Originally Posted by Sabreman
I had a similar arrangement on my last boat.
You need to fill the hole:
1. Taper the inside and outside of the hole so that it's no longer a 90 degree edge. Sort of like a sink drain.
2. Remove all traces of paint on the inside and outside. Wipe with acetone
3. Obtain fibreglass roving cloth. Cut several pieces the size of the hole
4. Tape over the outside of the hole with masking or duct tape to temporarily seal the hole.
5. Mix up some West Epoxy or equivalent, soak the roving, and insert in the hole. Repeat once more. Tamp gently to remove air bubbles (This is really important). Let stand until hardened
6. Remove the tape from the outside
7. Repeat #5 until desired thickness is achieved. Make sure to remove all air bubbles.
8. Cut and epoxy a larger cloth piece to overlap the edges of the hole by about 1" and apply to the inside of the hole.
9. Mix collodial silca to thicken the epoxy and fair the outside of the hole.
Following this repair method will result in a very week spot in your hull that could pop out under severe weather conditions. Don't do it! Do it right or hire it done by a professional! Doing it right involves grinding the hole back at a 12 to 1 ratio. That means that for your 3/8 inch thick hull the outer edges of your ground out area will be 4.5 inches away. Then gradually build up with larger and larger pieces of fiberglass roving until the final piece is the full 10 inches diameter. Actually Ferenc Mate says in his book "Shipshape, The Art of Sailboat Maintenance" that if the hull is greater than 1/4 inch thick then the ground out ratio should be 15 to 1 but everyone else that I've read recommend only a 12 to 1 ratio.
The rest of Sabreman's recommendations are quite good. That is cleaning the area very well, etc.