This crack seems to be really deep. I did not take a close look but there's no noticeable damage on the underside. I assume this is just a gelcoat crack?
I prefer stainless steel backing plates - but wood would work. I've also used starboard as backing plates - never used wood...This is on my new to me 94 Catalina 30. The O'Day coamings are built like a tank. It was tested one stormy day and the winch mounting bolts failed with zero visible marks in the fiberglass. I gained a ton of respect for fiberglass that day
I'm leaning more towards this being a deep gelcoat crack since there's no cracks on the underside. A large wood backing plate isn't a bad idea for the future. Still seeking out a few more opinions on this crack