Somebody should have pointed out that:
total vessel displacement is to the load waterline.
Emergency bouyancy is the minimum extra to keep the vessel from sinking.
Am I correct in thinking, that once completely swamped the boat reverts back to its actual total material weight? IE, using the classic illustration, pushing a basketball into the water requires more effort the deeper one pushes. But if the ball broke, it would take little effort (weight) to immerse.
Once totally flooded, awash, only actual total weight of vessel plus occupants needs to be kept afloat for rescue.
To float boat above the sheer much more flotation is required.
Therefor the reserve flotation airbags (noodles) idea put forth here should be attached below the sheerline and calculated to provide enough bouyancy so that the swamped vessel could pumped out and 'refloated' at sea. That's what I had in mind.
Poof!
As to the original problem. These days, after decades of forced air-holding boats and fabric, there shouldbe something, some air bag, that is tough enough to be expanded into a void with sharp protrusions such as the lazarette (viz ob motor) to act as space filling reserve bouyancy.
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