Raymarine DSM250, Owner'S Handbook Manual

Page: 79 / 122

Chapter 5: Basic Display Controls 5-19
When bottom is found, Hunt mode ceases and the sounder adjusts
to the range and frequency appropriate for the new depth.
While the DSM250 searches in Hunt mode, a constantly changing
‘stairstep’ pattern is displayed as the sounder switches depths and
frequencies. This situation is most likely to happen while the ves-
sel is travelling at a high rate of speed.
If you do not wish to view this stairstep hunt pattern, switch to
MANUAL RANGE
to turn off Hunt mode. Then, when you reduce
speed, return to
AUTO RANGE
to continue the hunt for bottom.
Selecting the Frequency
The DSM250 uses dual frequency sonar—50 kHz and 200 kHz—
and can be used in either auto or manual modes.
The DSM250 can automatically select the appropriate frequency,
based on the current display range. As the depth increases while
in Auto Frequency mode, the sounder module shifts from 200
kHz to 50 kHz at 250 ft (80 m). As the depth decreases, the
sounder module switches from 50 kHz to 200 kHz at 150 ft (50
m).
The Auto Frequency setting is also useful for automatically
searching for bottom, as described in
Hunt Mode on page 5-18 .
Alternatively, you can manually select either frequency, or both
frequencies simultaneously (split frequency). Each is suitable for
a particular purpose:
50 kHz Frequency
When using this frequency, the transducer scans a wide area. The
50 kHz signal penetrates water well, so is good for use in deep
water.
200 kHz Frequency
When using this frequency, the transducer scans a narrower area,
but produces a more detailed view. The 200 kHz signal is good for
finding fish near the bottom or close together. It is better for use in
shallow water.