Although generally accepted as a promising fuel saving solution, there are only a handful of vessels fully installed with the Air Lubrication System (ALS). Results from CFD simulations and towing tank tests indicate a change in performance, even up to 10% depending upon the hull form. For ALS it appears that having a large flat bottom surface area increases the benefits. ALS works on the basic premise that using air bubbles at the interface between the hull and water will reduce friction. The impact on streamlined vessels such as naval vessels is said to be smaller.
When it comes to the size of air bubbles, there are two schools of thought. One group states that larger air bubbles should be used, because they cling to the hull, better improving the benefits of ALS. However, the long-term effects are yet to be studied fully, such as impact on vibration of the vessel, impact on propeller cavitation and development of corrosion and fouling. The other group believes that these problems can be mitigated by using micro-bubbles, with a size less than 1mm. Both groups feel the bubbles should be of the same size, uniformly distributed on both sides of the flat bottom, irrespective of the type of propeller or hull form.
Perhaps some of the biggest challenges faced by both the ALS providers and the companies that install the ALS gear on board, are how to translate the increases in performance seen in CFD simulation or towing tank tests to a full life size vessel. For example do the air bubble sizes used on a 2 meter model have the same effect on a 300 meter real ship? The other challenge once installed on board, is to identify the savings in different speed and different dynamic conditions at sea. Finally, since energy is required for producing the air bubbles, how much is the net change in performance and hence what are the total savings.
Independent studies, based on data collected from vessels with ALS in real-life conditions and operations, have had the following results:
ALS brought a net increase in performance overall.
For high speeds the net improvement diminished as the effect of the bubbles appear to become smaller at higher speeds. In some cases additional air compressors were used.
ALS for some reason did not have much impact at very low speeds.
The ALS results didn’t seem to be impacted by the weather, shallow waters, trim, list, etc.
On passenger ships the stabilizer usage did not impact ALS performance.
It is recognized that there is still insufficient data on real life ALS usage, but even with limited data the results have so far been positive. It is perhaps time for the industry to take ALS seriously and design ships with ALS in mind, so that its entire potential can be capitalized on. However before that there are some questions to be asked.
What is the impact on the suction created by the propeller in the water ahead of it, such as impacts on squatting, effect on pumps using aerated seawater from sea-chests, etc.?
It seems to work best when going in a straight line and so what is the impact of continuous rudder adjustments when course keeping?
How much additional resistance does ALS openings on the hull create? What is their long-term Impact on hull performance if the system is not used?
What is ALS impact on the maneuverability of the vessel?
So how is such a study undertaken? The approach used in measuring performance is exclusively data driven – through real-time data collected from on-board systems and sensors. Data transmitted from the vessel is validated, enriched, mathematically modelled and then systematically analysed with relevant subject matter experts. With this fastidious method it is possible to validate the impact of any hydrodynamic investment such as ALS.
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