40,000 rpm........what's the problem

[ray schrauwen]

It is certainly possible to safely sport run at 40,000 rpm, but not that many do so successfully. Too many boaters (particularly newbies) seem to believe that the only way to go fast is to turn 40,000+ rpm because they read about it somewhere. Not true, I ran almost 90 mph last year in LA turning under 35,000. Going fast isn't all about high rpm, it's about putting together a system to reach your target speed. There are various ways to get there, some carry more risk than others.


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There seems to be a good number of Newbies watching youtube vids of boats turning 40,000 rpm and thinking they can do it too because a motor has a max spec of 60,00rpm. Problems arise because of things like stock props (unbalanced and or unsharpened being used or too big of props.
It's not the experienced guys I try to warn, it's the new guys.There have been a number of boats in the swap shop with interiors that show obvious signs of esc detonations from users that aren't to regular on here or it was "not their cup of tea" after a meltdown and they sell it off and get out of boats.
It's all good if you have the experience to do these things or take the right steps to prevent meltdowns but, it's an area that has a definite higher failure rate than the lower rpm range.

[Chilli]

Motors get to a certain level of RPM and load and their efficiency goes way down resulting in rapid heat build up. As mentioned above, it's all about the application. If you are doing a couple of short runs or running a SAW's event where run times are short, by all means crank up the RPM. But if you are sport running for five minutes plus, you better keep a close eye on the temperature of your gear. In these applications you may end up having to prop down so low that your prop efficiency suffers taking away any advantage of running a high rpm set up.

Like Ray mentioned, most of us want FE newbies to have a good first experience with RC Boating. So preventing them from burning stuff up is why we err on the side of conservative recommendations. BTW- Same thing goes for prop recommendations for a given set up. If someone wants a prop recommendation, the first thing I ask is "what's your application".

[kfxguy]

I take the practical view of running spinning things as low as RPM as possible to get the job done. I do it for my boats and my helicopters. I try to run the biggest prop I have in my box at the lowest RPM to get the output I want with correct handing characteristics. So far I have not needed more than 32K RPM (that's a 29" mono) to make the boat go faster given my power system and available props. Most of my boats are under 30K. Less RPM= less vibration issues (unless you *really* have a perfectly balanced driveline-- hard to achieve), less wear on strut and bearings.

If I were trying to build a boat for pure speed and ran out of prop, sure I'd think about cranking up the RPMs, but I would make that the last variable to change.

I guess I'd turn the question around and ask if there is any benefit on targeting a higher RPM than needed to achieve the speed target? If there were some benefit, then we could start looking at the vibration/wear factors more closely to realize that benefit. If there are no other options to hit the speed target than to increase RPM then we are back to my first point... now you are very likely trading reliability for speed which may be the tradeoff you want to make.

Chief

My main reason for higher rpm with a small prop vs low rpm with big prop is prop walk. I found I can go just as fast with no prop walk doing it this way.

[iridebikes247]

Driveline is very important. I personally try to run bearings in all of my drives but there are plenty of SAW guys setting records using bushings in their struts with no issues. I think its just a general guideline to stay below a certain rpm. Not everyone can perfectly tune a prop, setup a boat, etc. Less rpm, smaller chance of toasting stuff. I too prefer more rpm and smaller props because of how prop walk can really hurt how a hull handles on the water, no fun.

You can spin a lot of rpm safely but I do agree with Jay that the electronics will take an exponentially greater beating if things are setup poorly in a high rpm setup. They're great if you don't screw anything up!

[ray schrauwen]

There is a slight problem with running oval races with higher KV and smaller props. If you come off plane a smaller prop will take longer to bite and get back on plane than a larger prop so for ovals it's a balancing act.

[flraptor07]

Everybody is making great points and I've learned many other way of looking at the RPM issue already, plus no arguments yet! But we're all forgetting one thing, 40,000 + RPM Just sounds so BADASS!!!

[T.S.Davis]

The rpm doesn't automatically dictate failure. It's the beating everything else takes. The motor is the least of the worries.

Begs the question "why?"

Why opt for high rpm? Potential resonance issues, potential for amp spikes, probable heat issues until you find the right prop, flex cable whip, pick up some weeds......baked, plugged water line.....baked, the risk of throwing a blade on a prop that can't take the pounding? You only find the blade thing after you throw it. That has happened to me.

Sure you could address all those issues with careful planning and exacting detail but why bother when smarter less risky choices are available that are plenty fast? I could probably make it work but I wont on purpose. I used to run Hacker 9L on 12 sub C. Tiny props for a P Sport. I saw a 9L run on 24 sub C through a gear drive. Sounded cool until the rotor exploded. We don't do it anymore because there are easier ways to get where we want to go.

Plus, some new guy reads......."50k rpm to go really fast" and figures he'll just throw that in his Spartan. Runs it for 6 minutes and ends up having himself a nice bake sale. This is why you typically read guys not endorsing the approach and even discouraging it.

Another thing about motors......those ratings of 60k rpm are rotational ratings. That doesn't account for any of the axial thrust we push on the bearings. So when you run those kind of rpm you need thrust bearings. Yet another thing to have perfectly balanced. Is the new guy going to know to put the thrust bearing in there?

[kfxguy]

So basically what im gathering is a basic overloading of the motor is the result? What's the difference if I overload the motor at 30k rpm and burn up the esc or motor vs overloading it at 40k rpm and burning up the esc or motor? Is it going to explode into a nuclear like explosion? Lol j/j

[kfxguy]

There is a slight problem with running oval races with higher KV and smaller props. If you come off plane a smaller prop will take longer to bite and get back on plane than a larger prop so for ovals it's a balancing act.

Very good point! I like water wheelies!

[kfxguy]

Everybody is making great points and I've learned many other way of looking at the RPM issue already, plus no arguments yet! But we're all forgetting one thing, 40,000 + RPM Just sounds so BADASS!!!

Hell yea that rpm screaming sounds bad ass!

[ray schrauwen]

So do outrunners at 30,000rpm

Everybody is making great points and I've learned many other way of looking at the RPM issue already, plus no arguments yet! But we're all forgetting one thing, 40,000 + RPM Just sounds so BADASS!!!

[kfxguy]

The rpm doesn't automatically dictate failure. It's the beating everything else takes. The motor is the least of the worries.

Begs the question "why?"

Why opt for high rpm? Potential resonance issues, potential for amp spikes, probable heat issues until you find the right prop, flex cable whip, pick up some weeds......baked, plugged water line.....baked, the risk of throwing a blade on a prop that can't take the pounding? You only find the blade thing after you throw it. That has happened to me.

Sure you could address all those issues with careful planning and exacting detail but why bother when smarter less risky choices are available that are plenty fast? I could probably make it work but I wont on purpose. I used to run Hacker 9L on 12 sub C. Tiny props for a P Sport. I saw a 9L run on 24 sub C through a gear drive. Sounded cool until the rotor exploded. We don't do it anymore because there are easier ways to get where we want to go.

Plus, some new guy reads......."50k rpm to go really fast" and figures he'll just throw that in his Spartan. Runs it for 6 minutes and ends up having himself a nice bake sale. This is why you typically read guys not endorsing the approach and even discouraging it.

Another thing about motors......those ratings of 60k rpm are rotational ratings. That doesn't account for any of the axial thrust we push on the bearings. So when you run those kind of rpm you need thrust bearings. Yet another thing to have perfectly balanced. Is the new guy going to know to put the thrust bearing in there?

You make some good points friend. Your right, you can setup to do the same at lower rpm But I've gotten some mean prop walk with bigger props. I like my boats to go straight.....lol

[T.S.Davis]

I like the sound of high pole motors when you pull away from the shore.

When a speedo fails.....results vary. I've had them blow the deck off boats. I've had slow burns with a little puff of smoke and it's over. I've had them burn their way through the deck. The worse case scenario is the battery leads come off the boards and dead short the LiPo. That is truly nasty. That one I've been lucky enough to avoid thus far. Seen it though. yyyuck.

You can certainly overload a setup at 30k. The potential isn't as high though. There's also less risk of all those other annoying things I mentioned. Back in the brushed days it was always the armature that gave up the ghost for me. The endbells would melt right off the motors. I knew I had pushed too hard when I found plates from the armature smoldering in the bottom of the boat. "Toss another motor on the bar'bee."

One I didn't mention on high rpm setups, everything comes loose. You have to constantly check nuts and bolts. Motor screws to trim tabs. Better to Loctite everything. Still have to check them though. Manageable of course. Just an annoying PIA. Maybe for SAW where you run down and back and then check it all over anyway.

[T.S.Davis]

You have to find that fine line for which prop a boat likes. Each is a little different.

I have a P mono that I race that has a bit too high of RPM for my liking. All those RPM will make it swap ends on me coming out of a turn. Fix was a bigger turn fin on that side. Raised the strut a bit to keep more boat in the water too. Sometimes it's not just the big prop that makes the boat get all weird. Sometimes it's us.

[don ferrette]

You make some good points friend. Your right, you can setup to do the same at lower rpm But I've gotten some mean prop walk with bigger props. I like my boats to go straight.....lol

I can't even begin to think of how many "prop walk" issues I've seen over the years that were from poorly prepped props and nothing else. Just because it is "balanced" when you spin it and has sharp edges doesn't mean it's right. And that pales in comparison to poorly set up boats where the owner was blaming the prop for it's inability to track straight.