A Sustainable Boat?

I was reading an article about an electrically powered boat. Although the main aim of this boat is not to be purely powered by renewable energy, it could be. It is a sailing boat with an electrical engine.

But what really makes it interesting, is that the batteries can be charged by putting the propeller in reversed gear when sailing. This will charge the batteries. So in effect, this boat can be used as 100% renewably powered.

Please let me know if you know of any working examples or projects that are currently under way. I am always interested to hear about these projects!