Two of my friends and I set out yesterday for Titania's second sail. Temperatures were forecast to be 60 degrees and winds light at around 8 knots average, gusts to 15. The air was indeed quite warm, but the wind was WAY off - sustained 15-20 knots, gusts up to 30 or 35. Whitecaps were clearly evident in the middle of the lake and the waves were a good foot or more. We put one reef into the sail and set out but it was soon clear that we didn't even need that much sail to reach hull speed, and then put in the second reef.
Suffice it to say that the mast and removable tabernacle sustained a lot of pressure. About halfway through the day I thought I heard a "crack" and was understandably alarmed. Bulkhead 2 had separated from the cabin roof cleat right where the tabernacle is bolted in, formed a hairline crack. This crack extended along the cleat 2 inches along either side of the bolts. On the outside, I could see the bulkhead plywood bulging out slightly where the bolts go through, but the cabin roof/bulkhead joint was intact. Despite tightening the bolts with a ratchet wrench prior to setting out, fully 1/8" or more had opened up between the top of the tabernacle and the bulkhead. When the sail was dropped that space closed up. Confident that the bulkhead could handle the pressure, we hoisted sail again and didn't have any issues the rest of the day.
During construction I doubled-up the fiberglass cloth across the bulkhead 2/cabin roof joint. I don't believe the plywood would have separated from the roof. Rather, I think the flexing caused the cheap pine cleat to fail, causing the hairline crack.
The hairline crack is too small to fix. Rather, I will add a doubler of 9mm ply into the cabin, roughly a foot wide and extending from the cabin roof cleat to the lowest bolt. That should be sufficient to relieve some of the pressure from the 9mm ply bulkhead.
Summary for new builders:
I would recommend following Tattoo's advice from above: use high-quality wood for the cleat at that location. Double-up the fiberglass cloth across the bulkhead/cabin roof joint. Use a doubler inside the cabin, though I doubt it needs to be as large as his.