Raymarine L1250DRC, Owner'S Handbook Manual

Page: 133 / 246

Chapter 6: Sonar Operations 6-3
Bot
tom Ind
ic
ati
ons
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 spotted by this frequency gives a
broad arch; the 200 kHz beam is narrower 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.
Bottom Indications
The bottom usually produces a strong echo, enabling the Fishfinder to see
great detail. The Fishfinder can indicate many different 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.
D5017-2
50 kHz
200 kHz
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