Re: Help pinpointing a leak
I'd say yes that they are structural; there is lots of load on the keel stub due to the static weight of the keel; and when the boat heels The hull would want to deform in the bilge without the cross pieces. It looks like C&C made them large thin box sections in newer hulls to minimize weight while maintaining rigidity. My older C&C 41 does not have this construction method; the stringers are glassed over wood; and only ~2" wide. It seems to me that the foam used to core the box section is breaking down due to bilge water getting in. While the foam is not structural, any water that fills the void will be problematic if it freezes. I'd consider re-filling the void with expanding foam and then glassing in the limber holes so water can't get in from the surrounding bilge. But that's not a proper fix for the OP if there is a crack to the exterior. The crack needs to be repaired first.