What is the optimum RPM for a propellor

[Steven Bryant]

Hi guys

i have been talking to a friend about the propellors we use and the RPM we we are turning them at, he was quite shocked at the RPM we get out of the nitro engines and asked me a question which i could not answer:

WHAT IS THE OPTIMUM RPM FOR THE PROPS WE USE?

Does any know or are there any "white paper" on the topic?

Cheers

Steve

 

[Hydro Junkie]

I'm sure Mark Sholund will chime in on this one but for each prop, the optimum RPM will be dictated by the diameter, pitch, blade shape, hull type and weight as well as the amount of power being supplied to it. That all being said, what is optimum for one prop will be nowhere close for another

 

[Dasboatman]

Hey Steven,

Go to www.namba.com and find the the newsletter "Propwash". In the October 2013 edition, there is an article "Props - The Theory Part One by Lohring Miller" It is the first part of many to come I believe. CHEERS !!! Bob

 

[Steven Bryant]

Thanks guys

i'll have a look at the propwash articles

steve

 

[RonShaw]

RPM Theory- Whatever comes closest to the maximum RPM rating on the motor you are running, with the least slippage.... (My personal favorite)

Torque Theory- Whatever is the maximum diameter and pitch you can turn, and easily get "on the pipe"...........(What most people try to accomplish)

 

[Terry Keeley]

The rpm that makes you go the fastest!

 

[Tim Kish]

The rpm that makes you go the fastest!

Thats my theory. B)

 

[Joe Petro]

Answer with a question...

Why is the fastest boat electric???

Joe

 

[HTV Boats]

My best nitro .45's were in the 28000rpm range. .21's maybe 31000. FE on my data loggers show actual revs close to 40,000rpm and I know some SAW guys are spinning them higher. (50K+?) I would have thought we would have just had a cavitation issue until I actually ran them. I think part of the reason FE boats are so fast in SAW is they accelerate so much faster and you don't need as long a run to get wound up. That and a more balanced powerplant capable of spooling up with only 2 bearings for drag. Efficiency! As was stated before there is an optimum pitch and diameter applicable to each motor variety.

Mic

 

[JimZaremski]

electric motors make peak torque at stall (0 rpm) and have a linear torque curve above that.

 

[lohring]

I think there are two issues. If your engine is rpm limited. you probably need more pitch to go faster. This seems to be the case with all internal combustion engines in SAW riggers. There's a limit on how much you can cup and bend the blades of stock props. Since electric motors aren't rpm limited, efficiency, the ability to turn as much of the engines power into thrust power, becomes the issue. Now things like pitch to diameter ratios, blade areas, and blade shape start to make the difference. Guided cut and try is still the best way to sort all this out. There's a lot of this going on in the electric classes. It's the reason P spec riggers are around 10 mph faster than a few years ago. So far I don't see any clear answers.

Lohring Miller

 

[shoboat]

Steven,

There is no optimum rpm for each size of propeller we run.

I have electric customers that can go 85mph with an X-442 or

A nitro tunnel guy only going say 52mph with a X-442. So the

Propellers have a very wide range in which they can operate with

Different rpm loads. On the nitro side we cannot launch enough

Propeller off the beach because our set ups do not get into our pipe

and engine power band quick enough, so the pitch we would like to pull

We cannot launch off the beach. Now with an electric motor that is not a

Problem. Have you seen the speeds that some very high rpm 12 car motors

are pulling now? The record speeds are now well into the 90's for a 12 motor.

The power and rpm are two of the reasons that this is possible and a light boat.

So you can see engine rpm plays the biggest part to our ultimate top speeds that

we can achieve with our boats. Rpm equals speed! Now try in the off-season to find

more rpm out of your set up. Different engine timing number combinations and pipe

geometry and carb efficiencies will all play a bigger part on your set up for next year.

If you just want to go faster get an electric boat and see what rpm can do? This is just

one of the many engineering challenges that we face while having fun playing with our

Toy Boats?

Have Fun And Enjoy The Challenge,

Mark Sholund

 

[Arthur Apruzzese]

All of the replies are very accurate. Although I have only run Nitro boats, I've had some limited experience with Electric Cars, and also have flown R/C airplanes for about five years. The answer in a nutshell is both torque AND rpm. The maximum rpm one can get from two stroke nitro engines (and this is stretching it) is probably in the mid 30,000 rpm range, and only the smaller, shall we say .45 or less will turn over 30K........with the max torque occurring in the mid to upper 20K range. The brushless electric motors have RPM capabilities , shall we say "around" 60,000 RPM. That is just about double, and the torque is instantaneous! I personally worked in R&D on jet and rocket engines for many years, but the only internal combustion engine that can develop gobs of torque at relatively low RPM is the Rotary or "Wankel" engine. At my last recollection, there were only Three, yes, 3 full sized fighter aircraft capable of sustained acceleration in a 90 degree vertical climb! The good 'ol US of A has two of them. On the other hand, I would say that lots of Electric Model Aircraft have that capability, especially the aerobatic planes like I fly. I have a couple of Bi-planes with the drag of an extra wing and all, that will literally go out of sight in a vertical climb within seconds! The only way to describe the performance of Brushless Electric Motors is unbelievable! A friend of mine just purchased, a solely electric powered Sports Car, I don't recall the name of it but it is beautiful in appearance........and the "holeshot" it has is something to be remembered!