[mcsparkie]

I think that what they want to hear you say is that aircraft, ships, etc do not get "pushed" through the air, they get "pulled" through the air by the vacuum created forward of the proplusion

The propellor blades are slanted so that there is high pressure on the back of the blade and low pressure on the front. The difference, or net total force produces a thrust forward on the propellor. The vacuum on the front is limited to zero pressure, which is only 14 psi less than atmospheric. I would guess that the pressure increase on the back is more substantial.

[MonroeDNA]

Fluid dynamics deals in presure graduations, as in the presure in front of the propeller, vs. the presure behind the propeller. It works well, but it is a simplification.

Momentum is everything, and if the propeller was stationary, and yet managed to expell water, the boat would move forward.

Forward pressure just fights the movement. When they are equal, the boat cannot move. Vacuum is the absence of opposing force; in this case, forward pressure.

But what do I know? I'm just an electrical engineer.

[cinFLA]

The propellor is also an airfoil. The "faster" airstream on the front side produces significant lift.