From the OP's photo
it appears that the chainplate is attached to the raised cabin top instead of to the actual hull
. Very unusual and if you overtension the shrouds or overload them while sailing you will pull up on the cabin top and separate the tabbing.
- - I would classify that set-up as a major design fault. But you can work around it by cleaning
and opening up the area around the joint between the vertical bulkhead and the cabin top where the crack is located.
- - To fix the problem from re-occurring in the future it would be advisable to have a stainless plate made that will attach to the vertical bulkhead and then to the side of the cabin top below the chainplate. Then re-tab the bulkhead to the vertical bulkhead.
- - You need to check the bottom of that vertical bulkhead and see how it is attached to the hull at the other end. There just may be broken tabbing down there.
- - Another alternative is to relax the rigging, repair the tabbing, then have a polished stainless rod fabricated that will attach to the chain plate
on one end and to a structural hull member
down in the area underneath the furniture or whatever is right below the chainplate. I would prefer the bar idea as it would directly connect the chainplate to the hull where it should be attached - not to the cabin top.