Aerodynamics of Vehicles and Airplanes, and Hydrodynamics of Ships - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 112

Further, there are two main factors in hydrodynamic resistance:

c R ¼ f ð drag and wave resistance Þ : ð 7 : 2 Þ

The viscous resistance or drag is related to the Reynolds number and the
roughness on the hull length, and has an intensive impact on fuel consumption.
Silicon-based coloring on the hull with a very low hydrodynamic resistance
significantly contributes toward decreasing fuel consumption. The best example is
the world's largest container ship, Emma Maersk which saves approximately
1,200 t, i.e., 2.6 910 6 lb (0.01%) of fuel consumption per year through the use of
silicon-based color [ 15 ].
Besides smoothness, construction factors, e.g., the form of the hull, the structure
of propulsion system, and propeller also influence the resistance of the vessel.
A rectangular form is usually required for high transverse stability, e.g., for
tankers, and an appropriate depth of hull for longitudinal strengths in bulk carriers
or container ships. If the weather is windy, the resistance of waves primarily
influences fuel consumption [ 16 ]. On slow ships such as tankers and bulk carriers,
which have a Froude number smaller than 0.20, predominantly the friction
determines the resistance. On fast ships, such as ferries, refrigerator ships, con-
tainer ships, and war ships, which have Froude numbers over 0.25, the friction is
usually insignificant compared to the wave resistance [ 17 ].
In aerodynamic development in the last decades, the first improvement for fuel
savings has been the enlargement of the ship's hull; the second improvement has
been the regulated decrease in the ships' velocities with the introduction of
automated speed control systems [ 18 ].
The interaction between hull and wave defines the choice of propulsion type in
the construction phase and determines the required power of propulsion system in
operation.

7.3.1 Floating on a Cushion of Air

Large ships can save energy by floating on a cushion of air [ 19 ]. This technology
needs compressed air which is pumped through holes in the bottom of the ship.
A ''carpet'' of air builds up beneath the hull, reducing friction as it passes through
the water. The air is dispersed to either side of the propeller.
Decreasing friction between the hull and the water with an air cushion can
reduce fuel consumption up to 15%. On modern ships, wave radar sensors measure
the distance from the cavity ceiling to the water surface and level sensors auto-
matically control the height of the air cushion. The heeling of the ship is also
monitored, because the greater the heel angle, the higher the probability that air
will escape.