Mitsubishi reduces friction on ship hulls by blowing bubbles

In February last year, Mitsubishi Heavy Industries (MHI) and transport company Nippon Yusen Kaisha (NYK) announced plans to investigate the effectiveness of a system intended to reduce the frictional resistance between a vessel's bottom and the seawater using a layer of air bubbles. Now MHI has coupled the Mitsubishi Air Lubrication System (MALS) with a high-efficiency ship hull in the conceptual design for a container ship that the company claims would offer a reduction in CO2 emissions of 35 percent compared to conventional container carrier designs.

To verify the CO2 reduction efficiency of MALS, MHI has installed it on the "YAMATAI," a module carrier operated by an NYK subsidiary. A module carrier was chosen as the first permanent installation of the system because they have a shallow-draft hull that generates relatively low water pressure, which minimizes the amount of electricity required by an air blower to supply air to the vessel's bottom. Additionally, the flat, wide bottom is able to better retain the supplied air under the vessel's bottom.

With MHI expecting to see a reduction in CO2 emissions of around 10 percent on the YAMATAI thanks to MALS, the company is already looking ahead to the second application of the technology with the completion of the conceptual design of the "MALS-14000CS," a New Panamax size 14,000 TEU (twenty-foot equivalent) container vessel. New Panamax refers to the size limit of ships that will be able to travel through the Panama Canal after the completion of its planned expansion in 2014 - specifically, ships with an overall length of 366 m (1,200 ft), width of 48.8 m (160 ft) and tropical freshwater draft of 15.2 m (49.8 ft).

Like the YAMATAI, MHI expects the MALS-14000CS will realize a 10 percent reduction in CO2 emissions through the MALS. However, thanks to additional design features, MHI expects an overall cut in CO2 emissions of 35 percent. These include a new high-performance hull form that places the bridge relatively forward, exhaust funnels at the stern, and additional container space under the accommodation quarter. MHI says this design, coupled with a two-engine, two-shaft propulsion system, will provide a reduction in CO2 emissions of 24 percent, while the electronically controlled diesel engine and waste heat recovery system will provide another five percent.

Other environmentally friendly features include a Sox scrubber to remove sulfur oxide from flue gas, and a ballast water treatment system.

This isn't the first time a layer of air has been proposed as a way to reduce the friction between a ship's hull and the water. While MALS creates a layer of air bubbles by pumping air to the vessel's bottom, researchers are also looking at developing superhydrophobic surfaces modeled on the water fern salvinia molesta, which is able to remain completely dry when submerged by trapping a layer of air on the surface of its leaves using tiny hairs. Combining MALS with such a surface would mean the air wouldn't need to be pumped continually to the bottom of the vessel.

In the meantime, Wired has reported that grain conglomerate Archer Daniels Midland has ordered three dry bulk carriers that will be built with MALS.

Source: MHI