NorthStar EXPLORER 435, Installation And Operation Manual

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