Bottom Indications - Raymarine L750 Operating Handbook

L750 Fishfinder: Installation and Operation Handbook
38
the surface will have a larger arch. This is because the fish which is closer
returns more of the sound waves back to the Fishfinder.
Actually, the sound waves are usually reflected by the air sac near the
center of the fish, and not by the body of the fish. The size of this air sac
is different for different kinds of fish. This can also affect the size of the
fish arches on the display. A fish with a large air sac will produce a large
fish arch, while a fish with a small sac will produce a small arch. Several
arches together show a school of fish.
The return from a fish will have a different appearance, depending on the
frequency used. If a fish is spotted by the wide 50 kHZ beam, the arch
will be broad. When using the narrower 200 kHz beam, the same fish
will produce 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 a single vertical line.
When the Fish Symbol feature is turned on a fish will be indicated by a
fish-shaped symbol on the display. In general, a larger symbol means a
larger fish.
The symbol also indicates which of the two frequencies was used to
locate the fish. If the fish was spotted by the wide 50 kHz beam, the
symbol will appear to be hollow. If the fish was detected by the narrow
200 kHz beam, the symbol will be solid.

Bottom Indications

The bottom usually produces a strong echo, so it is easy for the
Fishfinder to see a lot of detail. The Fishfinder can indicate many
different kinds of bottom conditions. See Fig. 3-5. A hard bottom
appears as a thin line. An uneven bottom which is covered with rocks or
coral produces a complex display like that shown in Fig. 3-5c. The
bottom image is irregular, with long "tails" pointing upward. A wreck
produces a similar image.
A softer bottom or a bottom covered with seaweed will produce a wide line,
as the sound waves reflect from different layers of the bottom. In Fig. 3-5b,
the Fishfinder is getting a good echo from the bottom, so the upper layer of
the bottom is shown as a dark gray area. Notice that the lower layer of the
bottom is shown as a lighter gray area. This indicates a weaker echo.
Sometimes this means that the upper layer of the bottom is soft. Some of