You seem to want to make your boat an electric hybrid. Here are a few facts:
1. The area near the shaft log in a boat is not a real friendly atmosphere for continuously running electric motors, especially those that have commutators.
2. A hybrid system is usually less efficient. Think of it as the sum of INefficiencies. You have the inefficiency of the engine, the inefficiency of the generator, the inefficiency of the batteries, the inefficiency of the motor controls (analogous to the throttle), and finally, the inefficiency of the motor. If it were a car, you would make a significant gain in fuel efficiency whenever driving in the city because you would consume no energy when stopped. (That is the big advantage of hybrid autos over those gas burning engines at idle.)
Your conventional marine engine has inefficiencies too, but fewer; engine, transmission. (I am assuming that parasitic loads such as the alternator and pumps are figured into the engine performance.)
3. Electric advantages:
But there are a few factors not considered here:
1: We have motors (alternators, blowers, etc), in our current engine bays already.
2: Because of the flat torque curve, we need less of an engine than our current engines in a hybrid setup (the genset doesn't need to have a lot of power, just maintain enough to create enough charge capacity). Also you can peak your electric more than your charging capacity because of your batteries, without the torque curve you would have to gun your engine to get up to the needed rpms to get power.
Just so you know, I haven't installed one of these systems either, but I am interested.
Both of those factors may not weigh out enough to make an electric system worthwhile, but at least they do represent some positives that are often overlooked.
I am interested in these systems as well.