NorthStar Navigation EXPLORER 435 - Manual (Page 17)

Northstar Explorer 435/438 Installation and Operation Manual 17
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:
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 5-3
Sonar Bottom display.
Small school
of fish
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 600 ft (180 m) it takes about 2
minutes for each vertical line of pixels 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 3-2 Setup > Sonar.
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.
• 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.
Frequency and cone width
The pulse generated by the Explorer 435 and
Explorer 438 transducer travels down through the
water, spreading outwards to form a rough cone
shape. Inside the cone, the return signals are the
strongest. The cone width is dependent upon
the transducer design and the frequency of the
pulse: with Northstar’s transom mount supplied,
it is about 15°. The chart shows how the cone
width varies over depth for each frequency used.
Figures are approximate.
Depth 200 kHz
15°
50
100
150
200
250
0
13
27
40
54
67