Although air lubrication solutions have been in use in the maritime industry for over a decade, there are still some misconceptions surrounding this energy-saving technology. There are five common myths surrounding air lubrication, and this article explains why they are wrong.
Air lubrication changes the interaction between water and a vessel’s hull by creating a carpet of microbubbles that will coat the flat bottom of the vessel. Because the bubbles decrease the frictional resistance, it will take less power to push the vessel through water.
Less power needed for propulsion saves fuel and cuts emissions.
But what do people get wrong about this technology? Let’s quash the five most common myths about air lubrication.
When the tip of a propeller rotates with a velocity well above 100 km/h, a low-pressure region is created, which causes the cold sea water to turn into vapour. This is called cavitation. Once the vapour is out of the low-pressure region, it collapses violently, creating shockwaves and potential erosion.
The bubbles coming from air lubrication are filled with air, not water vapour, and therefore do not collapse at all. Moreover, in a mixture of water and air bubbles, the fluid properties change quite significantly, which can even suppress the development of cavitation to a certain extent. The air bubbles also act as a cushion during the collapse phase of the water vapour.
Because air lubrication does not cause cavitation, it also avoids the associated sonic boom.
It is already known that a small amount of underwater air has the potential to significantly dampen the transmission of URN – for example, bubble screens have been used to mask noise from offshore drilling operations for several years. Structure-borne noise from a two-stroke engine will be dampened by the air layer, as part of the transfer path from the hull plating to the sea water. The noise from cavitation phenomena will be dampened as well.
Air lubrication technology has been in use for more than a decade. It is also easy to measure its impact to fuel consumption in a ‘live environment’ simply by switching the air on and off. When the system is turned on, one will see a drop in shaft power and an increase in speed, as the frictional resistance is reduced.
Dry dock vessel inspections after multiple years of operation with air lubrication have shown little to no biofouling of system components. Leading coatings manufacturers have also confirmed that they do not expect air lubrication to cause any detrimental impact to hull coatings.
Air lubrication works well with other energy-saving technologies, such as optimised hydrodynamic designs, and EnergoProFin, EnergoFlow and GATE RUDDER™ from Wärtsilä.
Because the effects can easily be measured by turning the system on and off, one can check that the solution is indeed providing additional efficiency gains.
Air lubrication reduces the power needed for propulsion, and therefore saves fuel and cuts emissions.
Of course, the ship operator can use air lubrication and then choose either the normal speed – with fuel savings – or a higher speed to reach the destination faster.
Another benefit of air lubrication is regulatory compliance – it is a clean technology recognised under the IMO’s Energy Efficiency Design Index (EEDI) for newbuilds. And because it is simple to retrofit to a vessel during a planned dry dock, it is an opportunity for existing vessels to adopt and improve the efficiency rating on the IMO’s Energy Efficiency Existing Ship Index (EEXI).
Air lubrication is one of many energy-saving technologies that improve the propulsion efficiency of vessels. It can reduce emissions and increase operational flexibility for both retrofits and newbuilds.
In addition to air lubrication, there are at least 49 more ways to reduce greenhouse gas emissions. Download this fascinating eBook “50 great ways the maritime industry could cut its greenhouse gas emissions” to discover more options.
This article was first published in August 2022. It was reviewed and updated in February 2024.