I'd be leary about running a lightning ground to a single
bolt. Lightning has a habit of blowing large holes in things when it's concentrated.
I just had jumper cables and a largish zinc that I intended to use when lightning was about. Fortunately, never ran into any lightning. The jumper cables eventually ended up in my truck when we got back after living in a locker for our entire cruise
. Probably a bit more chance of encountering lightning in the Gulf and Atlantic off Fla.
You don't need washer on the outside if you countersink the bolt. Assuming you are talking a flat head
fastener. The hull will probably be 1/2" plus thick once you get much below the water line. Westsail used the same layup
schedule for the 42 as they did for the 32.