Your problem is a solved one. Up until now there were basically a bunch of startup electrical propulsion companies and there were DIY versions using floor waxer motors or golf cart motors (and batteries).
Now there is a mature looking system that has been picked up by Beta marine and Yanmar. I've emailed the designer of the engine/generator part and I've spoken with a salesman at Beta and the system seems really good.
Without getting into the whole diesel-vs-electric debate there are defiantly some good reasons to have a hybrid on a sailboat. Motor sailing is the main reason. You can be sailing along at 3 knots and with a silent push of the throttle you can now be going 5-6 knots and with very little amp hour expense. Personally, if I had the coin, I'd have one of these on my boat right now.
6 months ago when I contacted Beta and the motor designer you could get a brand new beta with the hybrid electric motor all set to go but Yanmar was not quite ready to ship. The electric motor could be bought by itself, or with a transmission, ready to mate to your existing engine (Volvo) shipped from the UK. I can dig up the prices and info if you like.
One nice thing about this engine is that you could also run the diesel engine without turning the prop, and it would turn the electric motor, creating a 10KW genset. So you had the option of running the diesel engine as you normally would for propulsion, running the diesel engine without turnint the prop and getting 10kw, turning the prop and using reserve HP to make electricity (up to 10kw) at the same time, or using the electric motor without the diesel engine as an electric drive. IT also had some nice features like an ability to sense when you were giving your engine near full throttle and it would automaticlally cut out the electric generator component to make sure you got full HP.
Here is the link:
Seagoing Hybrids - Hybrid Electric Marine Propulsion
For some reason the betamarine hybrid links are down today. Google shows them as an active link but it's not working today.