How Propellers Work

The "Push/Pull" Concept

To understand this concept, let us freeze a propeller just at the point where one of the blades is projecting directly out of the page (Figure 3-1). This is a right-hand rotation propeller, whose projecting blade is rotating from top to bottom and is moving from left to right. As the blade in this discussion rotates or moves downward, it pushes water down and 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. This action, of course, 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.

Thrust/Momentum

These pressures cause water to be drawn into the propeller from in front and accelerated out the back, just as a household fan pulls air in from behind it and blows it out toward you (Figure 3-2 below).

The marine propeller draws or pulls water in from its front end through an imaginary cylinder a little larger than the propeller diameter (Figure 3-3). The front end of the propeller is the end that faces the boat. 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.

This water jet action of pulling water in and pushing it out at a higher velocity adds momentum to the water. This change in momentum or acceleration of the water results in a force which we can call thrust.