The keel ballast in a P30 is IRON.
Exactly what do you SEE inside of this 'rupture'?
From your pic it looks like a previous BIG ding was repaired by slopping in thickened/mush epoxy, and the 'repair' maybe wasn't totally water tight. If there is significant RUST inside this 'cavity' ......... the formation of red rust in an enclosed space has the propensity to create GREAT pressure and that alone may have been the cause of 'rupture' of this possible/probable previous repair.
I dont 'think' the yard did this as it looks suspiciously like a blowout of a previous repair. Could even be a 'bondo' blowout. Bondo should NEVER be used below the waterline.
Its been quite a long time since I had a P30; but, if I remember correctly the fiberglass 'roving' in that section of the keel should be approx 3/8"+ or more in thickness.
You can probably check with Rudy at D&R Marine , Assonet, MA.,
http://www.drmarine.com for the thickness of that layup on the keel bottom, as he's probably one of the few remaining 'gurus' on P30s.
More info needed:
What's inside this hole?
How thick is the FRG ROVING layup adjacent to the 'hole'?
and most importantly - is that
thick RUST on the inside?