Can Devices Such as Propeller Nozzles, Fins, or PBCFs Improve Ship Propeller Efficiency?
Author: Jeroen Berger • Publication date:
Ship propellers have been progressively optimized over time, but propulsion efficiency is not determined by propeller design alone. The flow reaching and leaving the propeller is equally important. Unfavorable inflow, additional vortex formation and rotational energy in the propeller slipstream can cause energy losses, even with a well-designed propeller.
To limit these hydrodynamic losses, auxiliary devices have been developed in practice to direct, guide or deflect the flow around the propeller. These Energy Saving Devices (ESDs) are often applied as relatively straightforward measures, particularly on existing vessels where replacement of the propulsion installation is not technically or economically obvious. Depending on the selected system and the actual operational profile, this can lead to a measurable improvement in propulsion efficiency.
This article explains to what extent devices such as nozzles, guiding fins and Propeller Boss Cap Fins can contribute to higher propeller efficiency. It discusses the operating principles of these systems, the situations in which they typically add most value in practice and the limitations associated with speed, operational profile and installation conditions. Finally, these devices are placed in a strategic context for shipping companies, shipowners, technical managers, superintendents and directors, in which both operational savings and alignment with energy and emissions frameworks play a role.
Nozzles: More Thrust at Low Speed
A nozzle is an annular hydrodynamic structure installed around the propeller to guide and direct the flow through the propeller. The shape and positioning of the nozzle condition the inflow to the propeller so that a greater share of generated impulse can be converted effectively into axial thrust. This effect is particularly pronounced at low vessel speed and high disk loading, where an open propeller is relatively less efficient.
In this speed range, a nozzle can produce a clear increase in available thrust without requiring a proportional increase in propeller power. The device is therefore well suited to vessels that frequently operate at low speed under heavy load, such as tugs, dredgers and workboats. During maneuvers, against current and in laden condition, improved thrust can also support more controllable handling and a more robust working capability.
These advantages are balanced by a clear limitation at higher vessel speeds. As speed increases, the added wetted surface area and form drag of the nozzle become more significant. In this regime, the nozzle can limit overall propulsive efficiency, reducing efficiency relative to an open propeller. Applying a nozzle is thus explicitly profile dependent and requires a careful trade-off between gains at low speed and efficiency penalties at higher transit speeds.
The actual contribution of a nozzle is also strongly influenced by factors such as nozzle geometry, propeller design, hull form and local inflow conditions. An incorrectly sized or poorly integrated nozzle can partially negate expected benefits. In practice, application is most successful when nozzle and propeller are designed as an integrated set or explicitly aligned with the vessel’s intended operating window.
Guiding Fins: Inflow Correction
Propeller performance is strongly determined by inflow quality. In practice, inflow is seldom uniform. Hull form, the afterbody and any asymmetries in the flow often create a non-uniform velocity distribution and a rotating component in the water approaching the propeller. This leads to energy loss in the form of vortex formation, elevated blade loading and less efficient conversion of power into propulsion.
Guiding fins, also referred to as stators, are designed to correct these unfavorable inflow conditions. They are typically installed ahead of the propeller and function to align and redistribute the incoming flow, reducing swirl and making the velocity distribution across the propeller disk more uniform. Water then reaches the blades at more favorable angles of attack, which supports the propeller’s hydrodynamic performance.
The direct result is an improvement in propeller effectiveness. More uniform inflow produces more even blade loading, allowing available power to be used more efficiently. At the same time, the risk of local overload decreases, which can translate into reduced vibration, pressure pulses and cavitation phenomena. These effects contribute to lower fuel consumption, smoother vessel operation and longer service life of propeller, shaft line and bearings.
The extent to which guiding fins deliver real benefits is highly dependent on the specific vessel and the existing flow conditions aft of the hull. On vessels with relatively favorable inflow, the effect may be limited, while vessels with a pronounced non-uniform wake can show a more noticeable improvement. For that reason, guiding fins are often designed and positioned based on numerical flow analyses or model testing, so that geometry and placement match dominant flow patterns within the intended operational profile.
Guiding fins are therefore not a generic solution, but a targeted device to limit inflow losses and improve propeller performance within a specific operational context.
Propeller Boss Cap Fins (PBCF): Reducing Hub-Vortex Losses
A concentrated rotating flow, known as the hub vortex, typically forms aft of the propeller hub on conventional propellers. This vortex represents kinetic energy that does not contribute to effective propulsion, but is lost as vortex energy and increased turbulence in the propeller wake. The presence of this residual flow can also contribute to pressure pulses, vibration and adverse interaction with the rudder or other stern components.
A Propeller Boss Cap Fin is designed to reduce these energy losses. The system consists of a modified boss cap fitted with several small fins, typically with a specific angle and geometry aligned with propeller rotation and the flow pattern aft of the hub. These fins influence the rotating hub vortex and reorient part of the circular flow into a more axial component.
By reorienting the flow, the intensity of the hub vortex is reduced and overall propulsive efficiency can increase. In practice, this translates into a modest but measurable efficiency gain, typically on the order of a few percent, depending on propeller design, loading regime and inflow conditions. The gain does not result from higher installed power, but from more effective use of available power.
An important advantage of the PBCF is that it can be applied without major modifications to the propeller, shaft line or propulsion installation. Installation is limited to replacement or adaptation of the boss cap, so implementation is relatively straightforward and can often be performed during routine maintenance or dry docking. This makes the PBCF attractive as a retrofit solution for existing vessels.
Effectiveness is not universal and must always be assessed in relation to the specific propeller design and the operational profile. On propellers with an already significantly reduced hub vortex, additional gains may be limited, while other configurations can show a clearer effect. For that reason, application is increasingly substantiated with numerical flow analyses or in-service measurements, so that fin geometry and positioning match the dominant flow pattern in the relevant operating range.
The PBCF is therefore not a standalone optimization, but a targeted measure to limit residual losses in propeller flow and to make the overall propulsion system more efficient within existing design and operational constraints.
Practical Value and Strategic Application
Although individual efficiency gains from devices such as nozzles, guiding fins and Propeller Boss Cap Fins typically remain limited to a few percent, a carefully aligned application can still deliver a noticeable improvement in overall propulsive efficiency. It is important to emphasize that effects of different devices do not simply add up. Many systems act on overlapping loss mechanisms in inflow, outflow or stern flow, so the marginal return of a second or third measure can decrease. Conversely, combinations can be synergistic when the devices address different, complementary loss mechanisms and are correctly coordinated.
Model tests and practical applications regularly show that certain concepts can be consistently relevant when they are well matched to the ship type and the dominant operational profile. In particular for duct and pre-swirl-related solutions, in favourable configurations power reductions are reported in the range of a few percent, and in favourable cases up to around 8%, depending on factors such as hull form, propeller loading and design quality. These results are explicitly project specific and not directly generalizable.
For shipping companies and shipowners, the practical value of Energy Saving Devices therefore lies primarily in project-specific assessment. Economic feasibility is determined by the actual operational profile, loading level, speed spectrum, available installation space and the quality of the flow aft of the hull. Where a device demonstrably fits the vessel’s dominant operating point, or a broad operating envelope, it can reduce operating costs and directly lower fuel consumption.
For NOx emissions, a lower power demand can at most have an indirect effect via the engine operating point; NOx performance itself is primarily determined by engine type, settings and any aftertreatment systems. The contribution of ESDs to NOx reduction must therefore always be assessed in conjunction with the overall propulsion and energy system.
Strategically, nozzles, guiding fins and PBCFs are not universal optimizers, but targeted instruments. Their value arises when they are applied on the basis of a robust analysis of flow, loading and operational profile, and when the expected performance improvement demonstrably aligns with both operational and regulatory objectives.
Relevance Within Regulation
Applying devices such as nozzles, guiding fins and Propeller Boss Cap Fins does not stand alone, it aligns with how vessels are assessed within international and regional energy and emissions frameworks. Within MARPOL Annex VI and the resulting IMO instruments, demonstrable energy efficiency plays an increasingly explicit role, with roles for both design choices and operational measures. For existing vessels, this is reflected in the Energy Efficiency Existing Ship Index (EEXI), which requires that propulsion power and realized energy use be reasonably aligned with the ship’s transport capacity.
Attention is also shifting more explicitly to actual operational behavior. The Carbon Intensity Indicator (CII) assesses vessels based on annual CO2 emissions per unit of cargo carried and distance sailed. In this context, a structural reduction in fuel consumption, even when limited per measure, can contribute to a more favorable CII score over the vessel’s lifetime. Effectiveness remains dependent on operational profile, deployment intensity and the extent to which the vessel operates within its dominant operating point.
At European level, the Emissions Trading System (EU ETS) strengthens the economic incentive to limit fuel consumption and thus CO2 emissions. Although Energy Saving Devices are not direct compliance measures, by lowering energy demand they can reduce emissions costs under this system. Their role therefore lies mainly in supporting broader optimization strategies, not in single-handedly covering regulatory requirements.
Within this framework, Energy Saving Devices offer a feasible, relatively straightforward way to improve performance on existing vessels, particularly where major modifications to hull or propulsion installation are not desirable or feasible. Their contribution to regulation is, however, always indirect and conditional, only when the application demonstrably fits the operational profile and is correctly substantiated can they play a meaningful role within a coherent strategy for energy efficiency and emissions control.
About This Article
This article forms part of the background information on the propeller as a product and falls within the cluster Ship Propeller Design and Optimization. Its core premise is that devices such as nozzles, guiding fins and Propeller Boss Cap Fins are not generic efficiency solutions, but project-specific instruments that intervene in clearly defined hydrodynamic loss mechanisms. Their added value arises only when inflow conditions, loading regime and operating profile are explicitly incorporated into the design and assessment. For a project-specific elaboration, the page Custom Ship Propeller logically builds on this context.
For insight into the hydrodynamic principles these devices act upon, What Are Important Design Principles for an Efficient Ship Propeller connects directly. That article describes how inflow, blade loading, vortex formation and cavitation jointly determine propeller efficiency and thus provides the technical basis for understanding Energy Saving Devices.
Objective assessment of efficiency gains and performance retention is set out in How Is Ship Propeller Performance Measured and Validated. It explains how model testing, numerical analyses and onboard measurements are combined to document device effects in a traceable and verifiable way.
For the strategic context in which such optimizations are applied, How Does a More Efficient Ship Propeller Contribute to MARPOL Annex VI, EEXI/CII, and NOx Reduction is relevant. That article places efficiency measures within international regulation and shows how technical improvements affect energy and emissions indicators.
Together, these articles position Energy Saving Devices not as standalone devices, but as targeted optimizations that add value only when they demonstrably fit the operational profile, loading regime and the vessel’s broader energy and compliance strategy.