I agree there isn't movement of the chainplate itself, but if anything the bulkhead or what is referred to as a knee.
Actually I'm now not convinced it moved at all since I'm very confident there isn't any cracking where the knee meets the hull, but I will confirm this.
This image might do more harm than good but here you go. This picture is many years old (prior to me owning the boat) but you can see the damage doesn't exist here: https://i.imgur.com/zCY1J9b.png
Where the blue line is is the edge of the knee: in my first photo
it would be on the left side of the image. So the chainplates edge-to-edge are port to starboard. Meaning, the bolts face fore and aft.
Where the orange circle is is where the base of the shelf meets a piece of wood. There is actually a gap here: about 1/4" or more - meaning much more than the crack.
There isn't any support under this shelf running the length of that birth. So you have 5-6' of shelf and the only supports are on top of the shelf, being the knees.
I've never sat on the shelf (not really possible) but absolutely have leaned on it, grabbed it, and have had plenty of heavy things stored there - cases of beer
> Any difference in this crack on a port versus a starboard tack, such that the shroud is loaded/unloaded?
I didn't notice anything!
> Tension OK in the other shrouds?
Yes. Actually they are roughly same tension as when I tuned the rig, checked by loose gauge.
> Bolts tight? No sign of pulling at the bolt heads (evident none at the nuts from the photo).
I actually did not check this. I will do so. I just rebedded these 2 seasons ago
but the bolts all look fine. They are new bolts.
> Where does this crack go? Have you probed it with a wire? Odd that it would be on the inside of the hull but not evident elsewhere, or is it?
Unsure where it goes - have not probed it! Good ideas and I will do so soon. Only other evidence is on the shelf wood is pulling itself apart. I will take more photos of this when I get to the boat.
> You can apply a bead of what is called anti sabotage lacquer. A thin bead about toothpaste consistency that quickly dries hard and breaks easily, go for a sail and if it doesn’t break, there is no movement going on.
This is a great idea! I had my finger against it and didn't feel movement while sailing - but who knows. I will buy some and give this a shot.