Page 99 - Introduction to Naval Architecture

86 THE ENVIRONMENT

Table 5.3 Beaufort scale

Number/description Limits oj speed
(knots) (m/s)

0 Calm I 0.3
1 Light air 1 to 3 0.3 to 1.5
2 Light breeze 4 to 6 1.6 to 3,3
3 Gentle breeze 7 to 10 3.4 to 5.4
4 Moderate breeze 11 to 16 5.5 to 7.9
5 Fresh breeze 17 to 21 8.0 to 10.7
6 Strong breeze 22 to 27 10.8 to 13.8
7 Near gale 28 to 33 13.9 to 17.1
8 Gale 34 to 40 17,2 to 20.7
9 Strong gale 41 to 47 20.8 to 24,4
10 Storm 48 to 55 24.5 to 28.4
1 1 Violent storm 56 to 63 28.5 to 32.6
12 Hurricane 64 and over 32.7 and over

generation area they are termed swell. The wave form depends also
upon depth of water, currents and local geographical features. Unless
otherwise specified the waves referred to in this book are to be taken as
fully developed in deep water.
The strength of a wind is classified in broad terms by the Beaufort
Scale, Table 5.3.
Due to the interaction between the wind and sea surface, the wind
velocity varies with height. Beaufort wind speeds are based on the wind
speed at a height of 6 m. At half this height the wind speed will be about
10 per cent less than the nominal and at 15 m will be 10 per cent
greater. The higher the wind speed the less likely it is to be exceeded.
In the North Adantic, for instance, a wind speed of 10 knots is likely to
be exceeded for 60 per cent of the time, 20 knots for 30 per cent and
30 knots for only 10 per cent of the time.

WAVES
An understanding of the behaviour of a vessel in still water is essential
but a ship's natural environment is far from still, the main disturbing
forces coming from waves.
To an observer the sea surface looks very irregular, even confused.
For many years it defied any attempt at mathematical definition. The
essential nature of this apparently random surface was understood by
R. E. Froude who, in 1905,* postulated that irregular wave systems are