Nobody suggesting dishonest hatred of oil companies. However, not worth discussing further since that is off topic.
I am not sure I understood the relevance of your second point. You seem to be saying that if you run the motor consistent with the speed you think you are going, then at least you know when you will get there and that if you calculated right then your level of charge will be unchanged at the end of the voyage. ie you did not charge the batteries
My friend was trying to harness
the kinetic energy of the boat to charge the batteries so he had maneuvring power at his arrival without expending diesel to provide it. Converting kinetic energy into electrical energy without a loss of boat speed compared to letting the prop spin free (since the mass stays constant for all practical purposes) results in drag which slows the boat. In the limiting case, a prop stall, you will see the difference it makes to your boat speed. A wind turbine or (as you rightly say, now cheaper) solar cells result in less parasitic load. He went for the former -- a wind turbine.