Why are waterways fuel-efficient?

Four factor contribute to making water transport more
efficient than than either land or air transport. First, because of buoyancy of water,
the effective weight experienced by a transport vessel like a ship in water is less than
that by a land vessel or air plane for comparable
mass.

Second, because of the lower friction between water
and surface of water vessels, as compared to friction experienced by the wheel and axle
system of  land vehicles, the force required to move the same weight is lower for water
vessels than for land transports.

Next, land transportation
frequently usually takes place over roads with many upward and downward slopes and with
many turns. In comparison, water transport is usually on surfaces without any slopes.
Water transport, particularly that in seas, has many fewer turns in the transport route.
Finally, it is possible to make the water transport vessels very large to achieve
economies of scale.

It is worthwhile noting that air
transport also has the advantages mentioned in the second and third points above.
However, such advantages of air planes are more than offset by the need to make the
airplanes very light as compared to its size. This means that for every kilograms of
total goods and passenger weight transport a much larger weight of the transport vessel
must also be transported. This ratio of vessel weight to total weight is highest for
land transport.