A friend of mine turned me on to the idea of a dual prop arrangement. He's using it on his torpedo, and I don't see why I couldn't use it on my boat. This is the basic idea. I've read that if you use the radially aligned fins to reduce rotation in the generated horizontal water column you'll get more thrust out of it. At some point I'll have to actually build this and test it to determine how much thrust it can generate. I'm not sure if you can tell from this graphic (sorry for poor quality) but the props are meant to rotate in opposite directions.