Air cavity lubrication

The technique is to use air injection on the wetted hull surfaces to improve a ship’s hydrodynamic characteristics. The system, driven by auxiliary engine producing the power, creates an air cushion on the flat bottom part of the ship. Air-cavity systems are already in place today.

Fouling growth on the hull is reduced due to decreased wetted surface when operating an air cavity system, helping to minimize the drag resistance. The benefit from the measure will come in terms of reduced fuel consumption due to hull resistance and therefore the decrease in the main engine load.

Applicability and assumptions

How ships save fuel using air, Source: Silverstream Technologies

Air cavity lubrication is applicable for new buildings where the maximum reduction potential can be achieved for “low Froude number” ships for which frictional resistance dominates, as bulkers, tankers and containers.

The air cavity system requires installation of additional pumps and piping for the air in addition to changes in the hull shape in order to trap the air and create the air cushion. Depending on the design, such a system may require protected propellers or other means of avoiding air to stream to the propeller.

Air cavity systems will only affect the viscous part of the total resistance. Viscous resistance will account for 50% to 70% of the total resistance on most ships. Note, however, that speed vs. stability considerations should be considered.

Less than 3% of the total ship power is needed to support the air cavity system.

Cost of implementation

The cost of implementation is in the range of 2% to 3% of the new building cost for a vessel.

Reduction potential

Providers of the system claim to be able to achieve 15% to 40% 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 at 7% to 10% on the main engine, while for other ship segments it has been assessed at 3% to 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.

Other References

1. Air pollution and energy efficiency / IMO energy efficiency appraisal tool /
2. Silverstream Technologies / Description of the Silverstream system
3. Institute of Marine Engineering, Science & Technology (Imarest) / Air bubbles don’t float Maersk
4. Air Lubrication Drag reduction on Great Lakes Ships / Steven L. Ceccio, Simo A. Mäkiharju / Paper on air lubrication on Great Lakes vessels by Great Lakes Maritime Research Institute / 2012
5. On the energy economics of air lubrication drag reduction / Simo A. Mäkiharju, Marc Perlin, Steven L. Ceccio / Paper on air lubrication and drag reduction / 2012