Furuno GP-1650DF, GP-1650F manual Surface noise/Aeration, Surface noise/aeration

Surface noise/Aeration

When the waters are rough or the boat passes over a wake, surface noise may ap- pear near the zero line. As surface turbulence is acoustically equivalent to running into a brick wall, the bottom echo will be displayed intermittently. Similar noise sometimes ap- pears when a water temperature difference (thermocline) exists. Different species of fish tend to prefer different temperature zones, so the thermocline may be useful to help iden- tify target fish. 200 kHz tends to show shal- low thermoclines better than 50 kHz.

In rough waters the display is occasionally interrupted due to below-the-ship air bubbles obstructing the sound path. This also occurs when the boat makes a quick turn or reverses movement. Lowering the picture advance speed may reduce the interruption. However, reconsideration of the transducer installation may be necessary if the interruption occurs frequently.

Surface noise

Caused by aerated water

Thermocline

Figure 2-34 Surface noise/aeration

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