Advanced Navigator Functions

Effect of Frequency and Boat Speed

The return from a fish has a different appearance depending on the frequency used: 50kHz produces a wide beam, so a fish spotted by this frequency gives a broad arch; the 200 kHz beam is narrower so the same fish produces 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.

In Bottom lock mode the fish arches may appear distorted as the bottom topography is removed from the image.

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, as shown in Figure 5-12.

A hard bottom appears as a thin line. An uneven bottom which is covered with rocks or coral produces a complex display; 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 produces a wide line, as the sound waves reflect from different layers of the bottom. In Figure 5-12, the Fishfinder is getting a good echo from the bottom, so the upper layer of the bottom is shown as dark gray.

Notice that the lower layer of the bottom is shown as a lighter gray, this indicates a weaker echo. This could mean that the upper layer is soft; some sound waves may get through this layer and be reflected as a more solid layer below.

It is also possible that the sound waves are making two complete trips

hitting the bottom, bouncing off the boat, then reflecting off the bottom again; this can happen if the water is shallow, the bottom is hard, or gain is set high.

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Raymarine Marine GPS System manual Bottom Indications, Effect of Frequency and Boat Speed