Well, yes, of course. If it’s not fully submerged, then buoyancy changes with wave action. The degree to which this affects comfort and safety depend on the percentage of change. If the change is small enough you can call is “stable within a range”. Even completely submerged is not a constant equilibrium.
This can actually be pretty significant. Professional divers working shallow, for example scrubbing boat hulls, can experience more physiological problems due to the changing pressure gradient from waves than from submerging to deeper depths for longer periods. The rate of pressure change has a huge effect on bodies, but has some effect on machinery and structures as well. Diving shallow sucks because it is harder to maintain buoyancy when you move up or down a small bit in te water column, and it’s really hard on the body.
Picture a vessel floating in 100ft of still water, and submerge it to 67 feet from the bottom or 33 feet in depth (2 Atmospheres pressure). Wave conditions change from still water to 10 foot wave crests, you will note transient pressure changes equivalent to quickly moving from 23 ft (1.6945 atmospheres) to 43ft (2.3002 Atmospheres) or about 73.7% change.
The deeper it goes, the smaller the percentage of change, the higher the absolute pressure.
And masts provide leverage for righting moment. You’ve drawn a ballast that is not really a keel, because it’s not designed to provide a planing or cutting effect against the water for directional seakeeping. This is the damping effect you’re talking about, but damping is kind of a misnomer when applied to most hull shapes. Keels are designed to work against leeway using the difference between wind direction (or thrust from a motor) and the current; to lever the boat into a particular heading.
Precisely: what you’ve drawn might be better characterized as a ballast on an inverted mast rather than an extended keel. It’s not used for seakeeping. Sailboats rely upon the mast for more than hanging sails. They use the mast as inertial damping against buoyant and ballast-based righting moments. When a roundish-bottomed hull heels over due to wind or wave action, it does so at a certain speed which is not constant across the entire arc. As it reaches the end of the arc, more and more righting moment occurs because the ballast gets further out of zero (in relationship to gravity) and generally, because the hull shape is designed to float in a certain upright attitude so buoyancy is increased for that side of the vessel the further it heels, while buoyancy is simultaneously decreased for the upper side, opposite. As you reach the end of the arc, maximum accelerating force is applied to right the hull by moving back in the opposite direction. Sailors call this “stiffening up”. A de-masted sailing ship tends to “snap back” at this point with very little damping until nearly at the opposite end of the arc. A mast works as an inertial damper to slow the roll back. Otherwise the acceleration/deceleration at each end of the arc of the roll is VERY uncomfortable for passengers.
This is the crux of many arguments other folks have had with the proponents of this idea. Note- I do not personally think it’s a great idea, but I am not discounting it entirely. But if I were championing this idea, I would develop some basic formulae for scaling the dimensions. If one had been researching and developing a particular concept for years, it would be rather odd to not have these figures at one’s fingertips, have published them, and be able to cite them at a moment’s notice.
Entraining water should help with increasing inertial dampening, absolutely. The more surface area of the heave plate, and possibly shaping edges with caps, the more water resistance it has. The heave plate, the hull, and all internal fittings also serve as ballast to get the sphere down to hit the marks on waterline. At a minimum placement should be at or below the center line. The trade-off is the lower it is the better for ballast effect, the higher it is, the bigger it can be. You could have multiple heave plate rings at additional cost and complexity. At some chord of the sphere, depending upon gross overall size, the interior space becomes less usable for activity and better suited to using ti for ballast. The engineering of a ring around the sphere adds considerable stresses and complexity, I imagine. One might do better by building a ball-cupped-in-a-cylinder concept. The cylinder provides a skirt around a lower chord of the sphere for adding ballast weight, and simultaneously entraining more water for “free” inertial weight as well as significant water resistance due to surface area. During certain stages of construction, movement, and theoretically for placement in shallow areas, the cylindrical skirt also forms a gravity base. The additional weight of the skirt requires some significant engineering because it is a tensile connection rather than in compression (as is the “ballast keel” pictured above).
For really simple ballast, concrete can simply be poured on the inside, but concrete is not particularly excellent ballast because it is not dense enough- sailboat builders often include lead or scrap iron “pigs” in the concrete pour to increase the average density of their ballast.
A sphere also has a limited range of sizes where the emergent disc is useful. As you get further from the ‘north pole’ it gets steeper and steeper. The smaller the sphere, the faster that happens. If it only has a snorkel tube, you start to run into problems making it an attractive place to live and work.
I see the term “pressure vessel” in this thread. That is not a correct term. Pressure vessels contain pressure. Keeping pressure out is not the same thing. If you were pressurizing the interior to match outside pressure, then you have LOTS of problems not only with maintaining the interior pressure, but with transitioning from interior pressure to the outside. If you don’t pressurize the interior to match the outside pressure, then you have a force gradient from outside water pressure that is much greater down lower (adds another atmosphere of pressure roughly every 33 feet). The spherical nature of the structure is good for distributing that force as “evenly as possible”, but that is not actually the same as “evenly distributed”.