10-8
Ship´s hull
0
U
Ship´s
Speed
V
Inside
Boundary
Layer
Relative water Speed
Outside boundary layer
(Relative water speed = U)
Next to hull: Relative
water speed = 0
At 1/2 boundary layer:
Relative water speed = 90%U
At boundary layer edge:
Relative water speed = 100%U
Figure 3 Boundary layer
WATER TRACK U measures speed relatively close to the hull, and may thus measure a lower
speed. When the ship is in shallow water, the boundary layer may be different from normal.
This physical effect will affect all logs measuring relative speed.
The positioning of the TRU is very important. The water flow below the TRU must not be
turbulent or affected by skew water flows. Turbulent flow gives no common signal between the
two channels.
Calibration is needed to compensate the measured speed if measured within the boundary layer.
The calibration may be set at one speed, called single point calibration, or more than one speed,
called multiple point calibration.
3.3.2 Distance calculation
The speed value is integrated into distance. The distance information is sent in separate NMEA
telegrams.
3.3.3 Adverse conditions
It is important to remember that what WATER TRACK U measures is actually the speed when
passing a discrete number of reflectors within a small water volume under the hull. In order to
interpret the echoed signal it is anticipated that both transducer sensors are moving over the
same water volume. If the transducer has been installed at a non-preference location of the hull
the water flow might be turbulent at the site of the transducer. Under these conditions there is
no guarantee for the log to work. See section on installation of bottom parts for correct
transducer installation.
In the low speed range, the distance travelled per time interval is low yielding limited
information to the correlation function. Travelling in water with very high particle content
might give “foggy” reflections due to too many particles in the same water volume. Both these
conditions influences accuracy and may impose lost speed track conditions.
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