I don't know if it's true or not:
...but I was told the whole "Lift" idea in fin propultion is a myth because water doesn't compress like air. So with that in mind, the whole theory of lift with an uncompressable medium is false. With air the theory is a fact because an object will always be moved toward an area of low pressure because the medium (air) is trying to equalize, and this is only possible in a meduim that can compress. The wing just happens to be in the way of this attempt of air to equalize so it gets moved in the direction of the low pressure, which in the case of the design of an airplane wing is up. The top plane measurement of the wing is longer than the bottom measurement causing air to take longer to move over the surface of the top plane thus causing an area of low pressure.
If you were to put the same wing underwater and move it like it was moved in air it would just dive and go in circles because of the shape. Water fins, like on a sub or keel fins on sail boats or rudders are symetrical and to change directions they must be turned to cut through the water in a different direction. This has nothing to do with the concept of "lift".
With a prop on a boat, the screw action of the prop turning is essentially cutting a path through the water like it would cut a path through clay and the prop is secured in a semi dense meduim (water) similar to how screwing a screw into wood would be. Different pitch props are numbered 15 pitch, 17 pitch. That means that the prop will move 15" or 17" inches forward in one revolutuion - revolving a dense non compressable medium.
Fins do basically the same thing to a degree, your just pushing yourself off a semi dense non compressible medium. This whole idea that split fins create a spinning vortex and that's what propels you is not true. What the water does behind the fin after it has been agitated and sent into motion doesn't have any affect on any forward movement. The actual movement of the fin is what moves you created by it cutting through the water and the immediate pushing affect is where the propultion comes from.
Splits have two surfaces that cut through the water to the two inside planes and when the fin is moved up or down the cutting action kind of happens at a slight angle to the direction of travel off both egdes of the blades instead of straight off the back of one surface like conventional paddle fins.
Aircraft props on the other hand use both the concept of cutting like a screw and lift. The blades of an aircraft prop are angled like a boat propeller and the blade itself has a shape like an aircraft wing with one side flat and the other side bulged for lift, so it gets an added bonus.
This is just what I heard though.
