sufficient length of time, or when the duration time is
limited. Now let’s look at each situation.
FETCH LIMITED SEA.— When the fetch length
is too short, the wind is not in contact with the waves
over a distance sufficient to impart the maximum energy
to the waves.
The ranges of frequencies and wave
heights are therefore limited, and the wave heights are
less than those of a fully developed sea. The process of
wave generation is cut off before the maximum energy
has been imparted to the waves and the fetch is in a
steady state. This leads to the conclusion that for every
wind speed, a minimum fetch distance is required for
the waves to become fully developed, and that if this
minimum fetch requirement is not met, the sea is fetch
limited.
DURATION TIME LIMITED SEA.— When the
wind has not been in contact with the waves long
enough, it has had insufficient time to impart the
maximum energy to the waves, and the growth of the
frequency range and wave heights ceases before the
fully developed state of the sea has commenced. Such
a situation is known as a duration time limited sea. This
leads to the conclusion that for every wind speed, a
minimum duration time is required for the waves to
become fully developed; and that if this minimum
duration time requirement is not met, the sea is duration
time limited. The state of the sea, then, is one of three
conditions: fully developed, fetch limited, or duration
time limited.
Table 6-1 shows the various wind speeds, fetch
lengths and minimum wind duration times needed to
generate a fully developed state of the sea. When
conditions do not meet these minimum requirements,
the properties of the waves must be determined by
means of graphs and formulas.
DETERMINING THE WIND FIELD
As we have discussed, wind is the cause of waves.
It therefore stands to reason that in order to accurately
predict sea conditions, it is necessary to determine wind
properties as accurately as possible. Miscalculation of
fetch, wind speed, or duration will lead to inaccuracies
in predicted wave conditions.
In this section, we present methods of determining
the wind properties as accurately as possible with the
available data.
Table 6-1.-Minimum Wind Speed (V), Minimum Fetch Length
(F), and Minimum Duration Time (t) Needed to Generate a
Fully Developed Sea
V
F
t
WIND
FETCH
DURATION
SPEED (KT)
LENGTH (NMI)
(HR)
10
10
2.4
12
18
3.8
14
28
5.2
16
40
6.6
18
55
8.3
20
75
10
22
100
12
24
130
14
26
180
17
28
230
20
30
280
23
32
340
27
34
420
30
36
500
34
38
600
38
40
710
42
42
830
47
4
960
52
46
1,100
57
48
1,250
63
50
1,420
69
52
1,610
75
54
1,800
81
56
2,100
88
6-6
