Speed Sensor; Installing The Transducer; Speed Sensor Installation - Garmin Fishfinder 160 Blue Owner's Manual

Installation
OK
Adjust the speed sensor so it is parallel with the bottom
of the boat. If the speed sensor is not adjusted properly,
the sonar may experience inaccurate or degraded speed
readings.
Drill a 3/4" hole well above the water line.
Coat the inside of the hole with Marine Sealant.
Position Cable Clamps about one-third of
the distance from the hole to the transducer.
Drill 1/8" pilot holes 3/8" deep for the mounting screws.
Coat the screws with Marine Sealant before installing them.