Please Clarify

[GT1000000]

I have been searching through the Prop forum, trying to make heads or tails out of the terms over-propping and under-propping.
I am still confused...
Is it over-propped when you go over your recommended engine RPM's at WOT?
And, under-propped if your engine does not reach it's recommended RPM's at WOT?
Also, IF, a hypothetical engine, is equipped from the manufacturer with a 14" 17P prop and runs 5000 RPM at WOT, which is what the manufacturer recommends is ideal for this hypothetical engine, what would constitute over or under-propping?
Would changing the prop to a 14" 19P prop be considered over-propping? or under-propping? And, vice-versa?
Thanks,gt

 

[etracer68]

Re: Please Clarify

You would be over propped, if you cant reach recommended RPM's@WOT.

 

[Bondo]

Re: Please Clarify

I am still confused...
Is it over-propped when you go over your recommended engine RPM's at WOT?
And, under-propped if your engine does not reach it's recommended RPM's at WOT?

Ayuh,.... You've got it exactly Backwards....

 

[GT1000000]

Re: Please Clarify

So, the hypothetical engine from my previous post would lose approximately 400 RPM by changing from a 17P to a 19P? Right?

That should also translate to a slight decrease in torque, or put another way, a slightly slower hole-shot? Would that be correct?

Kind of like a gear ratio change in a vehicle? A 3.73 rear end would have a higher top speed than a 4.11, but the 4.11 would tend to accelerate faster.

As I understand it, you lose approximately 200 RPM for every inch of pitch increase and gain RPM for every inch of decrease in pitch.

Thanks again, gt.

 

[etracer68]

Re: Please Clarify

Now you have it correct, and yes about 200 per inch in pitch, depending on style of prop.

 

[Silvertip]

Re: Please Clarify

So, the hypothetical engine from my previous post would lose approximately 400 RPM by changing from a 17P to a 19P? Right?

That should also translate to a slight decrease in torque, or put another way, a slightly slower hole-shot? Would that be correct?

Kind of like a gear ratio change in a vehicle? A 3.73 rear end would have a higher top speed than a 4.11, but the 4.11 would tend to accelerate faster.

As I understand it, you lose approximately 200 RPM for every inch of pitch increase and gain RPM for every inch of decrease in pitch.

Thanks again, gt.

You now have this correct, but just like your axle ratio comparison, a 3.73 will ONLY be faster than a 4.11 IF the engine has enough power to twist those gears. The very same thing applies to props. Too much pitch lugs the engine and prevents it from reaching its maximum rpm so speed may drop. Conversely, two little pitch does the same thing as a 4.11 in the family truckster -- the engine would likely overrev at WOT.

 

[cyclops2]

Re: Please Clarify

I hope you have a good prop shop nearby. He will save you thousands of dollars.

Tell him your problem. He MAY LOAN you the correct props IF you leave money to cover you losing a prop while experimenting.
My prop shop has lots of new props, when people lose a prop as they shift into reverse.

 

[GT1000000]

Re: Please Clarify

Thanks to all for the information.

I also found, quite by accident, a very good explanation regarding props, on the "SOLAS props for sale" link at the top of the forum page.

As far as cyclops2 comment, "I hope you have a good prop shop nearby. He will save you thousands of dollars."

Thanks for the advice, but fortunately, at the present time, I was only seeking to get educated about props and their related terminology.

My set-up currently has a very good prop for the application.