Originally Posted by J36ZT
Negative Buoyancy means it will sink (at that depth).
There is no such thing as "negative buoyancy"; you are either buoyant (and have "reserve buoyancy"), neutrally buoyant (weigh the same as water) or the buoyant force is less than your weight and you sink.
In example if you put a block of solid aluminum in the water and it's weight is 10 grams, and the volume of water it displaced weighed (appx) 5 grams, it would sink because it needs 5 grams more buoyant force. If you had a scale on it it while submerged it would register 5 grams because the buoyant force is still there but only providing 1/2 of the needed force (it's not negative, it just is not big enough).
Take the same mass of aluminum as a foil and shape it into a shell or "boat" shape and it will float; because the shape allows there to be a displaced volume of water with air in it's place (much less dense than water). The displacement of the water allows the aluminum shell to float and it's the amount of water displaced is exactly the weight of the shell. If you add weight to the aluminum boat it will sink gradually with the additional displacement equaling the amount of weight added; and that's considered "reserve buoyancy".
The frame-only sailboat appears to be close to neutral buoyancy. The tubular carbon fiber displaces water and allows it to just barely float. The styrofoam pontoons are probably for reserve buoyancy (to be sure it won't sink).