transfer their hydrodynamic forces and moments through the attachments to the stern. Their shape and mounting position are therefore developed together with the adjoining ship structure. The assessment covers strength, stiffness, fatigue and possible resonance under the relevant operational loads.
Where necessary, stresses and deformations are investigated using the method known internationally as the Finite Element Method (FEM). Suitable load cases are used for the fins, connections and surrounding structure. Clearance from the propeller and accessibility for inspection and maintenance also form part of the integration.
For a retrofit, the propeller position, stern structure and existing appendages are largely fixed. The required reinforcements, installation work and access are matched to the available docking period. For a newbuild, the hull, stator, propeller and rudder can be designed together at an earlier stage. In both cases, the technical documentation and structural design are tailored to the ship-specific assessment and applicable classification requirements.
From Propeller Power to Annual Fuel Savings
The annual contribution of a PSS follows from the power difference at the speeds and loading conditions in which the ship actually operates. The calculated or measured differences are weighted by the corresponding operating hours. Both favourable and less favourable operating points contribute. The proportion of the design benefit that can be used in actual service thus becomes apparent.
To translate this into fuel consumption, losses in the drivetrain and the engine’s specific fuel consumption at the changed load are taken into account. The difference in propeller power is thus linked to the fuel consumption of the propulsion plant. The expected CO2 reduction follows from the calculated difference in fuel consumption and the type of fuel used.
The economic assessment weighs the expected annual saving against design, engineering, production, ship modifications, installation and any additional docking time. Maintenance and the ship’s remaining period of operation also count. Different scenarios for the operational profile and achieved saving show under which conditions the investment offers sufficient potential.
If the propeller, rudder or operational profile is subsequently changed, how the stator matches the changed inflow and propeller loading is reassessed. It is also established in this way which previous performance expectations remain usable and what further development is needed.
From Initial Hydrodynamic Analysis to a PSS Project
For the existing coastal vessels Marit and Myrte, the potential for hydrodynamic improvement in the propulsion configuration was investigated using a CFD-based Express analysis. An expected improvement in propulsion efficiency of more than 4% was calculated for both ships.
This outcome provided a sufficient technical and economic basis to develop the Pre-Swirl Stator further for both ships and ultimately install it.
Approach to Selecting and Developing a Pre-Swirl Stator
In the first project phase, it is established whether the PSS will be matched to the existing propeller or form part of a combined propulsion modification. The reference configuration, relevant operating points and necessary substantiation are recorded. The hydrodynamic analysis and stator design are developed further on that basis.
For a promising fin geometry, the hydrodynamic shape and structural integration are brought together. The technical proposal describes which components and work are included, the operating conditions on which the expected performance is based and which further assessments or tests are needed. The attachment and required modifications to the stern are also defined within that scope.
After the project decision, production, delivery and preparation for the work on board are coordinated. For a retrofit, implementation fits the available docking period and other planned work. If performance verification forms part of the project, the reference data, measurement method and acceptance arrangements are recorded in advance. After installation, the result can then be assessed on the agreed basis.
Have Your Pre-Swirl Stator Project Assessed
Are you considering a PSS for an existing ship or newbuild project? We would be pleased to discuss the opportunities that the stern and propeller configuration offer for targeted hydrodynamic improvement. For a technical assessment, we would welcome hull lines or usable three-dimensional hull geometry, stern drawings and data on the propeller and rudder.
For the propeller, relevant details include diameter, blade geometry, direction of rotation and usual rotational speeds. For a controllable pitch propeller, we would also welcome the pitch settings and loading conditions used. Speed and power data are linked to the corresponding draught, trim and loading. The operating hours in each sailing condition indicate which conditions should carry the greatest weight. Available wake measurements, model tests or previous CFD results can support the assessment.
For a retrofit, structural drawings around the possible attachment points, planned propeller or rudder modifications and the intended docking period are relevant to installation. During the project discussion, we agree on what further data are needed and how the hydrodynamic and structural design work can fit your ship’s schedule.
From Initial Hydrodynamic Analysis to a PSS Project
For the existing coastal vessels Marit and Myrte, the potential for hydrodynamic improvement in the propulsion configuration was investigated using a CFD-based Express analysis. An expected improvement in propulsion efficiency of more than 4% was calculated for both ships.
This outcome provided a sufficient technical and economic basis to develop the Pre-Swirl Stator further for both ships and ultimately install it.