1. ## prop 101

someone care to take the time to explain what the numbers mean in a prop. for example 15/20. what does the 15 and 20 mean respectively. and how does a 15/20 compare to a 14/19?. lastly if i add +1 mm to a 15/20 does that make it a 15/21?thanks

2. Originally Posted by vduzin12
someone care to take the time to explain what the numbers mean in a prop. for example 15/20. what does the 15 and 20 mean respectively. and how does a 15/20 compare to a 14/19?. lastly if i add +1 mm to a 15/20 does that make it a 15/21?thanks
(((NOTE... I have been corrected.. my information was for boat propellers and don't apply to Jetpumps)))

15/20 are numbers to indicate the number of inches the prop would travel through the water if it was perfectly hooked up. Its a progressive pitch meaning the front of prop is pitched to 15inches and the rear of blade is pitched to 20 inches of travel.

so a 14/19 should accelerate harder then a 15/20 but will turn more rpms at any given speed...

If you bend the prop to make it travel 21inches per full rotation at the back and left it alone at the front, then it would be a 15/21 and effectively lower your rpms at higher speeds

3. this is for boats

http://www.boatfix.com/how/props.html

4. Wow, and all these years I thought it was degrees.....

5. Originally Posted by RUNWME
this is for boats, but same in theory...

http://www.boatfix.com/how/props.html

Our pumps do not work like a boat prop. The numbers indicate the leading and trailing edge angles.

If you were to design a boat prop like a jet pump impeller, it would cavitate extremely badly due to the progressive pitch and would have poor performance. The increase in pitch towards the back of the prop would cause the water to separate from the front of the prop.

If you designed a jet pump impeller like a boat prop (no progressive pitch) it would have poor performance due to low pressure at the pump venturi by not having enough pitch. If you then pitched the whole prop more, it would excessively load the motor and start to cavitate before developing the desired pump pressure due to too much initial pitch.

Either way, the numbers on the impellers indicate the prop angles, not the screw ratio like they do on boat props.

6. And I learn something everyday

Thanks for pointing out my mistake ..

7. thanks for sharing the info on boat props !
I learned something today I didn't know

8. Since the prop number (i.e. 15/20) is in degrees, then adding 1 mm doesn't necessarily make it a 15/21. Though, in many cases depending on where you bend it, it turns out that 15/20 +1mm is ~15/21. Make sense?

9. Originally Posted by engineermike
Since the prop number (i.e. 15/20) is in degrees, then adding 1 mm doesn't necessarily make it a 15/21. Though, in many cases depending on where you bend it, it turns out that 15/20 +1mm is ~15/21. Make sense?
so w've not lernt alot off this thread