Furuno CH-37 Operator's Manual page 37

Sea surface reflections
To reduce sea surface reflections, set the tilt angle to 5 or
higher, so the upper edge of the sonar beam does not hit the
sea surface, or adjust TVG. When a narrow tilt angle is used,
sea surface reflections cover a large area as illustrated below.
15
15
Figure 2-19 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.
Other
ship
Own
ship
Own ship's
screw noise
Figure 2-20 Wake appearance on the display
Sea surface
Sea surface
Wake produced
by other ship
Own ship's wake
Own ship's (produced when
screw noise own ship turned)
Tilt angle
indication
Sea surface
reflections
2-21