The whole idea is viable if you are willing to go very short distances between charges. If you sacrifice top speed as well, then you will be able to have a pretty light setup.
Think about that statement.
Following the same logic, living in Death Valley without water is viable, if you are willing to accept a dramatically shorter lifespan. Skiing in Dakar is viable, if you are willing to accept much slower speeds.
If the reason to have an auxiliary engine is to provide auxiliary speed and emergency power, as well as the ability to recharge your batteries, then a straight electric power source is not viable at all, and in fact presents a whole new set of issues, such as recharging.
Don't confuse diesel electric boats/ships with electric boats. Whole different propulsion system, whole different kettle of fish.
Southern Cross, I am a nay-sayer, but I am not mad, I am amused. The ignorance of the realities of electric propulsion is entertaining, if nothing else.