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2. HORIZONTAL MODE
2-6
2.6.1 Tilt angle for surface fish
Sound emitted from the sonar transducer forms an oval-shaped beam with a width of
approximately 10° (–3 dB, full scan, vertical beam width on 85 kHz) in the vertical direction
(vertical beam width). The tilt angle is indicated by the angle between 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 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 7° 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
10°
10°
Tilt angle
2.6.2 Distinguishing fish echo from bottom echo in coastal waters
In coastal operations the bottom reflection is prominent so it is important to be able to
distinguish fish echoes from the bottom echo. The information below will help you to do this.
Tilt angle and bottom echo
Case 1 - Tilt angle 30° to 40°: The bottom echo, captured by the vertical beam width (10°
at -3dB full circle, 85kHz), is displayed “ring-shaped” and in strong colors.
Case 2 - Tilt angle 10° to 20°: The bottom echo is displayed wider and in weak colors as
the distance to the bottom increases.
Case 3 - Tilt angle 0° to 5°: The bottom echo is either weak or is not displayed even if the
vertical beam width captures the bottom echo, because the distance to the bottom is too far.