NorthStar Navigation Multi-Function Unit M84 - Manual (Page 36)

Northstar M121/M84 Installation and Operation Manual 35
CAUTION
Planing hulls at speed produce air bubbles
and turbulent water that bombard the
transducer. The resulting ultrasonic noise
may be picked up by the transducer and
obscure the real echoes.
The scroll speed can be set by the user to
display either a longer history with less fish
information or a shorter history with more fish
details (see section 19-3).
If the boat is anchored, the echoes all come
from the same area of bottom. This produces a
flat bottom trace on the window.
The screen shot shows a typical sonar window
with the Fish symbols turned Off. The sonar
pulse generated by the Instrument transducer
travels down through the water, spreading
outwards in a cone shape. The cone width is
dependent upon the frequency of the pulse; at
50 kHz it is approximately 45°, and at 200 kHz it
is approximately 11°.
The differences in the cone width affect what is
displayed. (See section 9-3)
Strength of echoes
The colors indicate differences in the strength
of the echo. The strength varies with several
factors, such as the:
• Size of the fish, school of fish or other object.
• Depth of the fish or object.
• Location of the fish or object. (The area
covered by the ultrasonic pulse is a rough
cone shape and the echoes are strongest in
the middle.)
• Clarity of water. Particles or air in the water
reduce the strength of the echo.
• Composition or density of the object or
bottom.
Bottom types
Mud, weed and sandy bottoms tend to weaken
and scatter the sonar pulse, resulting in a weak
echo. Hard, rocky or coral bottoms reflect the
pulse, resulting in a strong echo.
Frequency and cone width
Water Depth ft/m Cone width at 50 kHz Cone width at 200 kHz
45˚ cone 11˚ cone