I have been repairing and building boats as a marine
professional for over 10 years and you absolutely MUST REMOVE THE CORE in 99% of applications for any bolts and especially through hulls.
The removed core size should allow you to easily access all the fasteners required in the application you working on.
On your hull, based on the description, you will want to use a hole saw about 1-2 inches over sized of the through hull to remove the inner laminate skin and coring.
Then you will bevel your edges at a 3-1 ratio based on total thickness.
Last you want to measure: Top laminate thickness + bottom laminate thickness + 20% and add this laminate structure to the hull where the core was removed.
Although overkill I believe this job can be dramatically simplified by purchasing
pre-fabricated fiberglass panels
(or making your own) from Mcmaster Carr, FGCI, or anyone else.
the pre-fabbed solid fiberglass
panel the same thickness as your core- hole saw to remove core, same holesaw on the fiberglass panel, bed
in epoxy and glass it over to the same laminate thickness as your current
top laminate at the recommended 10-1 ration (10X the top laminate total thickness= distance of lamination coverage from cut edge).
There are also specially design coring for bolts/through hull installation
. Piedmont plastics sells a few. These are very high density solid plastics design for adhesion to common resin systems. Sometimes you can order samples and substitute this for your current
wood coring- which mean you do not have the tedious job of beveling your edges.
Only install bolts into a SOLID laminate structure, never a cored structure or you will crush the core, either instantly or over time and cause water
ingression, drips, core saturation, delamination
Just FYI: 16 layers of 1708 is 3/8" when done under vacuum bag or pressed under