Humminbird SINGLEBEAM 405SX Operation Manual page 12

USING THE 405SX
WHAT YOU SEE ON SCREEN
Thermoclines. Thermoclines are
sharp differences in water
Thermocline Second Return
temperature. These are easily
identified by the continuous
nature of the return.
Second Returns. When a sonar
signal is reflected off the bottom
back to the transducer, there is
often enough energy left in the
signal to be reflected off the
surface of the water back to the
bottom a second time. Second
returns appear as a slightly
weaker bottom representation
exactly twice the depth of the primary bottom return. The second return is most
likely to occur in shallow water and in areas of relatively hard bottom.
Fish Arches. With Fish ID off, schools of bait
fish as well as individual fish are clearly visible
on the 405SX display. Bait fish appear as
"clouds" having different shapes and sizes
depending on the number of fish and boat
speed. Individual fish appear as smaller black
Fish Arch Diagram
and gray lines often appearing as a "fish arch."
A fish arch forms as the boat move over the fish. Due to the transducer
beam angle the distance to the fish decreases as it moves into the beam,
and then increases as it moves out. When the window graphs this distance
change, an arch appears. The true depth of the fish is the top of the arch
when the boat is directly over the fish. Boat speed and movement of the
fish greatly affect the shape of the arch. When moving slowly, a fish creates
an elongated arch. With the boat moving fast the arch appears shorter. A
partial arch forms when the fish does not move through the entire cone angle.
The 405SX displays structure such as submerged grass, brush, trees and
wrecks on the bottom. Structure can be distinguished by comparing the
area just above and below the main bottom return. Usually structure shows
as areas of dark to light gray on top of a dark bottom contour. The
appearance of structure is greatly affected by boat speed and direction; to
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