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Concrete made with vermiculite? Not very heavy, which is why it's useful. Ballast may not be one of it's most applicable uses.

the factory version


another factory version.

high tech version.

While it is certainly conformable, and pretty strong, it doesn't weigh much, and wouldn't make good ballast. It would strengthen the hull much, though, and could be a good choice (depending on how it reacts to water, of course) as replacement for the sodden foam you are removing.

so many options....
lucky me I have quite some time to think on it before I need to decide
I wonder how an underfloor, foam embedded water tank would work?
Water is pretty heavy. Centerlined it'd act as ballast. When trailering (out of the water) less weight is a plus...drain the tanks.