If you take the average Igloo cooler, fit it with a rig, diesel engine, cruising stores, etc. it will sink like a stone when flooded.

Lagoons are CE certified to be unsinkable, yet two have sunk in 6 months. The Lagoon 42 and Alpha 42 are not that dissimilar in claimed displacement, and probably closer in actual cruising displacement. These cats are as heavy as monohulls by their own claims (which are suspect). The only thing in the boat less dense than water (as a broad statement) is the core and there just isn't enough of it. On my own 45' boat 2672 sq ft of core was used (with an all cored interior), about 1400 of it 1 inch and 1200 of it 1/2. That's 176 cu ft or 11,000 lbs flotation neglecting it's own weight. It seems unlikely that the Alpha would have twice that much laminate (if it did it would displace far more than what is claimed). The rest of the boat would displace a bit of water when flooded, reducing the flotation required, but not by much. Your Dragonfly is claimed to be less than 13,000, quite a bit lighter. Have you ever weighed it?

It is a difficult thing to calculate accurately. The only way we would know for sure on the Alpha or your boat is to flood them and see . You would think that if that Alpha was unsinkable, it would be ashore by now.