Prop rpm question

[Gary]

This may be an odd ball question but what is a average RPM loss at the prop as compared to a free rev. In other words say a hydro will spin at 50,000 free but under load at full speed it spins at 42,000 so the motor is dropping 8,000 rpm when running flat out. Is there a basic formula for this to know how much you should be loosing? Again could be way off with my thinking.

[properchopper]

Gary, good question; I'd too like to hear an answer from someone wiser than myself but just wanted to chime in let you know that I'm still in your corner. Formula should include Voltage drop under load as it interrelates to KV, which is a function of parasitic driveline drag, prop pitch/slippage which is a function of hull/water resistance. Jay ?

[Gary]

I also forgot to add this as well. I will see and average of a 3 volt drop/draw on the run as well. How much of a voltage drop is acceptable. I know if i could gain that voltage back with less drag somewhere {prop} the boat would be much faster with 7,000 rpm more.

[Gary]

One more thing. I forgot to figure in that under load the voltage is 18.5 so thats 18.5 multiplied by 2,500kv and thats the rpm at the prop. So there is a 2.5 volt loss "at the prop" so to speak and it started at 21.0 volts to begin.

[Fluid]

Prop speed depends on the motor characteristics, Kv, battery quality, terminal voltage, prop load, boat drag, even water conditions.

With Neu motors in oval boats, I like to see the average rpm to be 90% of the calculated rpm - 3.7 x cells x Kv. For SAW applications 80-85% is a target. This is adjusted by prop and boat trim changes.

With 2-pole motors like Hackers and Lehners I like the average rpm to be closer to 100% of the calculated value. The listed Kv for these motors is apparently taken at ~50 amps.

It is normal for any battery to lose voltage under load. The actual voltage lost depends on the cell quality, C rating, and total mAh capacity. I often see losses at the terminals of 0.5 volt per cell or higher with 30C cells. That means a 6S pack drops from 25.2 volts at full charge to below 22 volts under load.



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[properchopper]

Gary, here's two EagleTree pulls I did to compare a Turnigy to a Hyperion [ FE 30/Ul-1 motor] - just to show you some voltage drop samples & confirm what you're discussing.

[Gary]

I always run 45-90c thunder powers but came across some 50-100c Venom batteries so i will try those. I feel that i have found my perfect adjustments for my Phil Thomas 45 so i dont want to change a thing..."fear" LOL . If i can get back that rpm she should blow past her 88 mph max its stuck at. Jay .....Thats about where i am at 80% so i guess im on the right track. Thats amazing about reaching 100% at the prop. I cant see how thats done!!! That takes some big time work and time to get there !! Tony......... thats interesting with the chart . I never did that with my EagalTree. I will check that out.

[Shooter]

Interesting subject guys. So, if you are running 100%, your batts are not the limiting factor of the system, right? If you are running 80%, your batts are really working hard, and in essence cannot "keep up"??

You would think that the highest efficiency would be right at the cusp of 100% (any more and she would start running 99% and the batts would start to work too hard and create heat).

For SAW however, you probably want to run the most you can (not necessarily the most efficient, but the most ooomph you can get without trouble). It sounds like Jay has figured out optimum %'s for each set-up. Good stuff!

[Fluid]

Thats amazing about reaching 100% at the prop. I cant see how thats done!!! That takes some big time work and time to get there !!

Realize that the 100% rpm value has nothing to do with efficiency, it is just compared to the listed Kv and 3.7 volts per cell. If your packs have a higher or lower voltage drop, that will effect the rpm. And running too small a prop can give you over 100% but slow speeds.

And that "closer to 100%" value is for quality 2-pole motors because they (apparently) use a different amperage draw when the Kv is calculated. A 4- or 6-pole motor isn't making very much power at 100%. The lower the prop rpm (all else equal) the higher the amp draw and (usually) the more power delivered.


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