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2. HORIZONTAL MODE
2-8
Points to consider
• Normally, a vertically distributed fish school is a better sonar target than the bottom,
because it reflects the transmitted pulse back toward the transducer.
• In case 3, both fish schools “a” and “b” are presented. Generally speaking, however,
midwater fish schools tend to be larger than bottom fish schools and they are often
displayed near the bottom on the display.
• It is difficult to detect bottom fish when they are not distributed vertically.
2.6.3 Tilt angle for surface fish
Sound emitted from the sonar transducer forms an oval-shaped beam with a width of
approximately 8° in the vertical direction (vertical beam width). The tilt angle is indicated
by the angle between the center line of the beam and the horizontal plane. Then, if the tilt
angle is set to 0°, the center line is parallel with the sea surface and one half of the
emitted sound goes upward, toward the sea surface.
This causes one half of the emitted sound to be reflected toward the transducer and
displayed on the screen as sea surface reflections. When the sea is calm, since the sound
is reflected just like a light hitting a mirror at a narrow incident angle, it propagates away
and the sea surface reflections become negligible.
However if the sea is not calm enough, they will become dominant and interfere with
observation of wanted echoes. To minimize these sea surface reflections and to search
fish schools effectively, the tilt angle is usually set between 5° and 6° so the upper portion
of the beam becomes almost parallel with the sea surface. When the sea is rough, the tilt
angle is slightly increased to lessen the affect of sea surface reflections.
Tilt angle 0°
Sea surface
Tilt angle 5-7°
Sea surface
8°
8°
Tilt angle