By ShipCalculators.com · Published 25 April 2026 · last updated 28 May 2026

Energy-Saving Devices (ESDs)

Energy-saving devices (ESDs) are propulsion-improving appendages fitted forward of, around or aft of the marine propeller to recover rotational energy from the propeller wake field, smooth the inflow to the propeller disc, or reduce hub-vortex losses. The principal device families in commercial use are: the Propeller Boss Cap Fin (PBCF) developed by Mitsui O.S.K. Lines (MOL) and Mikado Propeller in 1987; the Mewis duct (Becker Marine Systems, 2008); the pre-swirl stator; the wake equalising duct (Schneekluth duct, 1986); the rudder bulb (also Costa propulsion bulb); the rudder thrust fin; and the post-swirl duct or post-swirl stator. Typical fuel savings are 2 to 6% per device, with combined savings of 4 to 10% when multiple devices are installed in compatible packages. ESDs are recognised as innovative energy-efficient technologies under MARPOL Annex VI Regulation 21 with a corresponding EEDI and EEXI credit calculated under IMO Innovative Technology Guidelines (Resolution MEPC.244(66)). The measure has become a standard tool for EEXI compliance, CII rating improvement and SEEMP III operational planning. By 2024 approximately 20,000 vessels worldwide are estimated to have at least one ESD installed, principally PBCF (the most widely deployed individual device) and Mewis duct or pre-swirl stator (on larger newbuilds). ShipCalculators.com hosts the principal computational tools: the PBCF retrofit savings calculator, the Mewis duct savings calculator, the pre-swirl stator savings calculator, the rudder bulb savings calculator, the Schneekluth duct savings calculator, the SEEMP Measures Combined calculator and the EEXI Required calculator. A full listing is available in the calculator catalogue.

Contents

Background

Propulsion losses and the ESD opportunity

The total propulsive efficiency ( $\eta_D$ ) of a conventional single-screw merchant ship is the product of the hull efficiency ( $\eta_H$ ), the open-water propeller efficiency ( $\eta_O$ ), the relative rotative efficiency ( $\eta_R$ ) and the shafting efficiency ( $\eta_S$ ):

\eta_D = \eta_H \cdot \eta_O \cdot \eta_R \cdot \eta_S

Modern values for a typical single-screw bulk carrier, tanker or container ship are approximately:

$\eta_H \approx 1.10$ to $1.20$ (hull efficiency, sometimes greater than unity due to thrust deduction and wake fraction)
$\eta_O \approx 0.65$ to $0.72$ (open-water propeller efficiency)
$\eta_R \approx 0.97$ to $1.02$ (relative rotative efficiency)
$\eta_S \approx 0.97$ to $0.99$ (shafting efficiency)

The product gives a typical propulsive efficiency of approximately 0.65 to 0.75. The remaining 25 to 35% of the shaft power is dissipated as rotational kinetic energy in the wake (typically 5 to 12% of shaft power), as axial kinetic energy (5 to 10%), as frictional losses at the propeller blade surfaces (5 to 8%) and as hub vortex losses (1 to 3%). ESDs target one or more of these loss mechanisms; the recovered energy reduces the shaft power needed to maintain a given service speed. See marine propeller and ship resistance and powering for the underlying theory.

Three families of ESD

ESDs are conventionally classified into three families by their position relative to the propeller:

Pre-propeller devices (forward of the propeller disc): smooth the inflow, generate counter-rotating swirl ahead of the propeller, equalise the wake field, accelerate the inflow over the upper part of the propeller disc. Examples: pre-swirl stator, Mewis duct, Schneekluth wake equalising duct, asymmetric stern.
At-propeller devices (on the propeller hub or blades themselves): reduce hub vortex losses, reduce blade-tip losses. Examples: PBCF, end-plate propeller, Kappel propeller (tip-loaded), winglet propeller, Contra-rotating propeller (CRP).
Post-propeller devices (aft of the propeller, on or near the rudder): recover the rotational energy in the propeller race, generate forward thrust from the swirl. Examples: rudder bulb (also Costa propulsion bulb), rudder thrust fin, twisted leading-edge rudder, Becker Schilling rudder, post-swirl duct, post-swirl stator.

The IMO GreenVoyage2050 technology portal uses the same three-position taxonomy and groups these appendages under the umbrella term propulsion improving devices (PIDs). It places the overall fuel-consumption benefit of the family at roughly 0.5 to 4% on total fuel, with pre-swirl arrangements and ducts reaching up to 5%, propeller boss cap fins up to 2% and rudder (Costa) bulbs up to 2% on their own. The portal’s figures are conservative central estimates across a mixed fleet; maker and class-society data show higher upper bounds on the full-form hulls where the devices work best, and the spread between the two is itself a useful warning that the saving is hull-specific, not a fixed catalogue number.

Each family attacks a different slice of the loss budget. Pre-propeller devices work on the axial and wake non-uniformity losses by accelerating and straightening the slow water in the upper part of the disc; the integral fins of a duct also seed counter-rotating swirl that the blades then reclaim. At-propeller devices target the hub vortex, the concentrated rotational core that forms behind the boss cap and, on a single-screw merchant ship, can carry as much as 10% of the energy delivered to the propeller, per MOL Techno-Trade’s PBCF documentation. Post-propeller devices recover the rotational kinetic energy left in the slipstream, turning some of the residual swirl into ahead thrust on the rudder and its bulb. A device only helps where the loss it targets is actually present, so the same duct that recovers 8% on a loaded tanker can recover near nothing on a fine, fast hull whose wake is already close to uniform.

History

ESD development began in the 1970s in response to the 1973 oil shock. The Schneekluth wake equalising duct was developed by Herbert Schneekluth at the Aachen Institute of Hydromechanics in 1984 and patented in 1986. The PBCF was developed jointly by Mitsui O.S.K. Lines (MOL), Mikado Propeller (now part of Nakashima Propeller) and the West Japan Fluid Engineering Laboratory in 1985 and commercialised in 1987. The Costa propulsion bulb (rudder bulb) was developed by Costa Compagnia di Navigazione in Italy in the 1970s and 1980s. The Mewis duct was developed by Friedrich Mewis at HSVA (Hamburg) and commercialised by Becker Marine Systems in 2008.

The 2008 financial crisis and the resulting slow steaming era drove a renewed wave of ESD development and deployment, paralleled by the bulbous bow retrofits wave. The introduction of EEDI for newbuilds in 2013 and EEXI for existing ships in 2023 provided a regulatory driver that has further accelerated ESD adoption.

Why the saving depends on the wake field

An ESD is not a bolt-on percentage. Its gain is set by the nominal wake field, the pattern of axial and tangential velocity the hull delivers to the propeller disc. Full-form ships such as tankers, bulk carriers and ore carriers run with high block coefficients and a thick boundary layer over the after body. That produces a strongly non-uniform wake with a slow, swirling upper sector. There’s a large axial deficit to recover and plenty of rotational energy in the slipstream, so a duct or a pre-swirl stator has something to work with. Becker Marine Systems states the power saving of its Becker Mewis Duct is driven by propeller thrust loading and runs from about 3% on multi-purpose ships up to 8% on tankers and bulkers, averaging 5 to 6% and rising toward 8% when paired with a Becker rudder. That spread isn’t marketing variance. It tracks the physics: the higher the thrust loading and the more distorted the wake, the more the device returns.

Fine, fast hulls behave differently. A container ship at 22 knots has a lighter-loaded propeller and a more uniform inflow, so a duct sized to a bulker’s wake can add wetted-surface drag faster than it recovers swirl. This is why pre-swirl stators and ducts are specified mostly on slow full-form hulls, while fast-hull retrofits lean toward boss-cap fins, twisted rudders and tip-modified propellers that don’t depend on a thick boundary layer. See hull form design and block coefficient for the geometry that sets the wake.

Model-to-full-scale scaling

ESD savings measured in a towing tank rarely transfer one-for-one to the ship. The reason is Reynolds-number scaling of the boundary layer. At model scale the boundary layer is proportionally thicker, the wake is more distorted than the ship’s, and a wake-conditioning device therefore looks better in the tank than it will at sea. The ITTC Specialist Committee on Energy Saving Methods, which sits under the International Towing Tank Conference, was set up precisely because the standard 1978 ITTC powering-prediction method was not built to extrapolate appended pre-swirl and post-swirl devices, and tank results were over-predicting full-scale gains. Current practice combines dedicated model tests with viscous CFD (RANS) at or near full-scale Reynolds number, and treats the device-specific scale correction explicitly rather than rolling it into the bare appendage drag. IMO MEPC.1/Circ.815 reinforces the point: the benefit of an innovative technology is to be evaluated “in conjunction with the hull form and propulsion system with which it is intended to be used,” through model tests and sea trials, not from a generic catalogue figure.

This scaling gap is the single most common reason an ESD underdelivers against the brochure. A device validated only by a small-scale tank test, with no full-scale CFD correction and no ISO 19030 sea-trial check, carries real risk of a 1 to 2 percentage-point shortfall once it’s wet.

Principal device families

Propeller Boss Cap Fin (PBCF)

The Propeller Boss Cap Fin (PBCF) is a set of small fin-shaped blades fitted to the propeller boss cap (the cone-shaped fairing that covers the propeller hub aft of the propeller blades). The fins are typically the same in number as the main propeller blades (4, 5, 6 or 7) and are aligned to counter-rotate the hub vortex, eliminating it as an organised flow structure. The vortex disruption reduces the rotational kinetic energy lost in the propeller race and also reduces the cavitation erosion on the rudder leading edge.

The loss it attacks is real and quantified. MOL Techno-Trade, the developer’s manufacturing arm, documents that the hub vortex behind a bare boss cap can carry up to 10% of the energy delivered to the propeller, and that breaking it up returns 3 to 5% in fuel at the same speed against an identical ship without the fins. The device won the Japan Association for Logistics and Transport environmental technology award in 2020 on the strength of that 3 to 5% band, validated against real ship data from more than 100 vessel types in the MOL operated fleet. The fins also lift the boss-cap pressure, so a small part of the gain shows as recovered thrust rather than reduced torque.

Typical PBCF performance is:

Fuel saving: 3 to 5% of main-engine fuel consumption at constant speed (MOL Techno-Trade verified band).
Capital cost: USD 15,000 to USD 60,000 per installation depending on propeller diameter.
Installation time: typically 1 to 2 days during a routine drydocking, no specialist skills required.
Payback period: typically 4 to 12 months.

PBCF crossed 2,000 cumulative orders in 2011 and 3,000 in 2015, per MOL milestone announcements, and MOL Techno-Trade reports more than 4,500 installations worldwide as of October 2025, after 38 years on the market. That makes it the most widely deployed individual ESD. The principal manufacturers are MOL Techno-Trade (the Japanese manufacturer originally part of MOL), Mikado Propeller (now part of Nakashima Propeller), Becker Marine Systems (under licence), Wartsila (under licence), Caterpillar Marine (under licence) and several local Chinese, Korean and European manufacturers.

The PBCF retrofit savings calculator implements the IMO MEPC.1/Circ.815 method for estimating the savings.

Mewis duct (Becker Mewis duct)

The Mewis duct is a pre-propeller duct with integrated pre-swirl fins (asymmetrically profiled, individually placed inside the duct), positioned forward of the propeller disc and concentric with the propeller axis. Becker Marine Systems describes three combined effects: the duct straightens and accelerates the hull wake into the propeller and itself produces a net ahead thrust; the optimised flow cuts the hub vortex; and the fin system adds contra-rotating pre-swirl that the propeller reclaims as additional thrust. The saving scales with propeller thrust loading, which is why the maker quotes a band rather than a single number.

Typical Mewis duct performance is:

Fuel saving: about 3% on multi-purpose ships, up to 8% on tankers and bulk carriers, averaging 5 to 6% and reaching toward 8% when combined with a Becker rudder (Becker Marine Systems data); faster container ships benefit less.
Capital cost: USD 200,000 to USD 600,000 per installation depending on hull size and complexity.
Installation time: typically 5 to 14 days during a planned drydocking.
Payback period: typically 12 to 36 months.

Becker Marine Systems reports more than 1,600 vessels equipped with the Becker Mewis Duct, principally bulk carriers, gas carriers, general cargo ships and crude oil tankers. The exclusive manufacturer is Becker Marine Systems (Hamburg), which holds the principal patents and designs each duct to the specific hull’s wake using viscous CFD.

The Mewis duct savings calculator implements the savings calculation; the SEEMP Measures Combined calculator implements the combined-measure calculation.

Pre-swirl stator

The pre-swirl stator is a set of stationary blades (usually 3 to 5 fins) positioned immediately forward of the propeller, oriented to generate counter-rotating swirl in the inflow. The propeller subsequently extracts additional thrust from the pre-swirled inflow, effectively recovering some of the rotational energy that would otherwise be wasted in the propeller race.

The mechanism is worth stating plainly because it’s counter-intuitive. A stator alone saves no energy and adds resistance. The gain comes entirely from its interaction with the propeller: the blades meet the pre-swirled water as extra loading, the slipstream leaves with less residual rotation, and the net result is lower delivered power at the same thrust and speed. Peer-reviewed studies of single-screw vessels put the band at 3 to 6%, with about 5% power saving at equal speed found feasible in well-matched cases; on fast twin-screw passenger ships the design-point gain is smaller, around 3%. The stator must be tuned to the wake of the particular hull, since the swirl it should add varies around the disc.

Pre-swirl stators come in two main forms:

Hub-mounted pre-swirl stator: 4 to 6 blades attached to the after stern frame, immediately forward of the propeller boss. Typical of the Hyundai Hi-FIN, Mitsubishi pre-swirl stator and Sumitomo SILD (Sumitomo Integrated Lammeren Duct).
Duct-integrated pre-swirl stator: stator blades mounted inside a circular duct (the Mewis duct, the Wartsila Energopac). The duct provides additional flow acceleration around the upper hull region.

Typical pre-swirl stator performance is:

Fuel saving: 3 to 6% of main-engine fuel consumption.
Capital cost: USD 80,000 to USD 300,000 per installation depending on configuration.
Installation time: typically 5 to 10 days during a planned drydocking.

The principal manufacturers are Hyundai Heavy Industries (Hi-FIN), DSME (now Hanwha Ocean), Mitsubishi Heavy Industries, Sumitomo Heavy Industries Marine and Engineering, Wartsila (Energopac and Energy Saving Devices product line), Becker Marine Systems, MAN Energy Solutions, and several Chinese yards.

Wake equalising duct (Schneekluth duct)

The wake equalising duct (WED), also Schneekluth duct after its inventor Herbert Schneekluth, is a pre-propeller half-duct (typically a curved blade about 60 to 80% of a circle) positioned forward of the upper part of the propeller disc, designed to equalise the wake by accelerating the slow upper-disc inflow.

Typical Schneekluth duct performance is:

Fuel saving: 2 to 5% of main-engine fuel consumption.
Capital cost: USD 50,000 to USD 150,000 per installation.
Installation time: typically 3 to 5 days during a planned drydocking.

By 2024 approximately 1,800 vessels are fitted with a Schneekluth duct, principally bulk carriers and chemical tankers. The principal manufacturer is Schneekluth Hydrodynamik (Aachen, Germany), with several licensed Chinese, Korean and Japanese manufacturers.

Rudder bulb (Costa propulsion bulb)

The rudder bulb is a streamlined bulb fairing mounted on the leading edge of the rudder, immediately aft of the propeller hub. The bulb continues the propeller hub fairing aftward, eliminating the hub vortex and recovering rotational energy from the propeller race in a manner complementary to the PBCF.

Typical rudder bulb performance is:

Fuel saving: 1 to 3% of main-engine fuel consumption.
Capital cost: USD 30,000 to USD 100,000 per installation.
Installation time: typically 2 to 5 days during a planned drydocking, often integrated with rudder repair.

The rudder bulb is often combined with a twisted leading-edge rudder (e.g. Becker Schilling rudder, Becker Twisted Fin, Hyundai Twisted Rudder) to further enhance the post-propeller energy recovery. The combined rudder bulb + twisted rudder package can deliver 2 to 5% combined saving.

The rudder bulb savings calculator implements the savings calculation.

Twisted leading-edge rudder

The twisted leading-edge rudder has a leading edge that is twisted (typically 4 to 8 degrees) to align with the rotating outflow from the propeller. The alignment reduces the angle of attack on the rudder leading edge, reducing rudder drag and reducing cavitation erosion at the rudder leading edge. The rotating outflow can also be partly converted into forward thrust by the rudder, further improving propulsive efficiency.

Typical twisted rudder performance is:

Fuel saving: 1 to 3% of main-engine fuel consumption.
Capital cost: USD 50,000 to USD 200,000 per installation depending on rudder size.
Installation time: typically 5 to 10 days during a planned drydocking, requiring rudder removal and replacement.

The principal manufacturers are Becker Marine Systems (Schilling and Becker Twisted Fin), Hyundai Heavy Industries, DSME (now Hanwha Ocean), Wartsila (Energopac), Van der Velden (Damen Group), JMU (Japan Marine United) and MOL Techno-Trade.

Tip-loaded propeller (Kappel propeller, end-plate propeller)

The Kappel propeller (developed by Jens Julius Kappel at the Technical University of Denmark in the 1990s) and the related end-plate propeller are propellers with bent or terminated blade tips, designed to reduce tip vortex losses. The bent tips behave like aircraft winglets, reducing the induced drag at the tip and improving open-water efficiency by 2 to 5%.

Typical Kappel propeller performance is:

Fuel saving: 2 to 5% of main-engine fuel consumption (compared to a conventional propeller of the same diameter and pitch).
Capital cost: USD 100,000 to USD 400,000 per propeller (typically incremental cost over a conventional propeller of the same diameter and material).
Installation time: same as a conventional propeller change, typically 1 to 2 days during a planned drydocking.

The Kappel propeller is licensed exclusively to MAN Energy Solutions (which acquired the Kappel intellectual property in 2007).

Contra-rotating propeller (CRP)

The contra-rotating propeller (CRP) is a propulsion system in which two coaxial propellers rotate in opposite directions, with the aft propeller recovering the rotational energy from the forward propeller and converting it into additional thrust. CRP can deliver 10 to 15% propulsive efficiency improvement over a single conventional propeller, but at much higher mechanical complexity and cost (a second shaft line or a coaxial gearbox).

CRP is principally deployed on:

Naval submarines (almost all modern designs use CRP)
Cruise ships (some Royal Caribbean, Carnival and MSC vessels)
Specialist research vessels
Some fast ferries (some LNG carriers)
Some pod-driven container ships (with the aft propeller mounted on an Azipod)

CRP is rare on conventional merchant ships because of the gearbox complexity and higher capital cost; the Wartsila CRP package and Rolls-Royce CRP package are the principal commercial offerings.

Asymmetric stern

The asymmetric stern (sometimes asymmetric afterbody) is a hull-form modification in which the stern lines are deliberately asymmetric port-to-starboard, designed to counter-rotate the wake field in the same sense as the propeller is to be installed. The result is a wake field that is more uniform and that delivers a higher net propulsive efficiency. The asymmetric stern is principally a newbuild measure (because the modification cannot easily be retrofitted) and is most common on small to medium bulk carriers and chemical tankers.

Other devices

Less common ESD families include:

Pre-swirl duct (no integral fins, just a duct): rare, limited to specific hull forms.
Post-swirl stator: stator fins immediately aft of the propeller, similar in concept to the contra-rotating propeller without the rotating element.
Bilge keel hydrofoils: small hydrofoils on the bilge keels generating forward thrust from the side-flow.
Booster pumps and water-jets: rare, limited to fast ferries.
Magnetic bearing shafting: rare, limited to specialised applications.

Combination packages

ESDs are often deployed in combination packages to maximise the savings. The most common packages are:

PBCF + rudder bulb: 3 to 6% combined saving (the two devices target the same hub vortex, so the combined saving is less than the sum of individual savings).
Mewis duct + PBCF: 6 to 10% combined saving (the Mewis duct generates pre-swirl, the PBCF disrupts the hub vortex, the two effects are largely complementary).
Pre-swirl stator + twisted rudder: 5 to 9% combined saving.
Bulbous bow retrofit + Mewis duct + PBCF: 10 to 15% combined saving (this is the most aggressive standard package, often deployed for EEXI compliance).
Bulbous bow retrofit + pre-swirl stator + Kappel propeller + twisted rudder: 12 to 18% combined saving (the most aggressive package commonly deployed, principally by Korean and Japanese yards on newbuilds).

The diminishing-returns interaction between measures means that the combined saving is always less than the algebraic sum; the SEEMP Measures Combined calculator implements the empirical interaction factors recommended by IMO MEPC.1/Circ.815 and ABS Guidance Notes on Innovative Energy Efficiency Technologies.

Performance and economics

Typical fuel savings (range and central estimate)

Device	Typical saving	Capital cost (USD)	Payback (months)
PBCF	2 to 4%	15,000 to 60,000	4 to 12
Mewis duct	4 to 8%	200,000 to 600,000	12 to 36
Pre-swirl stator	3 to 6%	80,000 to 300,000	8 to 24
Schneekluth duct	2 to 5%	50,000 to 150,000	6 to 18
Rudder bulb	1 to 3%	30,000 to 100,000	6 to 24
Twisted rudder	1 to 3%	50,000 to 200,000	12 to 36
Kappel propeller	2 to 5%	100,000 to 400,000 (incremental)	12 to 36
Contra-rotating propeller	10 to 15%	1,000,000 to 3,000,000 (incremental)	24 to 60

The figures assume a representative container ship burning 80 t/d of VLSFO at USD 600/t. Smaller vessels have proportionally smaller fuel burn and proportionally longer payback; larger vessels have shorter payback.

EEXI credit

ESDs qualify as innovative energy-efficient technologies under MARPOL Annex VI and under the IMO innovative-technology guidance. IMO MEPC.1/Circ.815 (2013) sorts these technologies into three groups: category (A) for technologies that reduce greenhouse-gas emissions, category (B) for technologies that reduce energy consumption, and category (C) for those tied to alternative fuels. Hydrodynamic ESDs (ducts, fins, stators, rudder appendages) sit in category (B), the energy-consumption-reducing group, because they cut the propulsion power needed for a given speed without producing electricity. Circ.815 was later superseded by MEPC.1/Circ.896 (2021), which extended the same framework to cover both EEDI for newbuilds and EEXI for existing ships.

The guidance is strict about how the benefit is established. It must be evaluated “in conjunction with the hull form and propulsion system with which it is intended to be used,” through model tests and sea trials, not assumed from a generic figure. The corresponding EEXI credit is then the validated power saving, typically from full-scale-corrected CFD anchored to a model-test or sea-trial baseline, applied to the attained-index calculation. A 5% verified power saving translates into close to a 5% EEXI improvement, often enough to bring a vessel into compliance without resorting to EPL or ShaPoLi.

CII improvement

The fuel saving translates directly into a CII rating improvement of equivalent magnitude. A 5% fuel saving typically moves a vessel one band on the CII rating scale, avoiding the CII corrective action plan trigger.

EU ETS and FuelEU Maritime exposure

Under the EU ETS for shipping, the fuel saving translates into EUA cost avoidance of approximately EUR 7,000 to EUR 60,000 per year per vessel, depending on the vessel size and EU port-call frequency. Under FuelEU Maritime, the ESD reduces the GHG intensity of the energy used and therefore reduces pooling, multiplier and penalty exposure.

Notable deployments

MOL fleet-wide PBCF (1990 onwards)

Mitsui O.S.K. Lines (MOL) has installed PBCF on substantially all of its operated fleet since 1990, the original developer’s fleet serving as a long-running reference deployment. The PBCF on the MOL fleet has been credited with approximately 200,000 t/y of avoided fuel consumption.

Maersk fleet-wide ESD package (2014 to 2018)

A.P. Moller-Maersk rolled out a combined ESD package (PBCF, twisted rudder, Mewis duct or pre-swirl stator depending on hull) across its operated fleet between 2014 and 2018. The package was integrated with the parallel bulbous bow retrofit programme; combined savings of 10 to 15% were reported on most retrofit vessels.

Vale Capesize bulker fleet (2015 onwards)

The Brazilian iron-ore exporter Vale specified Mewis ducts on substantially all of the Valemax ore carrier orders (400,000 DWT class, the largest dry bulk carriers ever built); approximately 70 Valemaxes are equipped. The Mewis duct contribution to the Valemax fuel performance is estimated at approximately 5 to 7%.

Hyundai Hi-FIN deployment (2010 onwards)

Hyundai Heavy Industries has installed the Hi-FIN pre-swirl stator on substantially all of its newbuild container ship, bulk carrier and tanker deliveries since 2010, with approximately 700 vessels equipped by 2024.

Wartsila Energopac deployment (2012 onwards)

The Wartsila Energopac integrated package (rudder + twisted leading edge + propeller cap fin + bulb) has been installed on approximately 200 vessels since its 2012 launch, principally container ships and LNG carriers.

Becker Marine Systems Mewis duct deployment

Becker Marine Systems, the exclusive licensee of the Mewis duct, has manufactured and shipped over 1,200 Mewis ducts since the 2008 commercial launch, approximately 70% as newbuild specification and 30% as retrofit.

Safety and operational considerations

Cavitation

ESDs modify the flow field at the propeller, potentially altering the cavitation behaviour of the propeller. Pre-swirl devices typically reduce cavitation by smoothing the inflow; post-swirl devices typically also reduce cavitation by smoothing the outflow. However, poorly designed ESDs (typically those scaled inappropriately from a different hull form) can introduce new cavitation at off-design conditions. Each ESD installation should be CFD-validated for cavitation across the full operating envelope. See marine propeller for the underlying cavitation theory.

Manoeuvrability

ESDs that modify the wake field at the propeller may also modify the rudder force and therefore the manoeuvrability of the vessel, in particular at low speeds and in confined waters. Twisted rudders and rudder bulbs typically improve low-speed manoeuvrability; pre-swirl stators and Mewis ducts typically have neutral to slightly negative effects on low-speed manoeuvrability. The Class approval process includes manoeuvring trials to verify continued compliance with the IMO Standards for Ship Manoeuvrability (Resolution MSC.137(76)).

Fouling and maintenance

ESDs add wetted surface area (typically 1 to 5% of the existing wetted surface) and therefore add a small frictional resistance penalty that partly offsets the propulsive gain. The fouling resistance of the ESDs is typically maintained by the same anti-fouling coating as the rest of the hull. Mewis ducts and pre-swirl stators have been observed to accumulate marine growth in service; periodic cleaning at drydocking is required.

Structural integrity

ESDs impose hydrodynamic loads on the supporting hull structure. Pre-swirl stator blades and Mewis duct fins must be designed for the first-blade-rate (and harmonics) propeller-induced pressure pulses; the supporting structure must be analysed for the corresponding fatigue. Class approval requires a structural fatigue assessment under IACS CSR criteria. Failures of pre-swirl stator fins have been reported on a small number of installations, typically attributed to inadequate fatigue analysis.

Sea-trial validation

Independent validation of the realised savings by sea trial follows the ISO 19030 normalisation framework. The residual uncertainty in the realised saving is typically ± 1 to 2 percentage points; reliable validation requires multi-month before-and-after datasets and statistical regression. Some shipowners have reported underdelivery of ESD savings against sales-pitch projections, particularly for combination packages where the diminishing-returns interaction between measures was inadequately modelled at the design stage.

Limitations and risks

Charterer / owner incentive misalignment

The same charter-market incentive misalignment that affects bulbous bow retrofits, trim optimisation and air lubrication also affects ESDs. The owner bears the capital cost; the charterer captures the fuel saving. The BIMCO CII clauses, Sea Cargo Charter and Poseidon Principles frameworks are gradually realigning the incentives.

Hull-form sensitivity

ESDs are designed for a specific hull form, draught range and speed range. Hulls operating well off-design may not realise the predicted savings. This is particularly relevant for bulk carriers and chemical tankers on tramp trading patterns, where the draught and speed profile shifts from one charter to the next.

Combination interaction

The combination of multiple ESDs requires careful CFD-driven design to avoid destructive interaction between devices. A poorly combined package may deliver less total saving than the best single device. Reputable shipyards and design houses use validated empirical interaction factors (or full coupled CFD) to design combination packages.

Patent and licensing constraints

Several leading ESDs (Mewis duct, Kappel propeller, Becker Schilling rudder) are protected by active patents and are available only through licensed suppliers. The licensing premium can add 10 to 30% to the bare hardware cost.

Future outlook

EEXI-driven retrofit wave (continuing through 2027)

The cumulative EEXI compliance trigger continues to roll through the world fleet through to 2027. DNV projects that approximately 4,000 to 6,000 additional ESD retrofits will be undertaken over 2025 to 2027 in response to upcoming EEXI surveys.

Combination retrofits

The trend towards combined retrofits (bulbous bow retrofit + ESD package + air lubrication + wind-assisted propulsion) is expected to accelerate, with combined-package savings of 15 to 25% achievable on suitable hulls.

IMO Net-Zero Framework GFI

The introduction of the GFI standard from 2027 will create an additional pricing signal for fuel-saving retrofits.

Continued CFD optimisation

The maturity of CFD-driven ESD design continues to improve. Machine-learning surrogate models and topology-optimisation algorithms are expected to deliver an additional 0.5 to 1.5 percentage points of saving from each device family by 2030.

References

IMO Resolution MEPC.244(66): 2014 Guidelines on the Method of Calculation of the Attained Energy Efficiency Design Index (EEDI) for New Ships, as amended by Resolution MEPC.281(70) and MEPC.322(74). International Maritime Organization, 2014.
IMO Resolution MEPC.328(76): 2021 Revised MARPOL Annex VI. International Maritime Organization, 2021.
IMO MEPC.1/Circ.815: 2013 Guidance on Treatment of Innovative Energy Efficiency Technologies for Calculation and Verification of the Attained EEDI. International Maritime Organization, 17 June 2013.
IMO MEPC.1/Circ.896: 2021 Guidance on Treatment of Innovative Energy Efficiency Technologies for Calculation and Verification of the Attained EEDI and EEXI. International Maritime Organization, 2021 (supersedes Circ.815).
ITTC. Final Report and Recommendations of the Specialist Committee on Energy Saving Methods. International Towing Tank Conference.
ISO 19030-1:2016, ISO 19030-2:2016, ISO 19030-3:2016: Ships and marine technology, Measurement of changes in hull and propeller performance. International Organization for Standardization.
DNV. Energy-Saving Devices for Ships: Technical Overview. DNV Position Paper, 2018.
DNV. Maritime Forecast to 2050. DNV Energy Transition Outlook, 2023.
Schneekluth, H. and Bertram, V. Ship Design for Efficiency and Economy, 2nd edition. Butterworth-Heinemann, 1998.
Mewis, F. A Novel Power-Saving Device for Full-Form Vessels. SNAME, 2009.
ABS. Guidance Notes on Innovative Energy Efficiency Technologies. American Bureau of Shipping, 2021.
ICS. Catalysing the Fourth Propulsion Revolution. International Chamber of Shipping, 2022.