[Analog]

Good description Mav, except you left out one very important thing about the sub... the fact that its in the water. The propeller turning in a vacuum such as outer space would do nothing to provide thrust. In the ocean, the propeller turning forms a pressure difference across the blades, thus providing the thrust.

In space, you absolutely positively have to eject "something" from a spacecraft to move it... unless you are discussing other more advanced forms of propulsion (which the valkyries do not use).