OK, you take a block of aluminum, attach a line onto it and drop it into the water. The block will sink until the line stops it. Now measure the force the block is pulling on the line and you have the force of NEGATIVE BUOYANCY!
Now, immagine you are a SCUBA diver in a wet suit. The wet suit contains small bubbles of air. You dive to a given depth and then begin returning to the surface. The closer you get to the surface, the less pressure is exerted on the bubbles and the more they expand (the more bouyancy your wet suit will have). In other words, the closer to the surface the faster your ascent will be. But, SCUBA divers have this thing called a BCD (Buoyancy Compensation Device) which is really nothing more than a vest that can be inflated or deflated. A SCUBA diver, wearing a wet suit, knows about buoyancy and will let air out if the ascent needs to be slowed.
SCUBA divers quickly learn about buoyancy--positive, neutral, and NEGATIVE--because it takes energy to overcome the forces.
KeelHaulin, not meaning to attack you...but you would have us believe a graph of buoyancy would be a parabola, or a curve (in the least) that would never reach zero. From having done diving, I can not accept this. As a matter of fact, just yesterday I was Neutrally Buoyant at a depth of eight feet. If a took a deep breath I ascended, and if I breathed shallow I sank...yep, I could make myself NEGATIVELY buoyant.
Back to our watercraft... Because the air contained within the craft can not compress, depth would not change its characteristics. Since the pictures show said craft, at rest, with a portion of it sticking out of the water (not just at the surface)...it has to have some Positive buoyancy in the liquid it was placed. It is possible the watercraft would have positive buoyancy in salt water but sink in fresh water. I wonder if there are warnings stating which type of water to launch the thing into?
Again, just my thoughts...as I've witnessed things...
Skipper, J/36 "Zero Tolerance"