Talk:PropellerEfficiency

Helicopters have large rotors for efficiency purposes: the large, slow-turning rotor lets them compensate the low flow speed of air through the rotor (meaning: parallel to the axis of rotation) by impelling a much higher mass of air through size. The same is applicable to boat propellers.

One of the main considerations with aircraft propellers though is that the tip speed should never approach or break through the speed of sound in order to avoid the formation of supersonic / transonic shockwaves that would ruin the aerodynamics (supersonic regime is very different from subsonic), waste a lot of energy into producing a deafening shriek, and litterally eat away the metal.

In the water, the speed of sound is much higher (4 or 5 times as fast), but because water, a liquid, does not expand with decreasing pressure, this consideration is replaced with that of cavitation: cavitation happens when the pressure on the extrados of a profile decreases enough that the water starts turning into vapor. Running the propeller deeper underwater (increasing the water pressure) can help a bit, but this phenomenon is why most boat propellers have very "fat" blades: they can harness the same amount of "pull" over a larger surface than thinner blades, reducing the local pressure decrease produced at the same regime.