Basics of prop dimensions

[kfa4303]

Hi all. I just had a few quick questions about the dimensions given for props. As I understand it, the first number given is the diameter of the prop and the second number is how far it will travel forward in one complete rotation. For example, a 9" x 11" prop is 9" wide and will travel forward 11" in one complete rotation. Is this correct, or do I have it backwards? If so, what are the general pros/cons/characteristics of props of varying size? The parts manual for my '66 20 hp Johnson lists 3 different factory prop sizes 9-1/4" x 11", 9" x 10", 9" x 9". Are some better for low end torque vs. high end speed, like gears in a transmission? If so, how? Many thanks.

 

[steelespike]

Re: Basics of prop dimensions

You have it right. diameter X pitch. Lower pitch has more power and reaches higher rpm easier may over rev lightly loaded.
Higher pitch has less power but can produce higher speeds if the motor can operate within its rated rpm.
lower pitch is like a lower gear in you're car higher pitch more like high gear or in the extreme like over drive.
Then there are various designs that help the prop perform better.

 

[kfa4303]

Re: Basics of prop dimensions

Gotcha. Thanks!