There is no core also within about 3" of the outer edges of decks along the entire length of hulls. Ask me how I know... you will drill all the way through even with a proper drill stop. Never see this important little bit of info mentioned on this subject...
But thankful for all the great info here! )
Be sure to get a drill stop, also. It is a very inexpensive bushing that tightens down on your drill bit to prevent it drilling any deeper than exactly what you want. Very hard to stop even a slow turning bit by "feel"..., as soon as it touches the bottom layer of FG, it wants to grab the cloth and punch all the way through.
Had to patch a lot of through-holes myself.... Ah but then I found I also had the dreaded split interior crack spilling all the epoxy into the boat. Agree that is the FIRST thing to check for! (never saw it mentioned till now either.)
Some time I will post photos on my unique " Epoxy Adventure " (nightmare).... Ended up with Second hatches near the rear crossbeams to get it all repaired with glass and epoxy from inside, then had to fair in around the 2nd hatches with light microballoon filler and epoxy, then a layer of glass on Top, thats right.... and finally some lovely cushy Seadeck... Bulletproof, Beautiful and nice on my Butt too, lol!
I know the uber racers may whine about the extra 8-9 pounds but with my custom insulated fat bags, thats about a nine-pack of beer and ice!