Test Procedure For The Stw Unit, Wtu Pcb; The Boundary Layer; Calibration Factors Stw - JRC JLN-900 Installation And Instruction Manual

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Test procedure for the STW unit, WTU PCB.

4.1 The boundary layer

The water moves slower close to the hull than it does further away. The layer with lower speed is
called the boundary layer, see figure 1.
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 1 Boundary layer speed distribution.
The WATER TRACK U, Water Track Unit, measures speed close to the hull, and may thus measure a
lower speed. When the ship is in shallow waters, the boundary layer may be different from normal.
This physical effect will affect all logs measuring relative speed.

4.2 Calibration factors STW

How to enter calibration values using the menu system is described in Section 10 "WTU Assembly
Technical Manual".
The STW part (WTU) has three different methods for calibrating the speed.
TRU calibration - a fix calibration factor (marked on the TRU and the TRU-cable)
compensating small differences in characteristics of individual transducers. To be set at the
speed log set-up at installation and when changing TRU only.
Draught calibration – a single point calibration factor compensating differences in indicated
speed, due to load and trim of the ship. Three different preset draught conditions can be
programmed. They are named: FULL LOAD, BALLAST 1 and BALLAST 2.
Ship´s hull
U
Relative water Speed
13-6
Ship´s
Speed
V
Inside
Boundary
Layer
0
Outside boundary layer
(Relative water speed = U)