Retrofit wave builds below the waterline

Propeller devices with energy-saving potential include propeller boss cap fins, rudder bulbs, stator fins and ducts. While each technology affects flow differently – either by improving water ingress to the propeller or smoothing exit flow – some can interfere with one another if installed inappropriately.

Consequently, selection and design optimisation are critical. The report stresses the importance of considering shaft alignment, torsional vibration and underwater-radiated noise (URN) when assessing retrofit options.

High-efficiency propeller retrofits are attracting renewed interest. Many vessels now operate at slower speeds than their original design point, particularly since the adoption of slow-steaming strategies. Propellers optimised for higher speeds may perform poorly under these altered conditions. Redesigning the propeller and pairing it with appropriately tuned ESDs allows for better alignment with the actual operational profile.

The uptake of propulsion retrofits is especially strong in sectors with high fuel consumption. According to data shared by Lloyd’s Register and sourced from Clarksons Research Services and IHS, rudder bulbs are fitted on 7% of the existing tanker fleet, rising to 17% for tankers currently on order.

Boss cap fins, ducts and stator fins show similar upward trends in the container ship and bulk carrier segments. Although the percentage remains lower in gas carriers, there is nonetheless evidence of growing interest in propulsive upgrades from this sector too.

Device cost and claimed savings vary. Wake-equalising ducts, for example, offer average fuel savings of around 3% and cost between US$300,000-$600,000 per unit, with estimated payback periods of up to six years based on fuel savings alone.

Conversely, stator fins and rudder bulbs offer higher savings with payback periods closer to 3 years. However, the report cautions some installations may not deliver the anticipated savings, especially where ship geometry is unknown or performance has been modelled rather than verified at full scale.

"Many devices still await a detailed physical explanation, which is needed to adjust them effectively for each specific vessel design," the report notes.

Lloyd’s Register global technical director Claudene Sharp-Patel noted, "Our research reveals propeller and ESD retrofits offer ship operators a proven pathway to significant fuel savings, extended regulatory compliance, and meaningful emissions reductions. 

"However, successful propeller and ESD retrofits require far more than simply bolting on additional equipment. They demand sophisticated analysis, careful integration with existing systems, and ongoing performance management.”

The report adds several owners do not have accurate hull geometry on file, especially for older ships that may have changed hands or undergone unrecorded modifications.

Geography is also playing a part in the retrofit trend. Shipyards in South Korea, Japan and the Middle East are handling a growing share of these projects, with relative activity in China declining despite overall numbers rising.

As of 2024, over 1,400 propulsion retrofits have been carried out on existing vessels since 2020.

The report calls for structured project planning, recommending initial vessel assessments, hydrodynamic analysis via computational fluid dynamics, structural evaluations and post-installation trials.

Underwater noise and biofouling are also addressed. ESDs may contribute to or mitigate URN depending on design and flow interaction.

The accumulation of marine growth on propeller blades and ESDs can nullify gains if not properly managed. A proactive cleaning and coating regime is therefore essential to preserve performance.

Lloyd’s Register concludes retrofitting offers a practical pathway for owners aiming to extend the operational and regulatory life of their assets.

"Retrofitting propellers and propulsion ESDs presents an opportunity to enhance fuel efficiency, reduce greenhouse gas emissions, and navigate the regulatory landscape," the report states.

However, it cautions a one-size-fits-all approach will not deliver reliable results and careful planning and post-installation validation are essential to ensure benefits are realised.

No single device or configuration can be universally applied. The trend now underway – towards ship-specific retrofits tailored to real-world operating conditions – suggests owners are becoming more sophisticated in how they address the propulsion efficiency challenge. With both fuel and compliance costs rising, attention is increasingly turning to what lies beneath the waterline.

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