A vessel’s resistance when moving through the water is made up of multiple components, of which frictional resistance is the most dominant. Injection of air into the turbulent boundary layer (between the stationary and moving water) can reduce the frictional resistance of the hull.
Outlined below are the main technologies tested in full scale:
ALS - Air Lubrication Systems: Unlike the below mentioned ACS method that attempts to maintain air in fixed compartments, Air Lubrication Systems (ALS) provide a constant flow of air bubbles to lubricate the flat bottom area of a ship’s hull which requires minimal structural changes. The main technologies currently available in the industry are:
- The Silverstream® System developed by Silverstream Technologies utilises 10-18 Air Release Units (ARUs) depending on vessel size and is currently commercially marketed for both new-build and retrofit installations. The first system was retrofitted on 40,000 DWT Tanker; MT Amalienborg in 2014 with net efficiency savings of above 5% from multiple sea trials in ballast and laden draughts of 7m to 11m with speeds ranging from 10 to 15 knots. Performance has been independently verified by Lloyds Register, HSVA, Shell and Southampton University. More recently, the Silverstream® System was installed on Norwegian Cruise Lines’s 163,000 GT newbuild; Norwegian Joy at Meyer Werft in 2016 and retrofitted to another cruise ship in 2017. The application of this technology for cruise installations has now been proven with net efficiency gains of more than 5% in draughts of 8m to 9m with speeds ranging from 10 to 25 knots, the results of which have been verified by reputable 3rd party organisations. Vessels like LNGC’s and Ro-Ro’s which tend to have larger flat bottoms can experience net savings in the range of 8-10%. In addition to energy savings, other benefits from use of the Silverstream® System include minimised noise and vibration and reduced fouling as confirmed from previous installations.
- The Mitsubishi Air Lubrication System (MALS) incorporates 3 outlets installed into the hull. The most recent installation of MALS has been on two AIDA Cruise vessels in 2016 and 2017. Efficiency savings of these cruise ship installations have not been published.
ACS – Air Cavity Ships: This method used large ‘cavities’ into the bottom of the hull filled with air. Full scale testing on the ‘ACS Demonstrator,’ an 83m multi-purpose vessel was executed by DK Group in 2008. The tests showed that in calm water, friction was reduced, but, in waves the energy needed to refill the cavities eliminated the overall savings. In conclusion, the associated significant structural modifications to the hull and cost rendered the idea impractical.
WAIP – Winged Air Inject Pipe: This concept involved 124 circular nozzles placed at the forward end of the hull expecting to lubricate both the flat bottom and vertical sides. The system was fitted to container carrier ‘Olivia Maersk’ in 2009 but after intensive testing, Maersk found that the balance between the energy and savings achieved was negative and the project was discontinued.
ACES – Air Chamber Energy Savings (similar to ACS): Developed as part of a series of Dutch national research projects with Damen Shipyards and MARIN. The system involved injecting air into large recesses covering the flat bottom of a vessel which claimed potential net power savings in calm water.