Geoscience Reference
In-Depth Information
If V
ˉ
< Vcos
there is an escaping wave. This situation corresponds to the case
when the frequency of collisions as in the case V
ˉ
> Vcos
ˇ
ˇ
can be lower but
opposite by sign, and ship will overhaul the wave.
For example, for a ship having length 152.4 m (0.5 < L
ˉ
/L < 1.5) with ratio
height/length = 1/48 under
ˇ
=30
°
and = 60
°
, V = 9.3 m/s when V
ˉ
> Vcos
ˇ
. In this
case, maximal lateral movement amplitudes can reach about 2.5
. Amplitude will
be growing linearly with an increase of relation wave height/wavelength, while
frequency will be changing intensively as L
1/2
and collision amplitude will be
increasing with growth of the ship velocity but will sharply decrease if the wave-
length becomes lower.
Practical navigation takes into account the physical forces which stimulate the
ship drift using an approximate model (Matusevich 1956)
°
2
sin q
W
V
a
¼ K
a
;
where K
ʱ
is the shift coef
cient, q is the course angle of apparent wind, W is the
wind speed. The coef
cient K
ʱ
is determined from an experimental formula:
K
a
¼
X
p
i
K
a
i
=
X
p
i
;
where K
ʱ
i
is a single value of shift coef
cient de
ned for shift angle
ʱ
i
in concrete
conditions
2
K
a
i
¼
a
i
W
V
sin
1
q
;
i
where p
i
is weight of single shift angle.
Problems of the ship management under its approach to the port and the cal-
culation of powers affecting it under berthing in the port have speci
c place in
navigation. The solution of these problems demands the study of wave energy
formation causes that is connected with different sources of water mass
fl
fluctuation,
due to the variety in physical in
uence of sea environment on the ship. Methods
based on the wave energy balance equations are used to solve the tasks arising here.
Elevation
fl
ʶ
(x, y, t) of free sea surface is described by the following function
(Krylov 1966)
f
ð
x
;
y
;
t
Þ
¼a
0
ux
ð
ch k
ð
exp
ð
i
;
x
t
Þ;
ð
4
:
53
Þ
where t is the time, a
0
is the wave amplitude, k=2
ˀ
/
ʻ
is the wave number, H is the
ʻ
ˉ
sea depth, x and y are the rectangular coordinates,
and
is the length and
frequency of incoming waves, respectively.
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