Second bottom 
True bottom
Manual range setting
Example of a second bottom signal. Unit is in 30 feet of water, with
range set at 80 feet and sensitivity set at 87 percent.
2.Next, take the transducer out of the water and place it in the water in the sump of the boat, face down. (The transducer face is shown in the figure on the following page.) Notice how the signal strength decreases. The second bottom signal will probably disappear and the bottom signal intensity will likely decrease.
3.Now move the transducer around to find the best location with the strongest possible bottom signal. If you find a spot with an acceptable bottom signal, mark the location and move on to step 4.
If you can't get an acceptable bottom signal, try turning up the sensitivity by three or five keystrokes and then move the transducer around once more. If you find a spot that works, mark it and move on to step 4.
If you have to turn up sensitivity by more than five keystrokes to get a good signal, the transducer should be mounted on the outside of the hull. This is especially true if you have to turn sensitivity all the way up to get a decent bottom signal.
4.Most people can get good results by following steps 1 through 3, so this step is optional. If you want to make an extra effort to be absolutely sure that your selected location will work under all conditions, make a test run
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