Air lubrication

Air Lubrication introduces air injected on the wetted hull surfaces to reduce the ship frictional resistance as it moves through the water. The system, which requires power from auxiliary engines or a shaft generator, creates an air cavity, air layer or a carpet of air bubbles along the flat bottom part of the ship. Air lubrication systems are well tested and available in the market today.

Air Lubrication’s main benefit is drag reduction, which manifests itself as reduced fuel consumption due to lower hull resistance, and therefore a decrease of the main engine load for a set speed. In addition to drag reduction from the bubble carpet itself, fouling growth on the hull may be reduced when operating an air lubrication system.

Credit: DNV

Applicability and assumptions

Air lubrication is applicable for both new buildings and retrofits. Key factors influencing the total saving potential include the size of the flat bottom area compared to the total wetted area, the vessel’s draft and, to some extent, its speed. At higher speeds, air lubrication may be more effective in reducing the frictional resistance. However, at higher speed, the relative importance of frictional resistance is reduced. Most of the air lubrication installations are done for LNG carriers, cruise vessels, RoRo, ferries and large container vessels,  but the technology is also used in other segments.

All air lubrication systems require the introduction of holes into the hull of the ship, which have the potential to introduce drag. Different manufacturers use proprietary air outlet designs and can usually provide an indication of the expected drag when the system is not in use. However, the air outlets are usually designed in a way to reduce any additional drag. Most air lubrication systems require the installation of additional compressors, which require electrical power to operate. Depending on the vessel type the compressor power can usually be catered for by the existing auxiliary generator configuration. In certain cases, depending on the system requirements and the vessel electrical load, additional electrical power generation capacity may be required.

Many academic studies – which do not look at any existing full-scale installations and only rely on laboratory tests – highlight issues such as unfavourable air inflow to the propeller, directional stability, and inability to maintain a steady flow of bubbles at high speeds. However, these issues have not been reported in any full-scale installation since 2015. and air lubrication systems have been developed notably in the last 10 years.

Cost of implementation

The costs are estimated to approximately 1-3% of ship newbuilding costs. The technology also requires some use of energy to function in the form of compressors driven by auxiliary engine. A certain maintenance cost should also be accounted for.

Reduction potential

Predicting savings potential (analytically or in a numerical CFD simulation) is difficult because of the challenges in simulating two-phase flow, bubble size and the boundary layer interaction. Similarly with other EETs interacting with the vessel resistance and propulsion, verifying the performance with in-service data can be challenging, as the changing environment (wind, waves, currents) and operational parameters (draft, trim, air supply, etc.) may affect the system’s performance and introducing noise that can affect the accumulation of equally comparable data points during performance measurements.

Providers of the system claim to be able to achieve 15 – 30% drag reduction and up to 10% fuel reduction on the main engine. The power reduction potential for LNG carriers cruise, RoRo and ferries have been assessed in the range of 7 – 10%, while for other ship segments it has been assessed to a range of 2 – 5%.

Amongst successful installations the most recognized measurements indicate a total net savings in annual energy consumption of 2-8%.

Other References

1. Ceccio, S. L. and Mäkiharju, S. A. (2012) Air Lubrication Drag reduction on Great Lakes Ships. Paper on air lubrication on Great Lakes vessels by Great Lakes Maritime Research Institute
2. Mäkiharju, S. A., Perlin, M., and Ceccio, S. L. (2012) On the energy economics of air lubrication drag reduction
3. Surveyor (2011) A Quarterly Magazine from ABS, pp. 10–15
4. Silverstream Technologies (2024) Description of the Silverstream system
5. Fitzpatrick, J., et al. (2017) Full scale applications of air lubrication for reduction of ship frictional resistance
6. Park, J., et al. (2018) Optimization of drag reduction effect of air lubrication for a tanker model
7. Fotopoulos, G., et al. (2020) Computational analysis of air lubrication system for commercial shipping and impacts on fuel consumption
8. Klaveness Combination Carrier (2022) KCC Concludes Milestone Contracts in Major Energy Efficiency Retrofit
9. 9) European Maritime Safety Agency (2026), Hull Air Lubrication System Study, EMSA, Lisbon