Bedienungsanleitung Navman FISH 4500 (Seite 19 von 36) (Englisch)

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4-3 Fish detection and display

Where to find fish

Underwater features like reefs, wrecks and rocky

outcrops attract fish. Use the 50 kHz or 50/200 kHz

frequency display to find these features, then look

for fish by passing over the feature slowly several

times using the Zoom display (see section 5-2 Sonar

Zoom display). If there is a current, the fish will often

be found downstream of the feature.

When fishing with the FISH 4500/4600 with the Fish

symbols Off, a weak fuzzy band may appear

between the bottom trace and surface. This

might indicate a thermocline - a rapid change in

water temperature, such as the edge of a warm

or cold current. The temperature difference

can form a barrier which the fish may not swim

through. In fresh water, fish often collect around

a thermocline.

Fish symbols

The fish symbol can be customized or switched

off altogether so that the echoes are not con-

verted to fish symbols on the display. See section

3-2 Setup > Sonar. The differences between Fish

symbol On and Off are:

Fish symbols On

Using Navman’s SBN-II sonar technology the

fishfinder analyses all echoes and eliminates most

false signals and clutter so that remaining targets

are most likely fish. Depending on the strength

of the remaining echoes, they are displayed as

either small, medium or large fish symbols - with or

without depth. While the SBN-II processing is very

sophisticated it is not foolproof - there will be times

when the fishfinder will not be able to differenti-

ate between

large air bub-

bles, rubbish

containing air,

fishing floats

etc. and gen-

uine fish.

The follow-

ing picture

shows the

Sonar display

with the Fish

symbol: On +

depth:

Fish symbols Off

For experienced users this always provides the

best information as every echo is displayed, wheth-

er it is surface clutter, a thermocline or a fish.

The picture in section 4-1 Interpreting the display,

shows the Sonar display with the Fish symbols

Off. The fish appear as arches.

Fish arches

In good conditions and with Fish symbols Off, a

fish passing through the cone-shaped ultrasonic

pulse is displayed as a fish arch. The 50 kHz

frequency uses a wider cone than the 200 kHz

frequency. This makes the fish arches easier

to see.

A fish arch occurs when a fish enters the weak

edge of the sonar cone, generating a weak echo

that is displayed as the first pixel of the fish arch.

As the fish moves closer to the middle of the cone,

the distance between the transducer and the fish

reduces and the echo is displayed at progres-

sively shallower depths, producing the start of an

arch. When the fish passes directly beneath the

middle of the cone, the echo becomes strongest

and thickest. As the fish passes out of the middle

of the cone the reverse happens with a progres-

sively weaker and deeper echo.

There are many reasons why fish arches may

not be seen. For example:

• Poor transducer installation (see Transom

Transducers Installation Guide).

• If the boat is anchored then fish will tend to

show on the display as horizontal lines as

they swim into and out of the transducer

sonar beam. Slow speeds in deeper water

give the best fish arch returns.

• Range is important. It will be much easier

to see fish arches when using zoom mode

to concentrate on a particular section of

water, rather than just displaying everything

from the surface to the bottom. Zooming in-

creases screen resolution and is necessary

for good fish arches.

• It is difficult to get fish arches in shallow

water as the transducer sonar beam is very

narrow near the surface and fish do not stay

within the beam long enough to display an

arch. Several fish in shallow water tend to

display as randomly stacked blocks of pixels.

• Wave motion may result in distorted

fish arches.

FISH 4500/4600 Installation and Operation Manual 19

NAVMAN

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