NorthStar Navigation Multi-Function Unit M84 - Manual (Page 35)

Northstar M121/M84 Installation and Operation Manual 34
A School of fish
B Single fish
C Hard bottoms such as rock and coral show
as wide bands
D Soft bottoms such as mud, weed and sand
show as narrow bands
9-2 Interpreting the display
The sonar windows do not show a fixed
distance travelled by the boat; rather, they
display a history, showing what has passed
below the boat during a certain period of time.
The history of the sonar signal displayed
depends the depth of the water and the scroll
speed setting.
In shallow water, the echoes have a short
distance to travel between the bottom and the
boat. In deep water, the history moves across
the window more slowly because the echoes
take longer to travel between the bottom and
the boat. For example, when the scroll speed
is set to Fast, at depths over 1000 ft (300 m)
it takes about 2 minutes for the data to move
across the window, whereas at 20 ft (6 m) it
takes only about 25 seconds.
The most recent echo appears on the extreme
right of the window, with the older echoes
being scrolled towards the left, eventually
disappearing off the window.
The scroll speed depends upon the water
depth and scroll speed setting. See sections
19-3 and section 9-2, for more information.
The appearance of echoes displayed are
affected by:
• The Instrument settings (see sections 19-3,
9-5 and 9-6)
• Echoes (different fish types, different bottom
types, wrecks and seaweed; see section 9-2)
• Noise (water clarity and bubbles; see section
9-2).
Cruise, Fishing and Manual Modes
The Instrument has three sonar operating
modes:
• Cruising mode: Use this when on the
move. The Instrument automatically adjusts
its settings to compensate for water clarity
and to display the bottom.
• Fishing mode: Use this when fishing. The
Instrument automatically adjusts its settings
to compensate for water clarity and to best
display fish, the bottom and other details.
• Manual mode: Use this to fine-tune the
Instrument settings by hand. Best results
are often achieved in manual mode, but
practice and experience are required to
obtain the optimum settings for different
conditions.
For more information about modes, see
sections 9-5 and 9-6.
! WARNING
Use the automatic Cruising or Fishing
modes when learning to use the Instrument
or when travelling at speed.
B
A
D
C