Page 67 - Practical Ship Design

38 Chapter 2

criterion. For vessels which are usually stationed in one location, the likely weather
is more easily defined than for a mobile ship. For most of the main production areas
records have been kept from which projections of wind speed and direction, wave
length and height can be made. These predictions are commonly made on the basis
of the likelihood of occurrence in a particular period, for instance the ten-year
storm or the hundred-year storm, although it must be said that the latter has shown
an alarming ability in recent years to turn up rather early in its century!
Where vessels are required to operate in ice, it is necessary to define whether the
ice is first-year or multi-year ice as well as stating the thickness. Lloyds have five
notations for first-year ice ranging in thickness from 0.4 to 1 .O m. For multi-year
ice they have another four notations for ice thicknesses ranging from 1 .O to 3.0 m.
For ships operating in Arctic or Antarctic waters the formation of ice on deck
can be a severe problem both because of its effect on the operation of deck
machinery and because of the loss of stability caused by the accretion of top-
weight. Inclusion in the statement of requirements of a need for ice accretion to be
considered can lead to both a reduction in the amount of gear arranged on the open
decks on which the ice can form and to the provision of systems that will speed ice
clearance.
A statement on the maximum and minimum air and sea temperatures in which
the ship will operate is required. This information has several uses and these need
to be remembered when the figures are being stated. The design of the accom-
modation heating system requires a knowledge of the ambient cold air temperature
which will be encountered in winter. The design of the air-conditioning system
requires knowledge of the ambient hot air temperature and associated relative
humidity which will be met in summer. In both cases the desired inside temp-
eratures and relative humidities must also be stated.
Air and sea temperatures also affect the power output and the fuel efficiency of
both diesel engines and gas turbines and it is desirable to give values of these
temperatures to be used as the basis of powering and of fuel consumption. It is
worth noting that these may or may not be the same as the temperatures on which
the air conditioning is to be based.

2.4 MERCHANT SHIP REQUIREMENTS AND TRANSPORTATION STUDIES

The main requirements that need to be set for a merchant ship are:
- the type and quantity of cargo to be carried;
- the service speed, the voyage route and distances.
The type of cargo and how it is to be stowed on the ship and handled on and off the
ship determines the ship type, whilst the quantity of cargo to be carried is obviously
the main determiner of the ship’s size.