I’m currently working on characterizing the dynamics of the OpenROV 2.7 and thought you guys would find these results interesting.
I have found the locations and magnitudes of the centre of mass and centre of buoyancy in my model using CAD. The centre of mass was found by incorperating the relevant weights and densities of all components into the model; and the centre of buoyancy through finding the centroid of the submerged volume. The magnitude of mass is the weight and the magnitude of buoyancy is the mass of displaced water.
In my model, I measured the mass as 2519g and the estimated total weight from CAD is 2513g, this agreement gives confidence in the results. The calculated buoyancy force is 24.47 N whereas the weight is 24.71 N. So in this case, the ROV is slightly heavier than water though in cases where water is denser (saliter, colder) it may be neutrally buoyant or positively buoyant.
In equilibrium state the centre of mass lies below the centre of buoyancy. The intensity with which the ROV returns to its equilibrium from a disturbance is dependant on the relative positions of the COM & COG, the inertia properties of the body and also the drag characteristics. From this analysis the COM & COG are seen to lie fairly close to one another meaning the restoring force is less than if they were further apart.
Another interesting thing is that the thrusters axes coincide with the center of mass, which is ideal to prevent a moment about the COM while thrusting. I’m not sure if this was a design consideration or a coincidence, does anyone know?