Simrad ES70 - INSTALLATION REV A Installation Manual page 117

Figure 21 Boundary water layer
A Turbulent flow
B Laminar flow
C Air bubbles in the water
Furthermore, air bubbles in the sea water are pressed down below the hull and mixed
into the boundary layer. The boundary layer is thin underneath the forward part of the
vessel, and increases in thickness as it moves towards aft. If the sides of the hull are
steep, some of the air bubbles in the boundary layer may escape to the sea surface along
the vessel sides. It is our experience that a wide and flat bottom, with a rising angle less
than around 13 degrees, is prone to giving air problems for the transducer. In any case a
transducer location in the forward part of the hull is preferred in order to minimise the
influence of the boundary layer.
Mount the transducer far away from the propellers
The propulsion propeller is the dominant noise source on most fishing vessels, research
vessels, merchant vessels and pleasure crafts. The noise is transmitted through the sea
water. For this reason, the transducer should be placed far away from the propeller,
which means on the fore part of the hull. Positions outside the direct line of sight from
the propeller are favourable. On small vessels with short distances it is advised to mount
the transducer on that side of the keel where the propeller blades move upwards, because
the propeller cavitation is strongest on the other side. The cavitation starts most easily
when the water flows in the same direction as the propeller blade, and that is to some
degree the case at that side of the keel where the propeller blades move downwards.
Mount the transducer far away from the bow thrusters
Bow thruster propellers are extremely noisy. When in operation, the noise and cavitation
bubbles created by the thruster make the echo sounder useless, almost no matter where
the transducer is installed. And when not in operation, the tunnel creates turbulence,
and if the vessel is pitching, the tunnel may be filled with air or aerated water in the
upper position and release this in the lower position. In general, all transducers must
be therefore placed well away from the bow thruster. However, this is not an invariable
343522/A
Transducer Installation
115