Furuno MODEL CSH-73 - Manual (Page 71)

9-3
Sea Surface Reflections
As ultrasonic pulses at both edges of the 180˚ sector beam are
always emitted horizontally along the sea surface regardless of
the tilt angle setting, sea surface reflections appear as a thick
line extending across the own ship mark. When the sonar is used
with a narrow tilt angle, the sea surface reflections cover an
extended area as illustrated below.
Sea surface
Reflections
Figure 9-3 Sea surface reflections
Wake
A wake produced by own ship or another ship can be a strong
reflecting object when a narrow tilt angle is used. As the wake
appears as a thick continuous line, it can be easily distinguished
from a fish school. A wake contains many air bubbles which
attenuate ultrasonic energy, making it difficult to sound beyond
the wake.
Wake produced
by other ship
Wake produced by
own ship when ship is
turned
Wake produced
by other ship
Figure 9-4 Wake appearance on the display