I've replaced and reinforced all the chainplate locations on the Cal 40 "Victoria!" . I chose to do this after a lower forward chain plate pulled up a few inches, while on the way to Canada (from PDX) two years ago.
Here's an outline of the process I used for the repair of the areas.

1. Map out the soft spot with a screw driver and Sharpie Pen.

2. Using a plumb-bob mark a line from deck to hull that includes the chainplate and all the soft spots,outboard the plumb. (if this area is too large, then maybe an inverted "L" shape might work. I had one bulkhead that required this)

3. Remove the chainplate, secure the stays so mast   is still supported.

4. Cut Bulkhead along plumb-bob line.

5. Using a sharpened 7in1 painters putty knife, and small   hammer peel off the tabbing from hull, remove piece.

6. Clean and bevel (or biscuit) the remaining bulkhead edge.

7. Using a piece of cardboard, and the removed piece, make a template for the replacement piece.

8. Make sure the new piece does not contact the hull, the gap   allows the hull / tabbing joint to flex. ( you might use the foam   caulking filler tubes to fill the gap).

9. Cut a new piece from Okume plywood. (Lloyds A-1 marine   plywood.) (Good boatyards have this material and can cut   pieces of it from a full sheet. Full sheet of 13 ply @3/4" is about $270+... If you can't find any, Cascade Yachts in PDX has stock, and will work with you.)

10. Match bevel (or biscuit) new piece to existing piece. I seal the outside edges of the new piece with a penetrating epoxy to help keep water intrusion from occurring. (Deckside and hull side of piece)

11. Glue new piece into place with floured or cotton fiber reinforced epoxy. Use wood wedges against hull to hold in place. You most likely need to secure the splice area so it doesn't slide out under the pressure of the wedges. I screw some greased fender washers at the joint as guides/keepers. The grease helps keep the epoxy from sticking.

12. Re tab against hull, 4-5 layers of 4" glass. I find it easier to dope up the 4-5 layers on a piece of foil covered cardboard,then lift the pieces into place on the bulkhead, and squeegee into place. Avoid square joints when tabbing. Maybe the radius of your thumb between the hull and bulkhead joint would be a guide.

13. I also tab the joint between the old and new bulkhead.

14. Tab both sides.

15. I added a "fish plate" or doubler of Okume plywood over   the joint, epoxied to, and secured with bolts. Longer bolts   for the chainplate and fender washered bolts on the wood   to wood connection.

16. I recommend only stainless bolts, with "never seize"   on the threads.

17. Everything described above is a "waste of time", if, you do not   stop the water intrusion that caused the problem in the first place.

dEmO
Roaring 40 #94
"Victoria!"