Air Lubrication Systems

What do you think about the Air Lubrication System? Is it an effective energy saving device?


Interesting question!
The air lubrication technology is not something new.
The concept was first developed in the 1950’s by the US Navy as a radiated noise reduction system. Air bubbles would be injected around the hull and the propeller to distort the acoustic signature of warships. Later on researchers in the US, Russia and Japan started thinking of air lubrication systems as energy saving devices.
In 2018, the IMO officially recognized air lubrication systems as Category B-1 “Innovative Energy Efficient Technology” as (MEPC.1/Circ.815 (2018)).
The operational principle of the air lubrication system is the following: compressed air is injected on the flat bottom of the ship, forming an air layer, hence reducing the direct contact of the ship’s wetted surface with the water flow i.e. reduction of skin friction resistance. The skin friction resistance is the predominant component of the total resistance of conventional hull forms

There are three major categories of air lubrication technology:

According to reported data, a vessel may achieve a net power saving of 4-5%, owed to an air lubrication system.

The installation of an air lubrication system is not simple at all.
Here is an example of the air lubrications system equipment for an LNG carrier.

The installation cost is circa 2 million USD and the yearly maintenance cost is around 25,000 USD (not accounting for the off-hire or extra docking days if it is a retrofit).

The performance of air lubrications systems is strongly affected by weather conditions.
Such systems would not operate efficiently in rough seas.
Higher gains are expected in larger vessels that operate at high speeds.
The installation cost is high. It would make sense to install it in a time chartered vessel where the cost could be shared between the owner and the charterer.


It would take a study to determinate the exact savings and the feasibility of the installation on the vessel of course. However I am thinking that for 5% savings and increased energy requirements/weight/maintenance for the operation of the air lubrication system why bother and not go for silicon hull paint?

Yes, I agree.
The above case is not a theoretical one. It is a real life installation on an LNG carrier newbuilding in a Korean yard. The savings were measured during sea trials and the feasibility installation was done by the building yard.
There is no straightforward answer in your latter question.
Both technologies have their pros and cons and the decision “what technology to install” is usually evaluated on a case by case basis.
What kind of ship? What type of chartering contract? What kind of commercial requirements? Etc. Etc.
Your concerns on air lubrication systems are correct but consider also the case where a shipowner invests a big amount on silicon paints (full SA2 blasting + expensive coating) and 6-months after the drydock, the ship remains idle for 40 days in tropical waters and barnacles grow in the flat bottom and vertical sides. What would be the result? The ship’s underwater parts will have to be cleaned using hard brushes and therefore the silicon paint will be irreversibly damaged.

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I hadn’t thought about that. That is interesting to look into.

Hi Paraschos,
You said that “according to reported data, a vessel may achieve a net power saving of 4-5%, owed to an air lubrication system.”, is it with respect to a conventional hull antifouling treatment or with respect to Silicon hull treatment?
What about the maintenance of the system? How often you must go to drydock? Does the injectors get ussually blocked? If enter in ports with small drafts??(seachests ussually have problems, so I could imagine what will happen to these inyectors)

Hello @jblanco It is with respect to conventional hull antifouling system.
The drydocking cycle should not be affected. Cannot think of a reason, linked to the installation of this system, that may lead the vessel to go to drydock more often than required. The maintenance concerns mainly the screw air compressors, the fresh water cooler and the cooling pumps, which are pretty straightforward systems.
As far as the blocking of the injectors is concerned, even if the hull gets fouled (due to the ships idling) these injectors should not normally be blocked, pressurized air blowing helps in keeping these injectors clean.


My understanding is that while there are some indicative figures as mentioned by @Paraschos.Liadis for LNGC (which is a ship type which ALS is applied and evaluated), its project need to be evaluated independently.
Points of concern
Effectiveness of the air lubrication film/ frictional resistance, depends on the range of reynolds number the vessel operating, effect on propeller inflow and efficiency of propeller at the new operating point (CPP could assist?).
Operating draught affects the pressure required to maintain the layer, thus dictating the compressors number and scale, definety affecting the total cost of installation.

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@Alexandros.Senteris your points are valid!
A thorough case by case study and ideally model testing would be required in order to take such a decision.

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The concept used currently for LNG-c is that the power used by the ALS compressors is delivered by the shaft generator (which usually has lower cost / kWh in the case of 2 stroke gas engine and permanent magnet rotor than diesel generators).
So you take out power on the shaft with generator to reduce hull resistance and thus power required by propeller for a given speed. If you start ALS the hull resistance will decrease and vessel speed increase. To maintain your speed you will thus reduce main engine power. You know the main engine power before and after so you know your gain.

For the propulsive efficiency,as per ISO 15016-2015 as part of model test a propeller load varitaion test should be conducted. This gives the relationship between propulsive efficiency and hull resistance for variation in the same order of magnitude than ALS saving. For LNG-c decreasing the resistance increase the propulsive efficiency.

The below paper gives a general overview of the relationships between vessel speed, propulsive and compressor power in order to establish a methodology to conduct a cost benefit analysis for such system.
SMOOTHALDRCeccioPerlinElbing.pdf (276.6 KB)


@Adrien56 thank you for your valuable insight.
As you mentioned LNG-c (and maybe containerships/roro) could have a sizeable benefit. What I have in mind is 3-7% net savings. As you also probably have experience operating such systems, can you confirm savings of this range?
Electricity generation from shaft generators/PTO is an initiative improving energy production efficiency, which can be utilized in many consumer (e.g. hotel loads).
What I tried to convey, maybe not extremely sucessfully, is that net power savings are questionable if the speeds are lower and reduce further in the influence of higher draught (pumping power exceeds the savings in friction drag/considering also the respective energy production efficiencies). Thus, such solution may not be applicable to other vessel types (bulkers or tankers).


Thank you for your valuable feedback. Does the vessel operate the ALS continuously or under certain conditions only (eg good weather, higher speeds)?
Do you have any experience with retrofits? It would be interesting to compare the performance and savings before the installation and after the installation/ hull modification during the days the ALS is not in operation.


ALS could be either turned on or off, on demand. Retrofits according to my knowledge are limited and most of applications are on NBs. Someone else may have some more experience on the matter.