Page 68 - Practical Ship Design

Setting Design Requirements 39

The type of cargo is generally the starting point, although some transportation
studies commence at a more fundamental level by looking at a country’s or
region’s economic forecasts to identify the cargo-carrying capacity that will be
needed in the future.
Even if consideration is limited to a particular cargo, a wide-ranging economic
study will usually be necessary to assess the quantity that will need to be carried in
the future and the loading and discharge ports which will best meet the trade.
At the next level it is necessary to assess what competition there will be and the
optimum shipment size. Consideration must be given to whether the ship should
cater for one cargo only or should be so designed that it can carry more than one
cargo at the same time or a different cargo or cargoes on different voyages. The
possibility of there being a suitable return cargo for the “homeward” leg of the
voyage avoiding a ballast leg must be investigated as this can appreciably improve
the economics.
In a simple case it may be possible to link the number of ships, their carrying
capacity and the number of voyages per year directly to the quantity of cargo
requiring transport. The aim may either be to ensure that the transport need can be
met or that the trade is a profitable one, or more usually both.
The number of voyages per year is clearly a function of time spent at sea and that
spent in port loading and discharging. The sea time is set by the voyage distance
and the ship’s speed; the port time by the cargo-handling arrangements provided.
In real life things are much more complicated and shipping companies require
more than a little luck as well as very sophisticated calculations to lay down
requirements that will result in ships which will operate profitably.
Once a preliminary decision has been taken to fix the ship’s cargo capacity, the
next step should take the form of a sensitivity study to optimise such things as the
speed and cargo-handling methods. When these have been optimised, further
refinement of the cargo capacity may follow.
For heavy cargoes the cargo capacity should be stated as the cargo deadweight;
for light cargoes the cargo capacity should be stated as the cubic capacity; for
intermediate cargoes both deadweight and cubic capacity should be stated. For
container ships, vehicle ferries and passenger ships the capacity should be a stated
in numbers divided into appropriate categories such as forty- and twenty-foot
containers, goods vehicles and cars, first- and second-class passengers, etc.
As well as the route between the cargo loading and discharge ports, the
transportation study should consider where the ship should load fuel. A short
addition to the voyage route may enable the ship to divert to a port where fuel can
be shipped at a lower price than prevails in the cargo terminal ports. If fuel can be
obtained cheaply at more than one port in a round voyage it may be wise to use this
option, thereby reducing the average displacement and hence the fuel consumption
per mile.