Pitch is the distance a prop moves through the water in one revolution. The second number stamped on the prop is pitch. A prop with a pitch of 18 inches would move 18 inches through a solid medium with each complete rotation. The reason a propeller moves something less than 18 inches is because it operates in a liquid medium, which creates slippage. So instead of moving 18 inches, a propeller in water moves maybe 15 inches. Some slippage is essential; without it, the prop can't move the boat, but too much or too little slippage reduces efficiency.
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Thanks, Stefan. This is an excellent article about the fundamentals in terms a layman can grasp. The terminology we use is important. I liked (and use myself) the term “geometric pitch” as Agus used in his comment. And when I began measuring in the method you describe I measured the hypotenuse rather than side B for the simple reason that my caliper can nail it to .001 inches. I use the term “bite” for side “a” and the term “sweep” for side “B” which you described as the shadow the blade would make if lit from above on the table. I will be sending links to this article for folks to read who are interested in discussing the nuances of measuring their own propellers!