This how I installed my unit.
Drilling thru Gel-Coat
Many of you may know this but I did not.
You must get through the gel-coat before drilling fiberglass. The gel-coat is very brittle and will crack, break or flake.
To drill a hole for bolts, I start with small bits and work up to the size I need. Start with drilling the hole backwards, it will wear through the gel-coat, if you start drilling there is a pretty good chance you will start flaking the gel-coat. Do this all the way up to the size hole size needed. Then chamfer the hole by running a larger bit backwards until the the chamfer is through the gel-coat.
Note if the screw or bolt is to big for the hole, there is a good chance that again it will flake a piece of gel-coat, I did this on my first screw hole to mount a transducer. After that I was more careful.
George