Hi
@LouisMK,
Did you upgrade to an O’Day 40 (your signature shows O’Day 34)?
I have an O’Day 322 built in 1988. I don’t know for sure about an O40, but if the interior side of the bulkhead is nice smooth gel coat, I would assume it has a liner. I know my O’Day 322 does. I had to drill large holes in the liner recently to replace some above-the-water line thru-hulls. None were in the area you want to install your instrument, I did not have much space between the outer hull and the liner. I also had the opportunity look in between the liner and top deck, and there is very little space there either.
Not sure if the construction between your boat and mine is the similar, but I bet if you have a liner, it is pretty close to the outer skin.
A lot of people mount gauges on those bulkheads, but I think most have to cover them on the inside.
Good luck,
Greg