Either 1 or more of the following happened:
1. The board slide down to far in the trunk (best case and most likely)
2. You ran over something (doesnt look like that happened)
3. The trunk/hull failed under the stress of sailing on a windy day (worst case)
It looks like the board slid down to far and the loads on the daggerboard increased and the trunk didnt have enough structure to hold the top of the board in the middle of the trunk, and it sheared off the trunk and then the second crack was the bottom part of the trunk tearing away from the hull.
Or you ran over something, which pulled the board aft, and the trunk sheared in the middle and then delaminated off the hull. I doubt this happened because the top of the trunk would most likely be damaged as well, and the board would be banged up too. Most of the time when you run over something the board takes most of the abuse, followed by the trailing edge of the trunk/hull. The impact usually james the trailing edge of the board backwards into the hull like a knife. It doesnt look like that happened.
I'm betting the board slide down to far, you didnt notice it and the loads from the board being to deep sheared off the bottom of the trunk.
If thats the case its good news, just glue it all back together and make sure the board cant fall below the deck in the trunk.
If the board didnt slide down into the trunk, and you didnt run over something, I would be really worried. If thats the case it means the trunk failed under normal sailing conditions. Thats a pretty serious design/build issue.
I agree with Bill here. From the pictures, it looks like the board slid way down in the trunk. These trunks typically have very thin walls. Once it was down there, the board probably punched through the trunk and that was the first crack you heard. At that point, the board was probably not aligned well with the direction of travel and fighting the rest of the boat/rudders/boards with a ton of lift which caused the failure of the exterior/bottom of the hull - i.e., the second crack you heard.
5oz glass would be a good medium weight to work with. Epoxy is easy and very strong and bonds well with cured plastics. Vinylester resins approach the mechanical properties of epoxy but don't bond quite as well to cured plastics. polyester is very low cost, doesn't bond well, and has the least strong properties. If you intend to finish with gelcoat, use vinylester. you can gelcoat over epoxy but it's tricky to get a decent bond.
I'm not sure if you should cut the hole in the deck of the side...though I'm thinking the side may be the way to go. Mike showed a good trick to replace the 'window; you cut out by glueing plates of fiberglass to the inside of the hull - and that should work well. I have also use a similar technique by bonding popsickle sticks to the inside of the hull and clamping until they cure. Then glue the window back in place. Then using a dremel, I grind through the entire perimeter of the seam to the inside glass. I glass in a few layers of 5oz on the inside layer, then I fill with Ultimate Bondo (yes, bondo - I've had it work very well...just ask Tad...but use the ultimate version), sand a nice taper on the outside glass layer, and put a layer of glass again on the oustide...fair to finish with ultimate bondo, and gelcoat to finish.
With mike's fiberglass plates, if you get them all the way around the hole, you may be able to save the step of grinding to the interior layer of glass.
