# Thread: Prop Pitch for Dumbies...especially me!

1. ## Prop Pitch for Dumbies...especially me!

How do the numbers relate to each other...for example a 4 blade 15/20...what does the 15 represent/measure...what does the 20 represent measure...

How does a 15/20 3 blade differ from a 15/20 4 blade...besides the obvious number of blades...! Or to be more exact how does a 3 blade with the same numbers differ from a 4 blade...besides blade count...?

Is there a calculation to determine 'optimal' prop/pitch for a given setup...

I am looking for a detailed...in simple terms...explanation of the numbers in a prop's pitch...please

Finally what this is all leading to besides a question that has lingered with me since joining Greenhulk 6 short months ago....

What would be the best (guess) 4 blade prop for a Turbo setup running to 8500-8550 rpm...I think Jerry has found the optimal 3 blade, but what 4 blade pitch are turbo guys thinking about...I mean do we retain the same five unit difference but just go steeper as in 17/22 or 18/23...or do we keep a certain ratio...which would put the two numbers further apart as pitch increased...?

I am confused...really confused...and really want to know this prop/pitch stuff as well as the best guesses for a 4 blade turbo kit prop...thanks

2. I've been told the first number is the pitch of the leading edge, while the 2nd number is the pitch of the trailing edge of the prop. The value is the number of inches forward the prop would travel for ever 1 revolution, assuming no slip (and jet drives have alot of slip).

I think the stock prop is a 12/20, versus the Riva stock recommendation of 13/18. The leading and trailing edges are different because the water passage gets smaller as you go back (look at the root diameter of the prop), so the velocity of the water must increase too.

I believe the only difference between, say, a 14/19 4 blade and a 14/19 3 blade is the number of blades.

In order to get a recommendation, I would just call IMPROS.

Mike

3. SIMPLE The LE pitch, the arch, the window,the root angle and the tail. What have I forgot?

Explain to me the inner workings of an automatic transmission? In simple terms in detail?

4. how about starting a database...
mods:3prop/mm+- :rpm's achieved
mods:4prop/mm+- :rpm's achieved

5. A database is useless to riders other than yourself, BECAUSE, there is no magical bend number...

EACH PROP must be CUSTOM BENT to their RESPECTIVE SKI's...

Your setup will dictate the results the prop will produce...

You could 2 skis that are set up EXACTLY the same, take 1 prop (swap between the two) and get 2 different results...