Air lubrication

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

Fouling growth on the hull is reduced due to decreased wetted surface when operating an air lubrication system helping to minimise the drag resistance. The benefit from the measure will come in terms of reduced fuel consumption due to lower hull resistance and therefore the decrease of the main engine load at similar speeds as without air lubrication system installed.

Credit: DNV

Applicability and assumptions

Air lubrication is applicable for both new buildings and retrofits. The maximum reduction potential can be achieved for low Froude numbers, meaning a low speed compared to ship length, ships for which frictional resistance dominates, such as bulkers, tankers and large container vessels. Air lubrication systems will only affect the viscous part of the total resistance. Viscous resistance will typically account for 50 – 80% of the total resistance on most ships. Air lubrication installations include cruise ships, LNG carriers, containers, RoRos, and may be applicable to most vessel types.

Some of the air lubrication systems require installation of additional compressors and piping for the air in addition to potential changes in the hull shape to optimize the effect of the air bubbles, in addition to make holes in the hull. The air lubrication system will, from an onboard power management system, be seen as a significant additional power consumer. Although less than 3% of the total ship power is needed to support the air lubrication system.

System technical specifications and the ship’s operational profile have proven to be very divisive regarding the potential for fuel savings. This indicates that the measure currently needs a large degree of tailoring, and one could expect to have large variations in effect from case to case. This divisiveness also stems from the difficulty of simulating an air lube system with CFD because of the bubble size and the boundary layer interaction or with data because of how the environmental conditions and ship operation may affect the system’s performance.

Downsides and scepticism towards the technology have long been reasoned with potential negative effects on directional stability and an unfavourable air inflow to the propeller. As of yet, this has not been reported as major negative effects. Depending on the design, such a system may require “shielded” propellers or other means to prevent air from flowing to the propeller and reducing its power. At higher speeds, it will also be difficult to maintain a steady flow of air bubbles, which is essential for the technology to work. The maximum reduction potential is achieved for ships where frictional resistance dominates, i.e. ships with large flat bottoms, long crossings, small draft and not too high a speed. This mainly involves bulk, tank, gas, cruise and container ships.

Cost of implementation

The costs are estimated to 2-3% 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

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 reduction potential for crude- and product tankers, and bulk vessels has been assessed in the range of 7 – 10% on the main engine, while for other ship segments it has been assessed to a range of 3 – 5% on the main engine. However, one ship owner in the container segment could not verify the savings, indicating that the reduction potential might be lower.

Amongst successful installations the most recognized measurements indicate a total 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. Institute of Marine Engineering, Science & Technology (Imarest) (2024) Air bubbles don’t float Maersk
6. Fitzpatrick, J., et al. (2017) Full scale applications of air lubrication for reduction of ship frictional resistance
7. Park, J., et al. (2018) Optimization of drag reduction effect of air lubrication for a tanker model
8. Fotopoulos, G., et al. (2020) Computational analysis of air lubrication system for commercial shipping and impacts on fuel consumption
9. Klaveness Combination Carrier (2022) KCC Concludes Milestone Contracts in Major Energy Efficiency Retrofit