I mounted mine, a huminbird platinum, on the bottom of the transom on the
outside of the hull. It also had a speed / temp wheel so that could not be
mounted "inside the hull". I used 5200 in and on the ss screws used to
drillinto the hull. the wiring was run up the back using sticky 3m holders
and into a hole in the top of the transom just under the rubber edge
bumper. The hole there I filled with 5200 then covered with a chrome
clamshell from westmarine for cosmetic effect. it works ok but I suspect
that motor vibration sometimes interferes with the ultrasonics and gives
false readings. It is rated to 600 feet. I see some pretty drastic
readings but in colorado lakes with canyon type underwater settings, who
knows...