Propeller Pitch

WestMarine.com

Pitch or face pitch is how far the propeller would move forward in a substance, without slipping (propeller slip), in one revolution much like a screw in wood. You can think of pitch in terms of a car’s gears; having a small pitch will take a small bite on the water for each revolution requiring less power to move forward but with limited top speed, it’s like low gear on a car. A larger pitch takes a bigger bite and will require more power to turn but allowing higher speeds for RPM’s like a cars higher gears.

In reality however the propeller is pushing a relatively large heavy object forward by pushing water back, so of course it will slip through the water, much like a car spins its wheels. Typically, propellers have a constant pitch across the face. This means that as you move out from the hub along the face of the blade the angle decreases consistently with the distance from the root. The reason for the use of a helix is, the reason for the propeller helix shape is the tip of the blade is moving faster than the root of the blade but the tip needs to move at the same speed though the substance as the root to make use of the entire blade surface, so the faster a portion of the blade is moving the flatter the angle.

Now the next question that springs to mind is how flat does this angle go? This brings up the term, Propeller Virtual Pitch, which is the average pitch angle along the entire blade; it is the real pitch of a propeller which of course is not published. We usually only get diameter and pitch but propellers with equal face pitch but different virtual pitch will give somewhat different results. To top it off blade thickness and width are also factors that will alter the results. There is however a way to measure for these differences which results in a number called propeller experimental pitch. It is the Speed of the propeller though the water over the revolutions per minute when thrust goes to 0. Think of a car going downhill and you push the accelerator to just before it starts accelerating.

The formula experimental pitch is as follows: P = 101.33*Va / N

  • P = Experimental Pitch
  • Va = Speed of advance of the propeller through the water in Knots
  • N = Shaft RPM at 0 thrust