Originally Posted by sailingdog
While electric power may make sense for a daysailer, it isn't a good idea for a cruising boat. If the boat requires moving under power in hazardous conditions for prolonged periods of time, the real problem is that the electrical bank can not be easily replenished. If the generator is large enough to power the motor without the aid of the battery bank, then there is little in the way of weight or energy savings.
Sailingdog is right. I have some knowledge of electric propulsion systems as I recently built and drive a 1974 VW Beetle that has been converted to battery-electric drive. I put 10,000 miles on it, and it has a top speed of 70 mph so it was highway capable. The best range I ever got out of it was 40 miles. I also investigated an electric outboard on a pontoon boat with solar panels on the roof.
Long-range cruiser? not a good idea.
The problem is simply the batteries. Current battery technology is just not a large enough storage medium (too small of a "fuel" tank). If you get caught in a hard blow, or a situation where you need sustained
auxiliary propulsion, you'll be screwed. You would have to positively upholster your boat in solar panels and wind turbines to get an appreciable charge into them without shorepower.
The batteries are expensive. I used 8 volt golf cart batteries. 16 of these ran me about $1500. Golf cart batteries are the ONLY batteries that can stand up to this application. "Deep Cycle" Marine batteries absolutely will not stand up to this kind of abuse, neither will 12v gelcell batteries.
If you have unlimited money to throw at this project, new Lithium batteries are smaller, lighter, and hold more energy. There are several chemistry varients: Lithium cobalt, Lithium sodium, etc. Unfortunately, Lithium batteries absolutely require a computer controlled management system to keep them balanced without destroying them. Lead acid batteries do not because you can safely (for a time) overcharge the "full" batteries while the laggers catch up and achieve full charge. The result is that the full batteries have boiled away some electrolyte and you just add de-mineralized water. You can't add water to a Lithium chemistry battery.
It's a good idea, and I'm not saying you shouldn't do it, but I am saying that you shouldn't do it on a blue water boat or long-range coastal cruiser. A day-sailer that stays near home is most appropriate under today's battery and charger limitations.
I can provide more information, but I think I've gone on long enough.