How The Fishfinder Works; Interpreting The Bottom Structure - Raymarine DSM25 Owner's Handbook Manual

Chapter 1: Overview

1.3 How the Fishfinder Works

The Fishfinder application uses the DSM25 to process sonar signals from a
suitable transducer and provide a detailed underwater view.
The transducer located on the bottom of the boat sends pulses of sound waves
into the water and measures the time it takes for the sound wave to travel to the
bottom and back. The returning sound echoes are affected by bottom structure
and by any other objects in their path, such as fish, wrecks, or reefs.
The sounder module processes these echoes and sends data to the A65 which
displays a visual interpretation of the underwater view. The strength of echoes is
indicated on the display by different colors. You can use this information to
determine the bottom structure, the size of fish and other objects in the water.
Notes: (1) The DSM can operate at a frequency of 50 kHz or 200 kHz.

Interpreting the Bottom Structure

The bottom usually produces a strong echo. These images indicate the bottom
conditions as follows:
A hard bottom (sand) produces a thin line
A soft bottom (mud or seaweed cover) produces a wide line. The dark
layer indicates a strong signal.
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A rocky or uneven bottom or a wreck produces an irregular image with
peaks and troughs
(2) Some transducers will also enable the A65 to display water temperature
and/or speed.
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