So as Mr Martini said, the bouyancy foam/chambers are pulling up, and the ballast weight is being pulled down by gravity. You could have 4 foam blocks an equal disatance apart on the top of the ROV, and they will pull up evenly. Ballast on the bottom will pull down. The heavier the ROV is, the better it will resist being moved by current, tugging on the tether, or the manipulator tugging on something, which in turn pulls on the ROV.
I designed my ROV to be big and heavy, which makes it harder to transport, but also makes it rock-steady in the water (like the Nautilus ROV). It also means i can tug on something buried in silt and not have the ROV jerk forward.
Another thing to keep in mind is this…
Designing things in CAD programs (such as Solidworks or Fusion 360) is good for prototyping without the cost of physically building it. But don’t fall into the trap of thinking computer modeling is the Only Way. Computer modeling can only take you so far. At some point, when you have a good design, you are going to have to build it in the real world and dunk it in the water, to see how it performs.
On another forum, a guy was having trouble getting his CAD program to let him make a hole in the side of a tube (his WTC). After some back-and-forth, we discovered he was intending to 3D print the thing. He wasn’t trying to learn CAD, he just wanted the physical object, and building it in CAD first was the only way to get it to print.
I told him to go to the hardware store, find a piece of PVC the right sixe, then use a hole saw to drill a hole in the side of it. Or a metal supplier if he wanted it in aluminum. Why on earth would you ‘print’ something so common as a piece of pipe? (Then have to deal with water leaking between the unfused layers.)
3D printing (or CAD software) is a fantastic technology, but don’t get so fixated on it that you start printing ink pens because you need something to write with.
Newton’s first law…
An object at rest will remain at rest unless acted on by an outside force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an outside force.
Newton’s second law…
Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).