How propellers work

As a propeller rotates or moves downward, it pushes water down & back, as is done by your hand when swimming. At the same time, water must rush in behind the blade to fill the space left by the downward moving blade. This results in a pressure differential between the two sides of the blade, a positive pressure or pushing effect, on the underside and a negative pressure, or pulling effect on the top side of the blade.

This action occurs on all the blades around the full circle of rotation as the engine rotates the propeller, so the propeller is both pushing and being pulled through the water. The boat propeller draws water in from its front end through an imaginary cylinder a little larger than the propeller diameter. As the propeller spins, water accelerates through it, creating a jet stream of higher velocity water behind the propeller. This exiting water jet is smaller in diameter than the actual diameter of the propeller and this water jet action of pulling water in and pushing it out adds momentum or acceleration to the water which results in a force which we call thrust.