Bottom Indications - Raymarine DSM250 Owner's Handbook Manual

Chapter 6: Sonar Mode Operation

Bottom Indications

50 kHz
200 kHz
Figure 6-1: Fish Indications
Effect of Frequency and Boat Speed
The return from a fish has a different appearance depending on
the frequency used: 50kHz produces a wide beam, so a fish spot-
ted by this frequency gives a broad arch; the 200 kHz beam is nar-
rower so the same fish produces a narrower arch.
The shape of the arch is also affected by the speed of the boat. If
the boat is moving slowly, the arches tend to be longer; if the boat
is moving more quickly, the arches are short and peaked. If the
boat is moving very fast, a fish may be indicated by vertical lines.
In Bottom lock mode the fish arches may appear distorted as the
bottom topography is removed from the image.
The bottom usually produces a strong echo, enabling the DSM250
to display great detail. The sounder module can indicate many dif-
ferent kinds of bottom conditions.
As illustrated in Figure 6-2 , a hard bottom appears as a thin line.
An arch-shaped image indicates a fish.
The same fish appears differently when shown by the
50 kHz or 200 kHz transducer beams.
A shallow arch is generated when the the boat is
moving slowly.
This indicates a group of small fish.
A fish moving under a stopped boat shows as a line.
When the boat is moving very fast, a fish appears as
two parallel lines.
6-3
D5017-2