Lowrance PRO X51 Installation And Operation Instructions Manual page 16

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.
Second bottom
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. 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 with the boat on plane and observe the bottom signal. You'll need to
figure some way to prop the transducer into position while you make
your test run. (A brick or two might be sufficient to hold it in place.)
5. When you're satisfied with a location, mark it and proceed with
the installation.
Shoot-thru-hull Installation
1. Make sure the area is clean, dry and free of oil or grease, then sand
both the inside surface of the hull and the face of the transducer with
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True bottom
Manual range setting