The fishBOAT: A swimming watercraft with only one moving part* and no hull penetration.

The non-human version is the fishBOATTM.  The next prototype will be a diver propulsion vehicle (gif, below; video you can pause).    A human powered version taught me how to make the fishBOAT. I call the human version the fishBOOTTM.   Why use a fin? Why not a propeller?  Here's a bit about the theory.   Curious regarding how this idea was developed? Check out early work on the first human-scale version.   *Ok, there is more than one moving part.  The fluke is also a spring which deflects, which is movement. Certain versions also have a harness, which moves a bit.  "One moving part" refers to the torque reaction engine. The fishBOAT is an idea of Martin Garthwaite. Martin is developing the fishBOAT with Billy Roeseler through Fishboat, Incorporated. “Our inventions mirror our secret wishes.” – Lawrence Durrell