I can give you my 2c on the topic.
I built a steel boat and launched it 1981.
Though the boat was sand blasted inside and out, I could not get into the keel voids...no worries, I thought....as these will get filled with ballast...and sealed over with coal tar epoxy, which they did...
All but the most rear void. The plans did not call for this to be filled with ballast, additionally it was located under the engine
, but I had to do something about protecting the steel inside that void, so I filled it with cement.
As I was still in the build process, I drilled a small 1/4" weep hole in the keel right at the very bottom rear to allow water to escape as the concrete cured. That weephole oozed water for quite some time...I can't recall
how long anymore, but it was several weeks. When I noticed the weephole had dried up, I welded it close.
After the concrete had dried, I slathered the top with coal tar epoxy as was the rest of the inside.
In the 15 odd years I had that boat, I never had a single
problem with that keel cavity, despite having salt water
in there from time to time.
Interestingly, that boat is now about 40 years old, and is still around, but with a new owner, who stripped it down, wanting to make some interior
modifications. But the inside of that hull
has remained rust and corrosion free.
Had I to do it again, I would consider using the two part expandable insulating foam to fill that void, and would coat it with coal tar epoxy as before. I don't recall
the weight of the concrete, but seem to recall it was around 700 lbs...about 320 kg...which affected the trim of the boat a little....
hope this helps...