Page 110 - Introduction to Naval Architecture

THE ENVIRONMENT 97

selected wave height and wave period bands. They show a spread of
period for a given height and of height for a given period. This 'scatter'
is not due to inaccuracies of observation but to the fact that the sea
states observed are at various stages of development and include swell
as well as sea components.
The data can be combined in many ways. They can, for example, be
averaged over the North Atlantic or world wide. Doing this confirms
the popular impression that the Atlantic is one of the roughest areas:
21.4 per cent of waves there can be expected to exceed 4 m whereas the
corresponding percentage worldwide is 16.8.

OTHER EXTREME ENVIRONMENTS

In addition to the conditions of wind and waves to which all ships are
subject, there are other extreme conditions the ship and equipment
may need to allow for. These include driving rain, dust and sand which
can abrade exposed surfaces, chemical deposits (including salt from
spray) and fungi which can harm surfaces and eat away certain
materials. Sea-spray and snow can cause icing up in cold climates. Ice
impedes the operation of moving items and can pose a serious stability
problem. The conditions upon which designs of ship and equipment
should be based are laid down in various specifications. These also
define suitable tests and should be consulted by the designer.

INTERNAL ENVIRONMENT

Besides the external environment in which a ship may operate, the
naval architect is concerned with the environment inside the vessel.
The vibration levels, for instance, must be kept low for comfort and
efficient functioning of machinery. Noise must also be kept below
certain levels. Vibration and noise are discussed in Chapter 11. Vertical
accelerations associated with ship motions must be minimized to
reduce the likelihood of motion sickness.
Other features of the internal environment the naval architect will
control include:

(1) The air quality in terms of temperature, humidity, purity and
3
odours. Typically about 0.3 m of fresh air are introduced for
each person per minute. A person generates about 45 watts of
sensible heat and 150 watts latent heat, depending upon the
level of activity. These and the heat from machines, must be
catered for by the air-conditioning system. The aim is to