Page 182 - Technology Roadmap Transportation

TECHNOLOGY VISION 2035

the potential to reduce fuel consumption and are opened on the hull’s bottom. An
consequently emissions. air injection system delivers air to the
cavity through a system of automated
(c) Hull air lubrication system: Air lubrication has compressors and valves. A control system
attracted some attention in recent years as it monitors the volume and pressure of the
has the potential to significantly reduce viscous air and establishes and maintains an optimal
drag of a ship. The principle of operation is and continuous air cavity. The steel-seawater
that a film of air on part of the hull reduces interface is thus replaced by a more
friction and in turn fuel consumption. The basic slippery air-seawater interface, effectively
concepts include: reducing the hull’s wetted surface area and
thereby the viscous drag. The technology
• Air bubbles lubrication system: This system is still under trials. Air injection system
is based on the powered injection of air requires about 1~3% of additional power.
beneath the ship. Several small holes on A decrease in fuel consumption of around
the hull’s bottom are used for injection of 15% is claimed. Negative side effects include
micro air bubbles into the flow stream. By the generation of a destabilizing free surface
interfering with the generation of vortices, under the hull and loss of energy both due
the transition to the highly dissipative to formation of gravity waves on this free
turbulent flow regime, which typically surface and by dispersion of bubbles into
occurs around the hull, is delayed. This helps the propeller inflow.
in reduction of viscous drag due to lower
friction forces associated with laminar flow 4.2.2 Use of hybrid lightweight material for ship
as compared with turbulent flow. However, construction
the concept is not without problems. By reducing the weight of a ship’s hull,
Uncertainties in the physical mechanisms
and the scaling and technical feasibility consumption of fuel and therefore emissions can
be reduced. Steel can be replaced by lighter weight
of this system need to be resolved. alternatives in non-structural elements or by lower
In particular, the potentially negative weight high-tensile steel. At present, lightweight
interactions of the dispersed bubbles with
the propeller must be eliminated before materials such as aluminium, carbon fibre or
glass-fibre sandwich constructions are being
use of the concept on board ships is mainly used for high-speed crafts. Hybrid materials
popularized.
can be formed from multiple layers of metal
• Air cavity lubrication system: In air cavity sheets and piles of polymer composite laminates.
lubrication system, large indentations Fibre-metal laminates combine the qualities of
FIG 4.3: AIR CAVITY LUBRICATION SYSTEM[3]

HOW ACS WORKS
Compressed air is pumped through
1 holes in the bottom of the ship

5 The air is dispersed to either
side of the propeller

2
The compressor unit
uses less than 1% of the
ship's engine power
3
A "carpet" of air builds up
beneath the hull, reducing 4 The bottom of the hull has
friction as it passes through a recess which helps the
the water air carpet to form

180 WATERWAYS