Have you ever noticed how bacteria have rotory propulsion,
one might consider that having had billions of years to develop,
they are likely to have one of the best concievable examples available.
Let’s examine:

looks like it functions on alternating current, uses ball-bearings hinged in multi-layered wall.
has longer and safer propeller, on a universal joint.
here is a video of how the flagellum constructs itself:
http://www.youtube.com/watch?v=0N09BIEzDlI&feature=fvwrel
here is a video about how the flagellum evolved:
http://www.youtube.com/watch?v=SdwTwNPyR9w
It seems that current engines are analagous to the “poor-rotation” version, which is good as dispersion
Maybe we can also learn something about the layering of the hull from the membrane structure.
Though it would be interesting to see if anyone has attempted to make a propeller like the flagellums.