7-1
7. Fish Finder
This chapter describes the functions of the fish finder (requires a network sounder or
Color LCD Sounder FCV-1150). A fish finder shows the images of underwater objects,
like fish and the seabed on the display. Topics include
• Display selection
• Automatic and manual operation
• Frequency selection
• Picture advance speed
• Interference removal
• How to measure depth to an object
7.1 How the Fish Finder Operates
The fish finder calculates the distance between its transducer and underwater objects
like fish, lake bottom or seabed. The results are shown in different colors or shades of
gray according to echo strength.
The ultrasonic waves transmitted through water move at a constant speed of approx-
imately 4800 feet (1500 meters) per second. When a sound wave “hits” an underwater
object like fish or sea bottom, part of the sound wave is reflected toward the source.
To find the depth to an object, the fish finder calculates the time difference between
the transmission of a sound wave and the time the reflected sound wave is received.
The picture displayed by the fish finder contains a series of vertical scan lines, one line
for each transmission. Each line is a "picture" of the objects under the vessel. The se-
ries of pictures are put side-by-side across the screen to show the contours of the bot-
tom and echoes from fish. The amount of history of objects that have moved under the
vessel can be less than a minute to more than one minute.
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