Re: Repair a LARGE hole ???
The standard way to repair this would be as a couple people have suggested, to place a section of mold on the outside of the hull in the shape you want, then overlap the layup from inside the hull. Done right with a good mold you should be able to make it look like original work.
It's easiest to pull this off with flat surfaces, that's just a matter of placing a flat panel against the outside of the boat.
But, since you said it's curved, you'll probably need to make a mold from either the opposite, good side of the boat or if you're lucky and it's not too acute a set of curves you can flex a mold to match the hull shape.
How that works... you use a piece of thin flexible material... it can be thin plywood, hard plastic sheeting, fiberglass, metal, whatever. Make it as large as will fit against the side of the hull. Lay it in place over the hole, and press it against the hull so that the mold curves to match the hull... imagine it like you're holding a playing card against a soccer ball, if you push the sides against the ball the card follows the curves of the ball in the vertical direction exactly. If you push the top and bottom in after that, the card comes close to approximating the 3d curve of the ball. You can do a layup against that... if you have shallow enough curves, you may be able to match the hull curve exactly by hand.
Another way to do it is to pull a mold of what's there and finish it by hand. You patch the hole from inside the hull with hard plastic sheeting or some other flexible material so it matches the hole as closely as possible, and make sure the edges are sealed against the hole edges. Then you spray PVA mold release from the outside over the hole and surrounding hull. Once that's ready you can wax the same area several times. When that dries, you do a mold layup against the side of the hull, basically creating a negative mold from the hull and hole area. Make it larger than your hole by a couple feet per side if possible. Lay it up thickly, at least 4-5 layers of mat or cloth, and use wood framing on the outside to provide structure.
If all goes well, you can then pull the mold from the outside of the hull, then fair it much like you would a hull. Basically you fill or grind down the section of the mold which was over the hole until your mold is fair.. it'll be pretty obvious where the hole is on the mold. Once the mold is fair, you PVA and wax it. Remove the material you had covering the hole from inside the hull and put the mold back in place. If you faired it properly it should now form exactly the curve you want on the hull, and all you have to do is layup the repair from the inside hull and do a little touch-up once it hardens and you remove the mold. Don't forget to mark the mold's position on the outside of the hull when you originally create it to ensure you get it back in the same spot.
The hardest way to do this repair would be to tape cardboard or wood support members outside the hull, do a thick layup against that from inside, then grind the outside fair and smooth with microbubble putty. You'd have to be a real artist to make that look good though.
Final thought... if you're investing in the boat, I'd have this part of the repair done by a boat shop... it's not something that's easy to do at home, and it's easy to screw up, and would be a safety problem. Have the shop do the hard stuff, then you re-wire, install new controls and a new interior, and go float.
Erik
