NorthStar Navigation Explorer 660 - Manual (Page 32)

Northstar Explorer 660 Installation and Operation Manual 32
8-2 Interpreting the display
The sonar displays do not show a fixed distance
travelled by the boat; rather, they display a
history, showing what has passed below the boat
during a certain period of time.
The history of the sonar signal displayed depends
the depth of the water and the scroll speed
setting.
In shallow water, the echoes have a short distance
to travel between the bottom and the boat. In
deep water, the history moves across the display
more slowly because the echoes take longer to
travel between the bottom and the boat. For
example, when the scroll speed is set to Fast,
at depths over 1000 ft (300 m) it takes about 2
minutes for the data to move across the display,
whereas at 20 ft (6 m) it takes only about 25
seconds.
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 15-3).
If the boat is anchored, the echoes all come from
the same area of bottom. This produces a flat
bottom trace on the display.
The screen shot shows a typical sonar display
with the Fish symbols turned Off.
Bottom
Large
school of
fish
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.
Note: 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.
Single fish
Kelp / Weed
Hard bottoms
such as rock
or coral show
as wide bands
Soft bottoms
such as mud,
weed and
sand show as
narrow bands
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. See section 9-3.
Small
school of
fish