But what if we place a micro hydro turbine generator "down stream" from the propeller to recapture the energy? Something similiar to this.
Yes, there is some thrust sacrificed. Propellers work on Newton's Third Law, Equal and Opposite force. The initial force of the water being pushed away from the propeller isn't decreasing, so the opposite force pushing it forward is the same. However that water is then pushing on the turbine in some form, thus, creating another force on the ROV that is negating, to some degree, the forward thrust created by the propeller.
All of that considered, what happens if there is an easily spun turbine(so as to lose as little thrust as possible) with the water flow somewhat funneled into the generator, so the water is hitting as little of the casing as possible? Is it possible to recapture a percentage (albeit probably only 5-10%) of the energy? Or is this resulting in only more NET energy output in the system than normal?
If you can't make out my ramblings above, I guess I'm basically talking about putting a windmill on an airplane behind the engine. All logistics aside, I'm asking if I can capture a large enough percentage of that energy back that the extra energy output is made up for, PLUS some.