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Propeller Boss Cap Fins (PBCF)
Propeller Boss Cap Fins (PBCF) are fins fitted to the boss cap behind a ship’s propeller, together forming an Energy Saving Device (ESD). The fins rotate with the propeller and influence the flow around the hub and the formation of the hub vortex. A suitable design can reduce losses in this region and thus lower the required propeller power at the same ship speed. In suitable applications, a fuel saving of around 2% is a realistic indication, with a corresponding reduction in the associated CO2 emissions; higher values may also be possible in specific projects. The shape of the cap and fins is matched to the propeller geometry, local flow, available clearance from the rudder and actual operational profile. In cooperation with a specialist international partner, we support shipping companies, technical managers and shipyards in selecting a PBCF and coordinating the project, from the initial technical assessment and hydrodynamic analysis through design and engineering, production and delivery to preparation for implementation on board.
Available worldwide
Influencing the Hub Vortex and Pressure Behind the Propeller
Vortices originating at the blade roots may merge behind the hub into a concentrated hub vortex. The low pressure in this region contributes to resistance on the boss cap. The PBCF fins influence how these vortices develop and merge. Suitable geometry can increase the pressure behind the cap and thus increase the net thrust of the propeller and boss cap.
The fins also generate their own hydrodynamic forces and influence torque. The design choice therefore weighs the reduction in hub loss against the additional losses. What matters is how much thrust the complete propeller, with its cap and fins, delivers for the required power.
Matching Fins and Boss Cap to the Propeller
Propeller diameter, number of blades, blade geometry near the root, direction of rotation and hub dimensions form the basis of the PBCF design. The height, profile shape and setting angle of the fins are matched to these characteristics, together with their position relative to the propeller blades. The shape and length of the boss cap also influence the flow. Cap and fins are therefore developed together.
The local flow around the hub also depends on the wake field behind the hull: the distribution of water velocity and flow direction in the inflow to the propeller. Rotational speed, propeller loading and ship speed determine the conditions under which the fins operate. The assessment therefore covers the operating points that matter to the ship. When application to a controllable pitch propeller is investigated, the pitch settings used are also taken into account.
If the existing propeller is retained, the improvement achievable with a modified boss cap and fins is investigated. If there is damage or heavy fouling, or the propeller is less well suited to the operational profile, the necessary maintenance or modification is established first. That choice provides the basis for assessing the additional value of a PBCF.
When Does a PBCF Offer Sufficient Hydrodynamic Potential?
A PBCF may be particularly attractive where a relevant proportion of the hydrodynamic losses arises around the propeller hub and the existing boss cap offers scope for targeted improvement of this flow. A distinct hub vortex, low pressure behind the cap or boss cap geometry that could be better matched to the flow at the blade roots may justify further investigation of a PBCF.
Suitability therefore depends less on ship type than on the propeller itself and the flow in which it operates. Two comparable ships may have different potential for improvement because of differences in propeller geometry, hub ratio, boss cap, rotational speed, loading and rudder arrangement.
The space available behind the propeller is also decisive. The PBCF fins and modified cap must retain sufficient clearance from the rudder or any Rudder Bulb fitted. At the same time, the geometry must be matched to the propeller so that the reduction in hub loss offsets the hydrodynamic loading and resistance of the fins themselves.
An initial assessment therefore brings together the existing propeller and hub geometry, local flow, rudder arrangement and main operating points. It can thus be established early whether optimization around the boss cap offers sufficient potential or whether another modification to the propeller, inflow or rudder addresses the dominant loss more directly.
PBCF and Rudder Bulb in the Same Hub Flow
A PBCF rotates with the propeller and acts directly around the boss cap. A Rudder Bulb is attached to the rudder and influences the transition of the flow from the hub towards the rudder. Both can act on the same hub vortex. Selection therefore connects the local loss mechanism with the existing cap shape, rudder geometry and distance between them.
If the rudder already has a favourable influence on the hub vortex, the additional contribution of a PBCF may differ from that found when assessing the propeller without a rudder. A combination is therefore investigated jointly for its hydrodynamic performance and available space. The effect on power is established for the complete arrangement, taking the interaction into account.
A change in inflow ahead of the propeller may also alter the flow around the hub. In combination with other ESDs, the modified hull–propeller–rudder configuration is therefore used as the starting point. The PBCF is matched to the flow in which the cap and fins must actually function.
Assessing Hub Flow, Thrust and Torque Together
Computational Fluid Dynamics (CFD) can be used to investigate the flow around the blade roots, boss cap and fins. Variations in cap shape, fin profile, dimensions and position are compared for their influence on pressure distribution, vortex formation and propeller loading. It thus becomes apparent how the geometry affects efficiency.
The power assessment includes thrust and torque on the propeller blades, cap and fins in the same comparison. The change in the local vortex pattern is thus linked to the performance of the complete propeller configuration. A change in thrust is assessed together with the corresponding torque and rotational speed.
The existing boss cap forms part of the reference configuration. If the cap shape changes as well as the fins, the comparison covers the full modification. The stated performance then relates to the combined modification of cap and fins.
From Propeller Testing to Required Power on Board
An open-water test investigates the propeller under specified inflow conditions, independently of the hull’s influence. Such a test or corresponding calculation can help to compare PBCF variants. To translate the results to the ship, the performance of the selected design in the actual wake field and in combination with the rudder is then investigated. Open-water efficiency is thus linked to the ship’s propulsion performance.
For the ship comparison, the required power with and without a PBCF is established at the same ship speed and corresponding draught, trim and ship condition. The self-propulsion point, at which the propulsive and opposing forces are in equilibrium, is established. The required rotational speed may differ between the two arrangements. 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.
The calculation or test method is tailored to the performance that needs to be substantiated for the project. It remains clear which results concern the propeller alone and which concern the complete propulsion configuration.
Cavitation and Pressure Fluctuations Around the Hub
A suitable PBCF can reduce the hub vortex and associated cavitation. The influence on cavitation on the propeller blades and at the blade tips is assessed separately where it matters to the project. The different cavitation phenomena thus remain linked to the flow regions in which they originate.
The fins may also generate their own vortex structures, which influence the further development of the propeller slipstream. Where relevant, pressure fluctuations, vibration excitation and underwater noise are therefore investigated. The design choice combines the expected power benefit with acceptable cavitation behaviour and suitable loads. The necessary calculations and tests are tailored to the propeller, rudder and relevant operating conditions.
Scale Effects and Verification of the Saving
At model scale, frictional resistance on the fins, pressure behind the cap and inflow from the hull may differ from conditions on the actual ship. These scale differences affect the balance between the reduction in hub loss and the additional fin losses. When translating the results to full scale, both contributions are therefore assessed together.
Full-scale calculations can support this translation. The outcome depends on factors including the flow models used, the representation of local vortices and the input data. Comparison with suitable measurement data helps to assess the calculations. The uncertainty is compared with the magnitude of the predicted power difference and taken into account in the design and investment decision.
After installation, the result can be assessed using sea-trial or operational data. Measurements of speed, power and rotational speed are linked to draught, trim, loading and relevant weather and sailing conditions. For a retrofit, hull cleaning, propeller maintenance and other work during the same docking are also recorded. The reference and measurement setup are adjusted accordingly, so that the PBCF contribution can be assessed separately as far as possible. Calculated expectations and measured results remain clearly linked to their own conditions and assessment method.
Mechanical Connection, Clearance and Maintenance
The mechanical connection is matched to the propeller hub, cap attachment and available space around the shaft end and propeller nut. The distance from the rudder and any Rudder Bulb fitted also determines which cap length and fin geometry are feasible. These connections and clearances are checked before the design is finalized for production.
The fins are rigidly attached to the cap, so the assembly rotates with the propeller. Cap, fins and attachments must be able to withstand the loads at this location. Material selection, corrosion protection and any coating are matched to the propeller and chosen design. The required drawings and technical assessments follow from the specific ship and system configuration.
For a retrofit, the existing hub and shaft arrangement form the starting point. Installation and access are coordinated with the maintenance or docking schedule. For a newbuild or propeller replacement, the propeller, cap and rudder can be developed together at an earlier stage. Inspection of the fins, cap and attachments can be combined with routine propeller maintenance, with attention to fouling, damage and deformation.
If blade geometry, pitch, diameter or the rudder is modified later, the suitability of the PBCF for the changed flow and available space is reassessed. A substantially different operational profile may also justify reassessment of the original matching and expected performance.
From Power Difference to Annual Fuel Savings
The annual contribution follows from the power difference at the speeds and loads under 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. It thus becomes apparent how much of the design benefit can be used in actual service.
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 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 saving against design, engineering, production, installation and any additional docking time. For a retrofit, the additional PBCF costs are distinguished from maintenance that is planned even without the project. Maintenance during the service life and the ship’s remaining period of operation also count. It thus becomes apparent under which operating and cost conditions the improvement around the propeller hub offers sufficient value for the investment.
Propeller Boss Cap Fins (PBCF)
In cooperation with a specialist international partner, we support shipping companies, technical managers and shipyards in selecting a PBCF and coordinating the project, from the initial technical assessment and hydrodynamic analysis through design and engineering, production and delivery to preparation for implementation on board.
Available worldwide
What Are Propeller Boss Cap Fins (PBCF)?
Propeller Boss Cap Fins (PBCF) are fins fitted to the boss cap behind a ship’s propeller, together forming an Energy Saving Device (ESD). The fins rotate with the propeller and influence the flow around the hub and the formation of the hub vortex.
A suitable design can reduce losses in this region and thus lower the required propeller power at the same ship speed.
In suitable applications, a fuel saving of around 2% is a realistic indication, with a corresponding reduction in the associated CO2 emissions; higher values may also be possible in specific projects.
The shape of the cap and fins is matched to the propeller geometry, local flow, available clearance from the rudder and actual operational profile.
Influencing the Hub Vortex and Pressure Behind the Propeller
Vortices originating at the blade roots may merge behind the hub into a concentrated hub vortex. The low pressure in this region contributes to resistance on the boss cap. The PBCF fins influence how these vortices develop and merge. Suitable geometry can increase the pressure behind the cap and thus increase the net thrust of the propeller and boss cap.
The fins also generate their own hydrodynamic forces and influence torque. The design choice therefore weighs the reduction in hub loss against the additional losses. What matters is how much thrust the complete propeller, with its cap and fins, delivers for the required power.
Matching Fins and Boss Cap to the Propeller
Propeller diameter, number of blades, blade geometry near the root, direction of rotation and hub dimensions form the basis of the PBCF design. The height, profile shape and setting angle of the fins are matched to these characteristics, together with their position relative to the propeller blades. The shape and length of the boss cap also influence the flow. Cap and fins are therefore developed together.
The local flow around the hub also depends on the wake field behind the hull: the distribution of water velocity and flow direction in the inflow to the propeller. Rotational speed, propeller loading and ship speed determine the conditions under which the fins operate. The assessment therefore covers the operating points that matter to the ship. When application to a controllable pitch propeller is investigated, the pitch settings used are also taken into account.
If the existing propeller is retained, the improvement achievable with a modified boss cap and fins is investigated. If there is damage or heavy fouling, or the propeller is less well suited to the operational profile, the necessary maintenance or modification is established first. That choice provides the basis for assessing the additional value of a PBCF.
When Does a PBCF Offer Sufficient Hydrodynamic Potential?
A PBCF may be particularly attractive where a relevant proportion of the hydrodynamic losses arises around the propeller hub and the existing boss cap offers scope for targeted improvement of this flow. A distinct hub vortex, low pressure behind the cap or boss cap geometry that could be better matched to the flow at the blade roots may justify further investigation of a PBCF.
Suitability therefore depends less on ship type than on the propeller itself and the flow in which it operates. Two comparable ships may have different potential for improvement because of differences in propeller geometry, hub ratio, boss cap, rotational speed, loading and rudder arrangement.
The space available behind the propeller is also decisive. The PBCF fins and modified cap must retain sufficient clearance from the rudder or any Rudder Bulb fitted. At the same time, the geometry must be matched to the propeller so that the reduction in hub loss offsets the hydrodynamic loading and resistance of the fins themselves.
An initial assessment therefore brings together the existing propeller and hub geometry, local flow, rudder arrangement and main operating points. It can thus be established early whether optimization around the boss cap offers sufficient potential or whether another modification to the propeller, inflow or rudder addresses the dominant loss more directly.
PBCF and Rudder Bulb in the Same Hub Flow
A PBCF rotates with the propeller and acts directly around the boss cap. A Rudder Bulb is attached to the rudder and influences the transition of the flow from the hub towards the rudder. Both can act on the same hub vortex. Selection therefore connects the local loss mechanism with the existing cap shape, rudder geometry and distance between them.
If the rudder already has a favourable influence on the hub vortex, the additional contribution of a PBCF may differ from that found when assessing the propeller without a rudder. A combination is therefore investigated jointly for its hydrodynamic performance and available space. The effect on power is established for the complete arrangement, taking the interaction into account.
A change in inflow ahead of the propeller may also alter the flow around the hub. In combination with other ESDs, the modified hull–propeller–rudder configuration is therefore used as the starting point. The PBCF is matched to the flow in which the cap and fins must actually function.
Assessing Hub Flow, Thrust and Torque Together
Computational Fluid Dynamics (CFD) can be used to investigate the flow around the blade roots, boss cap and fins. Variations in cap shape, fin profile, dimensions and position are compared for their influence on pressure distribution, vortex formation and propeller loading. It thus becomes apparent how the geometry affects efficiency.
The power assessment includes thrust and torque on the propeller blades, cap and fins in the same comparison. The change in the local vortex pattern is thus linked to the performance of the complete propeller configuration. A change in thrust is assessed together with the corresponding torque and rotational speed.
The existing boss cap forms part of the reference configuration. If the cap shape changes as well as the fins, the comparison covers the full modification. The stated performance then relates to the combined modification of cap and fins.
From Propeller Testing to Required Power on Board
An open-water test investigates the propeller under specified inflow conditions, independently of the hull’s influence. Such a test or corresponding calculation can help to compare PBCF variants. To translate the results to the ship, the performance of the selected design in the actual wake field and in combination with the rudder is then investigated. Open-water efficiency is thus linked to the ship’s propulsion performance.
For the ship comparison, the required power with and without a PBCF is established at the same ship speed and corresponding draught, trim and ship condition. The self-propulsion point, at which the propulsive and opposing forces are in equilibrium, is established. The required rotational speed may differ between the two arrangements. 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.
The calculation or test method is tailored to the performance that needs to be substantiated for the project. It remains clear which results concern the propeller alone and which concern the complete propulsion configuration.
Cavitation and Pressure Fluctuations Around the Hub
A suitable PBCF can reduce the hub vortex and associated cavitation. The influence on cavitation on the propeller blades and at the blade tips is assessed separately where it matters to the project. The different cavitation phenomena thus remain linked to the flow regions in which they originate.
The fins may also generate their own vortex structures, which influence the further development of the propeller slipstream. Where relevant, pressure fluctuations, vibration excitation and underwater noise are therefore investigated. The design choice combines the expected power benefit with acceptable cavitation behaviour and suitable loads. The necessary calculations and tests are tailored to the propeller, rudder and relevant operating conditions.
Scale Effects and Verification of the Saving
At model scale, frictional resistance on the fins, pressure behind the cap and inflow from the hull may differ from conditions on the actual ship. These scale differences affect the balance between the reduction in hub loss and the additional fin losses. When translating the results to full scale, both contributions are therefore assessed together.
Full-scale calculations can support this translation. The outcome depends on factors including the flow models used, the representation of local vortices and the input data. Comparison with suitable measurement data helps to assess the calculations. The uncertainty is compared with the magnitude of the predicted power difference and taken into account in the design and investment decision.
After installation, the result can be assessed using sea-trial or operational data. Measurements of speed, power and rotational speed are linked to draught, trim, loading and relevant weather and sailing conditions. For a retrofit, hull cleaning, propeller maintenance and other work during the same docking are also recorded. The reference and measurement setup are adjusted accordingly, so that the PBCF contribution can be assessed separately as far as possible. Calculated expectations and measured results remain clearly linked to their own conditions and assessment method.
Mechanical Connection, Clearance and Maintenance
The mechanical connection is matched to the propeller hub, cap attachment and available space around the shaft end and propeller nut. The distance from the rudder and any Rudder Bulb fitted also determines which cap length and fin geometry are feasible. These connections and clearances are checked before the design is finalized for production.
The fins are rigidly attached to the cap, so the assembly rotates with the propeller. Cap, fins and attachments must be able to withstand the loads at this location. Material selection, corrosion protection and any coating are matched to the propeller and chosen design. The required drawings and technical assessments follow from the specific ship and system configuration.
For a retrofit, the existing hub and shaft arrangement form the starting point. Installation and access are coordinated with the maintenance or docking schedule. For a newbuild or propeller replacement, the propeller, cap and rudder can be developed together at an earlier stage. Inspection of the fins, cap and attachments can be combined with routine propeller maintenance, with attention to fouling, damage and deformation.
If blade geometry, pitch, diameter or the rudder is modified later, the suitability of the PBCF for the changed flow and available space is reassessed. A substantially different operational profile may also justify reassessment of the original matching and expected performance.
From Power Difference to Annual Fuel Savings
The annual contribution follows from the power difference at the speeds and loads under 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. It thus becomes apparent how much of the design benefit can be used in actual service.
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 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 saving against design, engineering, production, installation and any additional docking time. For a retrofit, the additional PBCF costs are distinguished from maintenance that is planned even without the project. Maintenance during the service life and the ship’s remaining period of operation also count. It thus becomes apparent under which operating and cost conditions the improvement around the propeller hub offers sufficient value for the investment.
Approach to Selecting and Developing a PBCF
In the first project phase, it is established whether the PBCF will be matched to the existing propeller or included in a propeller replacement. The reference configuration, relevant operating points and mechanical connections are recorded. The hydrodynamic analysis and design of the cap and fins are developed further on that basis.
For a promising design, hydrodynamic shape and mechanical design are brought together. The technical proposal describes the cap and fin design to be supplied, the connections that apply and the operating conditions for which the expected performance is valid. Installation work, any modifications and the necessary additional assessments or tests are also recorded.
After the project decision, production, delivery and preparation for the work on board are coordinated. For a retrofit, the schedule fits the ship’s maintenance or docking period. If performance verification forms part of the project, the reference data, measurement setup and acceptance arrangements are established in advance. After installation, the result can then be assessed on the agreed basis.
Have Your PBCF Project Assessed
Are you considering a PBCF for your existing propeller or as part of a newbuild or propeller replacement project? We would be pleased to discuss the opportunities that the flow around the hub offers for targeted improvement. For a substantive assessment, we would welcome the propeller drawing showing its main dimensions and blade geometry, supplemented by drawings of the boss cap, shaft end and propeller nut.
An arrangement drawing of the propeller shaft and rudder, with the distances between them, helps to assess the mechanical possibilities. Please also mention any Rudder Bulb fitted or other modifications around the propeller hub. Available propeller tests, CFD results and information on the current propeller condition can support the technical assessment.
For the expected saving, rotational speed, engine or shaft power, ship speed and operating hours for each sailing condition are important, with the corresponding draught, trim and loading. For a controllable pitch propeller, the pitch settings used are also relevant. For a retrofit, please indicate when maintenance or docking is planned and which modifications to the propeller or rudder are being considered. During the project discussion, we agree on what further data are needed and how the hydrodynamic and mechanical development can fit your ship’s schedule.
Discuss whether a PBCF suits your propeller and hub configuration >