Page 186 - Practical Ship Design

152 Chapter 5

capacity and sometimes the speed are determined by the design instead of being
the main factors that determine it.
The design processes for these ships are essentially non-standard and give the
naval architect a chance to exercise his ingenuity. Some methods and ideas relating
to these types may, however, be of interest.

5.7, I Container ships
As the design deadweight of most container ships can be carried at a draft less than
that obtainable with a type B freeboard, deadweight cannot be used directly to
determine the main dimensions.
On the other hand, as container ships carry a substantial proportion of their
cargo on deck, it is not possible to base the design on the required cargo volume as
this is indeterminate. In these circumstances, stability considerations take over the
primary role in the determination of the main dimensions.
For maximum economy in the design of any container ship, the dimensions
should be such that containers can be stacked up in tiers to the limit permitted by
stability. To maximise numbers, the upper tiers, subject to the owners agreement,
should be reserved for relatively lightly loaded (or even empty) containers, whilst
heavier containers are directed to the lower levels. It may also be desirable in the
interests of maximising container numbers and therefore revenue to design the
ship to carry ballast, either permanent or water or both even in the load departure
condition - something that would be a heresy in the design of most ship types!
For each number of tiers of containers carried there is an associated breadth of
ship which will provide the KM necessary to ensure stability, whether the tiers are
enclosed below hatch covers or carried on deck being a second order effect. Ships
designed to achieve a particularly high KM for a given breadth obviously have an
advantage provided any penalty incurred in powering or seakeeping is acceptable
(see Chapter 8, 58.6)
Longitudinal and torsional strength require a proportion of the breadth of the
ship to be devoted to structural decks with the balance of the “open” ship providing
space for a number of container cells with their cell guides. There is therefore a
direct relationship between the number of container tiers and the number of
container rows in the breadth.
A first approximation to the length of the ship is then generally determined by
what is thought to be an economically desirable value of the lengthheam ratio.
This is then adjusted in association with the length required for the engine room,
peak tanks, cell guides,bulkhead stiffeners etc. so that the cargo spaces are tailored
to a multiple of container lengths.
With the number of container tiers “fixing” the number of rows in the breadth
and this in turn “fixing” the number of container cells in the length, there is the