Hi,
I have noticed over my time working that some boats have a cassette
keel, where the keel is actually hollow then filled with ballast (lead shot or lead block etc) then resin poured into the cassette to seal in the ballast and glassed over to strengthen it all up.
This could be what your looking at, I don't imagine that the
builder would have buried the bolts but I could be completely wrong as well.
Any chance of loading up a
photo?
Shipwright by
trade, adventurer by nature