A is the leading or cutting edge. The height at B is the trailing edge.


A propeller acts like a screw cutting through water as it turns. The amount of distance that a propeller can cut through in one full revolution is known as the pitch of the propeller. Pitch is usually marked in inches. This is the theoretical pitch. (Propellers are not as efficient as marked)

A propeller with a pitch of 16 " for every revolution (360 degrees) should travel forward 16", but due to its inefficiency when cutting through water (A propeller is not as efficient as it's pitch) a slip factor is usually added into any propeller size equation. Slip is measured in percentage. Ranges of slip from 12% to 20% is common for outboard motors depending on boat, motor applications. 18% seems average.


Slip is the lost percentage of theoretical boat speed compared to actual boat speed.



By altering 1" of pitch we could expect a change of approximately 200 rpm.
A loss of 1/4" Diameter could increase rpm by 200 revs.
Adding diameter will result in rpm loss.



If the above propeller was marked 16" it would theoretically travel forward 16" in one revolution.

Book Suggestions

 Propeller Handbook: The Complete Reference for Choosing, Installing and Understanding Boat Propellers

How Boat Things Work: An Illustrated Guide

Speedboats (Fast!)

Drag Boats of the 1960s (Photo Archives)

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