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I just sold my pedal boat projects, but they were winning races right out of the box. For me, the key was propeller efficiency. While studying the possibilities, I decided to try out a model aircraft propeller, and stayed with them. For motor power, you'd probably want to copy one in metal or carbon fiber, but if you are going for a dragon tail, there's room to gear it right.
I was recently appalled to learn that marine architects are still using the Betz limit in propeller calculations. This is usually quoted regarding windmill efficiency. It applies to tugboats, and pulling contests against stern wheelers, but once a vessel is under way, we need the Froude numbers, which take into account the steady supply of undisturbed water. By pushing gently on a large mass of water, we do much better than with a smaller diameter prop, which is always pushing on water that is running away. The situation is even worse with jet drives, but both are selected for their ability to run in shoal conditions. A dragon tail is easy to lift, or even operate surface-piercing at need.
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