Not having read the aforementioned thread on rudder
repair (but just having built a new rudder due to a similar case of water
penetration into a fiberglass-encased wood rudder with similar splits in the fiberglass
skin), and just going otherwise by the attached photo
, it appears that the fiberglass/resin has been penetrated by moisture, the wood has swelled and/or rotted, and the leak has grown into a fissure by the wood expanding with moisture and splitting the glass coating.
The problem with fiberglass
encased rudders is that you can't easily tell the condition of the wood inside without tearing them apart. In the least case there is localized rot
near the fissure(s). In the worst case the entire inside of the rudder is a punky, waterlogged, pile of wood mush.
Stick something sharp and pointy into the crack and poke around. If it's soft then there's rot
inside. If not, maybe not.
What I did was bought a slab of white oak at the local hardwood supplier and used the existing rudder as a pattern to make a new rudder. I now have the option of simply patching up the fissures in the old rudder and repainting it as a 'spare' rudder (which I'll probably do, if for no other reason than to save as a pattern for future new rudders if needed), or just s**t-can it.