The same instrument displays that created the gelcoat work also necessitated below-the-waterline modifications. The new Raymarine depth and speed transducers required a 2″ hole, whereas the old Nexus transducers were 1-1/2″. Enlarging a hole by sanding or grinding is a sure way to create just about anything but a perfect circle, so I planned to use a 2″ hole saw. Without something for the pilot bit to bite, however, the drill will wander. No problem. I drove a tapered wooden plug into each hole, then cut it flush with the hull (in the case of the depth transducer, I cut it parallel to the horizontal). Now my pilot bit had something to bite to center the hole saw, and both holes turned out great.
I applied a thin layer of unthickened epoxy over the edges of exposed laminate in each new cut to prevent water migration, then bedded the transducers in plenty of 4200. Ariel splashed a few days later with nary a leak. By the way, hull thickness in both areas is 1″.