NorthStar Navigation 435 Bottom types Strength of echoes, Frequency and cone width, Depth, 015

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.

Single fish

Large school of fish

Small school of fish

Bottom

•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.

Kelp / Weed

Soft bottoms such as mud, weed and sand show as narrow bands

Hard bottoms such as rock or coral show as wide bands

Bottom types

Strength of echoes

The colors indicate differences in the strength of the echo. The strength varies with several factors, such as the:

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.

Frequency and cone width

The pulse generated by the Explorer 435 and Explorer 438 transducer travels down through the

Depth 200 kHz

015°

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.

50

100

150

200

250

13

27

40

54

67

Northstar Explorer 435/438 Installation and Operation Manual 17