Furuno FCV-668 Operator's Manual page 13

B/L (Bottom-Lock) mode
The bottom-lock mode shows a compressed normal
picture on the top 2/3 of the screen and a 5 or 10 meter
(10 or 20 feet) wide layer in contact with the bottom is
expanded onto the bottom 1/3 of the screen. This mode
is useful for bottom discrimination, essential for bottom
trawling.
This mode requires that the seabed be steadily and
distinctly plotted in red or reddish-brown. Adjust the
gain if necessary.
Note 1: The bottom-lock range can be selected on the
system menu [3].
Note 2: The zoom marker can be turned on/off on the
main menu.
B/Z (Bottom Zoom) mode
The bottom zoom mode expands bottom and bottom
fish echoes two to five times to vertical size of the
screen (the amount of expansion can be selected on
the system menu [3]), and it is useful for determining
bottom hardness. A bottom displayed with a short
echo tail usually means it is a soft, sandy bottom. A
long echo tail means a hard bottom.
Note 1: The amount of expansion can be selected on
the system menu [3].
Note 2: The zoom marker can be turned on/off on the
main menu.
Zoom marker (yellow)
T h i s s e c t i o n
i s z o o m e d
Normal
display
Bottom-
lock
display
220.7
Bottom-zoom display
15
20
25
36.4
Bottom
Normal
display
0
10
20
30
40
5
Fish
4
3
2
1
0
0
10
20
Zoom
marker
(yellow)
30
40
2-3