energy efficiency and electric propulsion
disclaimer: I'm not an engineer. I hope one comments on this thread.
The problem I see with a boat equipped with a diesel reciprocating engine generator, battery bank, and electric propulsion motor is one of efficiency. Let's start with the premise that in using these devices, you are converting one form of energy into another, as in chemical to thermal to mechanical to electrical to mechanical to kinetic. Next, we must admit that each time a form of energy is converted, there is a certain loss of eficency. For instance, published efficiencies for diesel reciprocating piston engine generators is on the order of 25-40%.
The amount of energy that can be stored in diesel fuel can be calculated. For various diesel-powered propulsion systems, we can estimate a "miles per gallon" efficiency of propelling the boat. Comparing diesel propulsion with an engine coupled directly to a propeller shaft versus that produced by a diesel generator to batteries to electric motor to propeller raises the issue of efficiency loss at each conversion. The direct diesel propulsion has to be more efficient in moving the boat, thus more "mpg" as there are less steps of energy conversion. This is one of many reasons why I think electric boats are not yet commonplace. Now, of course if you add additional power generation capacity from generating power from boat movement via the propeller shaft while under sail, it gets more interesting. There are many intriguing possibilities for the future, including fuel cells as storage devices. I wait on the sidelines with interest