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Propellers in water are smaller in diameter. They also move more slowly. On the other hand, aircraft propellers are larger in diameter, have narrower blades and operate at very high speeds. An aircraft propeller would break apart in water, while a water propeller would produce little to no thrust in air.

Rotational speed can be easily adjusted while moving between environments. However, is there such a thing as a propeller shape that is halfway between the two designs? Or would it simply be extremely inefficient in both environments?

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Interesting, it implies water to air and air to water "craft" which is a target to engieers in the present time.. – Force Apr 14 '13 at 20:32
up vote 3 down vote accepted

There's two important differences between air and water: Air is compressible, and the densities are about a factor of ~1000 apart - 1 kg/m³ vs 1 t/m³!

For most concerns where you use propellers, compression plays no role because the pressure diferences are very low. The densities, however play a large role.

The thrust can be described as $F = \dot m * \Delta v$, with $\dot m$ beeing the mass flow - kg&/s or such - and $\Delta v$ the difference in velocity a volumeelement of fluid is accelerated.

So, to achieve a similiar thrust, the same propeller would have to move 1000 times more air than water by volume. Hence the often larger and faster spinning propellers for planes.

On the other hand, in a heavier medium each wing of the propeller is subject to stronger torque (all else beeing equal):

$$Q = \rho V_{a}^2 D^3 f_q(\frac{ND}{V_a})$$

(Source) $\rho$ is density, $V_a$ rate of advance (how much the propeller moves forward per revolution), D is diameter and N number of revolutions.

Without going into the math it can probably be shown that in a heavier medium the propeller will experience somewhat more torque for the same thrust - I'm to lazy to try now. The propeller will be built with more robust (and possibly heavier) material than would be the case id it's an only air propeller.

That said, I believe that a propeller for both media is entirely possible, though challenging.

However, a propeller for a both media will need a drivetrain that can accomodate speed roughly a factor 1000 apart (that's not trivial).

One other reason why don't see a propeller for both media is that there is no vehicle that could make use of one.

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Who said that the propellers in water are small? If you have a look at a nuclear submarine, you're likely to see that the height of the single propeller is almost the height of the submarine. The size of propellers doesn't matter at all. It's completely based on how denser the fluid is (enough to break the propeller)..!

Water propellers have a lot of challenges to face. A ship is a lot of mass (to be thrusted) compared to aircrafts. Moreover, the blades also experience cavitation (a bubble-formation phenomena) that causes wear of the material. So, the propellers should be strong and also be constructed in such a way that the equilibrium of the ship is maintained.

On the other hand, aircraft propellers have no (or less) problems to face when thrusting through air compared to ships. The pressure exerted by the blades is far enough to lift the plane. The issue is different is water because "a whole mass" of water has to be pushed back.

You're right about them when the function of both are replaced. The aircraft can't even lift a submarine's propeller. So, there can be no such thing as "inter-propellers" which can function depending on fluids (like the one in X-men). But, there are a lot of similar models like some sea planes which can move at a moderate speed in both air and water since it slides easily. But, no hetero-propellers...

Let's take a table fan with four blades. If its moving in the anti-clockwise direction, then the side along the direction of its motion is curved inwards so that they can push the fluid out. Let's assume this as thrust. Now, imagine how the fan swirls in both fluids. In air, it requires less mass of the fan itself so that it can thrust itself forward. In water, it requires more mass to push the denser fluid back and so thrust front. Hence, the mass & inertia matters here...

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