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

[Doby]

I would think that the best way to eliminate prop walk is to adjust the boats setup, not install a higher RPM motor.

[NativePaul]

This is a carbon wrapped rotor from a Neu 1415/1.5d 2700kv that came apart on 4s (circa 45,000rpm unloaded), it's last season was all on the same prop so I know it was not overheated, all I can attribute it to is the RPM. Do not attribute it to its obviously rusty condition, it lives in my tool box and occasionally gets used to dredge for lost flex shafts with a length of string tied to it, it looked OK when it came out (apart from the fact that 2 of its magnets had broken and been thrown).

On the other hand I bought it cheaply second hand from an unknown person on the net, so I have no idea of its history and I did not take it apart to check its condition on arrival (it gave me 2 seasons and for the price I paid I can not blame the seller, nor regret the purchase)

Having said that all my race boats and sport boats still run around 40,000 unloaded and that wont change any time soon, I replaced the motor above with a Leopard 3674/1700kv on 6s (circa 43,000 unloaded), 40K is not unusual amongst racers here at all either, I only race with one guy that keeps it under 30k and the radar gun says he is an average of 12mph down at the end of the straights (I'm not saying it is the low RPM slowing him down, there are a lot of things that can slow a boat down so it could be a coincidence ) while 50k is fairly unusual here we have had 2 outstanding rigger guys in the UK in recent years and both with very light boats, both were spinning around 50k unloaded and both had motors that were just lukewarm after a 5 minute race.

A word of warning, with high RPMs the tolerance to overpropping drops massively, at 25k, going up a prop size may mean just a few degrees more heat, whereas at 50k going up a single prop size can be the difference between a motor that runs a race and comes out warm and one that blows or melts its ESC after 30 seconds, the 50Kers have several props of the same purchased size cut to slightly different sizes, shapes and bends to optimise in a much finer way than would be possible over the counter.

I can dispel some myths too:
40k wont unscrew your bolts unless your doing something wrong, I do my props as best I can, use good couplers and Nylock nuts on hardware, as I think everyone should whatever the RPM (and as I did with my first "FE" boat which you would laugh at now and was probably under 15K rpm) but I don't locktite anything apart from my wires in my stubs and motor screws, I have never tried to balance a coupler, stub or drive dog and I cant remember ever having a bolt on my hardware coming undone.

Small props do have to mean spending an age to get up on the plane and sluggish acceleration, few of my boats can take a full throttle start without ending up on its back awaiting rescue, even some boats that reputedly need a big prop to push, for example my Sprintcat .40/.80 with its Neu 1521/2.5D 1200KV on 8s (circa 40,000 unloaded) needed a detongued X438mm prop to get the 5 minute race time needed on the 5000mAh cells that were at the 1140g weight limit when I bought it, and it still needed partial throttle starts to avoid flipping. (I say needed as technology has marched on in the last 5 years so I can now get 8S 6600mAh under 1140g and it runs a detongued X440 now, it is faster and I have to use even less throttle for the hole shot).

40K does not have to mean heat, I have a Mega 16/15/3 3000 on 3s (circa 40k unloaded) in a JAE Minisprint, and after a 5 minute race it comes out colder than it was when I put it in.

Even a tiny prop can cause prop walk if it is blunt so everyone should sharpen and thin (not just bevel) their props, but it will still walk less than a blunt big prop, small props produce less torque roll too which means little to the cats and riggers I mostly run but can be a big deal on a mono (especially a stepped one with its very narrow contact patch).

It is a combination of RPM and diameter that breaks props, I have folded several Octura props and thrown 2 blades when propping up for SAWs, but have never folded or thrown a blade while racing or fun running despite spending the VAST majority of my time doing that. Graupner carbon and CNC alloy props are more popular here despite being weaker than BeCu and most of us running around 40k.


I suspect the biggest advantage of high RPM is that for the same power there is less torque so if you run a flex it will wind up less, and whatever your shaft is made from you can use a smaller diameter which is lighter and will have less drag if it meets the water. (I suspect drive drag itself is 6 of one half a dozen of the other, as the higher RPM gives higher surface speeds but so does the fatter diameter of the low RPM setup)


Finally I have a conundrum that maybe one of you can help me with, I have been thinking about it on and of since it happened about 18 months ago and still have not come to a satisfactory conclusion. I tried a Mega 22/3/2 1780kv motor in my JAE.21FE on 6S, with a x438 (smallest Octura 3/16" prop) detongued and cut down to 35mm I couldn't quite get my 5 minutes race time so I swapped drives down to 1/8" and fitted a detongued X435 with the same area and diameter as the cut X438 and it would not get on the plane(while slower as expected it ran fine with a shove), I did up an X435 leaving the tongue on and still no dice, went back to the x438 cutting it further to 33mm and reducing its area more. The X433 ended up having no tongue, less area, less diameter, more pitch and more drag (from the fatter stub and strut), all of which my mind says should hurt acceleration especially at low speed compared to a stock B/S/T X435, but the acceleration was in fact brutal at all speeds and it definitely still needed partial throttle starts.

[properchopper]

What appears to be correctly emerging here is the need to first determine the application and separate the discussion into segments within applications . For the apparent mandatory initiation ritual for those just starting out, the "let's see how fast I can make this thing go regardless of durability and control" guys, high rpm setups seem like the easy way out. I'll leave that hang there and discuss another application : oval racing.

As an avid oval racer, I need to do a mill + 6 laps on a 1/6 mile course plus have the ability to do another penalty lap if earned without over-stressing the components and over-drawing the batteries {which in P-Mono is limited to 10Kmah on board}. Douggie Twaits once said that races are won in the turns. My racing partner Mark F and I have two identical P-Monos except I run an 1890KV motor and a big 3-blade wheel; He runs a 2300KV motor with a smaller 2-blade. He's maybe 1-2 mph faster than me in the straights but when we exit a turn together I dig in and blast out of the turn way ahead of him (and hope he doesn't catch up to me at the next turn entry ). That's where lower rpm with a big prop with lots of instant thrust seem the preferred choice for me and many oval racers as well.

While I have many SAW Time Trial records, I'll leave the "optimum rpm" for all-out top speed application discussion to those better suited to comment. Carry on

[Luck as a Constant]

[jkr]

Nice thread!

Well 40k is not a problem but you need to be more careful on what you do with it and how you do it.
My boats run <35k and i also tried 40k+ only for short runs.

[kfxguy]

Native paul, very good info...that's for taking the time to write that. That's my purpose of this thread, I want real world information....not just "you shouldn't do that because everyone else is saying it" mentality.

[Fluid]

Unloaded rpm is often pretty meaningless. Example - a Neu 1527/1D has a Kv of 2300, so the theoretical "unloaded rpm" on 6S is over 51,000 rpm. The motor will not see this rpm in use. Pulling 230 amps - near this motor's best efficiency - puts the shaft speed under 42,000 rpm - 10,000 rpm less than the "unloaded rpm". Other motors may be closer or further from the theoretical "unloaded rpm". Most high rpm fans are actually running lower rpm than they believe. But regardless, the margin of error is still small and that is the reason for conservative recommendations.

I love comments like "the motor was cooler after the run than it was before the run". This is of course impossible on the face of it - trust me, no motor is over 100 percent efficient, 10 to 20 percent of the energy running through it is turned into heat. Physics does work. The only ways this could happen are:

- the cooling water is very cold, well below ambient temperature
- the motor was artificially hot to begin with, or
- the motor temperature was not correctly taken (bad temp gun, aimed wrong, after the motor had cooled, etc.)

This kind of comment is misleading on the face of it and confuses beginners. They will not see this happen running higher amp set ups in hot Summer water.


.

[NativePaul]

KFX guy, I am wondering if when you made the thread were you referring to loaded or unloaded RPM?

Jay, I know loaded RPM are normally lower than unloaded hence why I stipulated that they where unloaded. I see so many different RPMs on my logs throughout the run, even when only looking at full throttle periods that the only RPM I feel safe quoting is the theoretical unloaded RPM.

this is a graph of my JAE.21FE with leopard 3674 1700 on 6s showing just throttle and RPM, you can see there is circa an 7,000RPM difference between full throttle accelerating out of a corner and full throttle top speed, there are a great many spikes above 40Krpm and rather surprisingly 3 above 44Krpm given that 4.2Vx6Sx1700KV=42,840 Theoretical Unloaded RPM, average for the run was circa 30Krpm including the partial throttle corners.

I am not suggesting that my motor is over 100% efficient (Mega only claim 85%, so not quite up to Neu, Lehner or Plettenberg standards), but in England where I live lake temperatures are indeed lower than ambient temperatures during the race season (The situation reverses at night times, but I don't run at night). I know Texas is a lot hotter than England, is that not the case there? For sure the temperature was not taken correctly, I don't own a temp gun, but I can tell if something is significantly colder than A: my self, and B:everything else I am touching, which that motor is after a race. FYI in case you didn't catch it from my post above, the racing I do is for 5 minutes duration not sprints, I test to see if I am pushing too high an amp draw not with a heat gun but with a stopwatch.

Jay, I am not sure what you mean by "misleading on the face of it" it is not my intention to mislead, and everything that I have said is true. Indeed I have used higher amp setups in my minisprint and my 16/15/3 does not cool down during a run if with a higher amp setup than my race one, when I prop up to an X632 for example for some hairy fun it heats up, but then it won't get the 5 minutes I mentioned either, so it has no relevance to what I said.

[kfxguy]

Paul, I was just referring to rpm in general. Let me clarify. I see people post all the time that going over 40k rpm is bad and you should keep it under that. Most setups won't see the unloaded rpm anyway and like stated above may not even be close.

[srislash]

Not sure if I should chime in here or not but... The loaded RPM can be quite a bit lower than theoretical as Jay stated, I think we had an average of 7000 RPM lower on mild setups aiming for the 40,000 mark. I will be putting this to a test on my Fantasm with both Electrical RPM measuring as well as 'at the hub' with my Eagle Tree this year.

Boy, good thread here Travis, already on page 2

[gsbuickman]

Thanks for the thread, a lot of good info here

I guess I fall into the old school category with proper chopper. As a general rule of thumb I keep my boats between 37k & 40k theoretical. I really like saw runs; I've only tried to spool up 45k two or three times, but w/o namba in ID. or anyone to run against locally, there's no need for it. I guess that's why I haven't played with wire drives yet, I can't put it to good use, so no sense in changing from the pro-boat 3/16 I like.

It wasn't in a boat, but I dropped a castle sidewinder v3 5700kv, in a 1/10 scale Traxxas tc3 sprint car setup on a 45c 6800 mah 3s2p. I wound that thing up for everything it was worth on a nice long straight away a few times. It was screaming like a banshee & I was waiting for it to grenade, but she never did.

[bracka69]

Love this thread. Me and a few of my buddies run our boats hard. Most of our boats run over 50,000rpms. Have had a motor go yet only just a few cheap esc's and that's it. For example I have a CF Genesis that has a Neu 1521 1.5D 1900kv motor and I run that on 8s. Also a Fighter Cat Lynx with a Neu 1515 .5Y 4400kv on 3s and also a Miss Bud Hydro with a Neu 1451 1D 4100kv on 3s and I've had no issues at all. Even thinking of stepping up the two smaller motors to 4s just to see what happens. Probably not a smart idea but just curious to see how well they hold up to the high rpms.

[Brushless55]

Cool info in this here thread!

[T.S.Davis]

I can dispel some myths too:
40k wont unscrew your bolts unless your doing something wrong

Guess it didn't happen. Someday I'll figure out how this FE stuff works.

[urbs00007]

Well mate I run a twin rigger with 2 Neu 1527 .5y on 6s. This is a saw setup only for me but the main issues I have experienced are parts being out of balance. You can get away with pretty good balance at 30k at 47k it is so so critical. I think this out of balance helps motors let go with the excessive vibration. Running high rpm is fine if you pay a lot of attention to balance of the whole driveline. Most don't so 30k is a more reliable setup. Just my thoughts anyway.

any videos of that twin? whats the speed and set-up? weight? have 2 neu 1D sitting around. thanks

[larryrose11]

I have seem pics of LMT motors, and they are a completely different build.
Instead of manets bound on the surface of the rotor, held in place with some sort of wrap like CF, Kevlar and the like, the magnets are discs. The have a good track record at high RPM
http://www.kbbforums.com/viewtopic.p...6c878146ccec6e

[ls1fst98]

very interesting read. like the fact everyone is discussing for the most part and not going overboard. I would think at any rpm, balance is the primary issue, and at higher than what FE "normally" runs this is even more so (hence check and double check bolts to insure they are snug as any vibration will loosen a botl that is not tight to begin with regardless of rpm) . i would imagine it also depends on what motor you are running, as i would imagine (if already stated i apologize) a given motor has a point where more rpm just means more heat and not more power or speed. just like in my real life experiences with diesel engines, there is a sweet spot in the rpm/efficiency/power range. i certainly will not claim to know as much as anyone here, as i have no race experience. so this is just my opinion.

[kfxguy]

Not sure if I should chime in here or not but... The loaded RPM can be quite a bit lower than theoretical as Jay stated, I think we had an average of 7000 RPM lower on mild setups aiming for the 40,000 mark. I will be putting this to a test on my Fantasm with both Electrical RPM measuring as well as 'at the hub' with my Eagle Tree this year.

Boy, good thread here Travis, already on page 2

Thanks. I'm interested in seeing this.
I'm glad no one is arguing here and this thread has turned into something with a lot of meaningful and good information. Keep it going guys!

[T.S.Davis]

"5 minute race time needed on the 5000mAh"

That's 48 amps average if you draw to 20%. About 1300 watts out.....ish. Seems light for an 8s boat. What kind of speeds do you get with so few watts out on an 8s boat?

My 4s 4 minute boat averages about 110 amps or 1540 watts.....ish. I'm still working on my 8s offshore setup but I'm figuring for about 3000watts output to push that hog.

[jkr]

Also more appropriate would be ''where and when is better to use 40k rpms''.

A mono like the pursuit can run 4s 2560kv m445 62mph+ but if i want to go faster i can either prop up or run 5s.
My stepped 29 inch mono runs 4s 2200kv 40mm prop 52mph ~32k loaded but i can't prop up cause it is narrow and it is better to spin faster.

40k loaded rpms aren't necessary in most of the cases but i can't imagine a rigger to spin lower than 40k.

Ps.I like outrunners for <35k cause the sound is awesome.

[properchopper]

One important variable not to be overlooked is the means in which ones desired terminal rpm is not only achieved but maintained. Whatever the desired rpm is, the batteries need to sustain their voltage number to achieve this and Voltage DROP Under Load (VDUL) is worthy of consideration. For example (using round numbers to make the math easier to understand) if you have a 2000KV motor running 4S packs that start out at 16V nominal and under load they drop one volt (not unusual and mostly more) you lose 2K rpm right then. I've graphed the VDUL of lots of different brands of batteries over the years and found that the better packs sustain voltage way better than less expensive ones. Whatever your target rpm is, use the best batteries you can to get (and stay) there.

[iridebikes247]

I'm with properchopper on this, good batteries sustain voltage way better than cheap ones and this certainly benefits all of us when going for high speed high rpm. This is a sick thread but has anyone made the distinction between single and twin high rpm setups?

A lot of guys going fast with twin boats will run mega-rpm motors and they will last...for some time hopefully but I for one have NEVER built a super fast single motor boat. A twin cat or mono for example will accelerate much faster and have less prop slip than an equally fast single-setup but singles are a different animal completely when it comes to spikes, prop slip, etc.

Just thought I would throw this one out there as there is surely a difference between high rpm setups in singles vs twins when it comes to any and all electronic components. Also as Fluid said things change quite a bit depending on water temperature. In NY we run boats year round over the summer it can be 95 degrees and in the winter 25 degrees with the water a few clicks away from freezing. Motor and esc temperature change drastically so what works in March for 5 minutes of running may come in HOT in August.

[NativePaul]

The amp draw of that cat would be a little higher than 48 as I would have been leaving less than 20% left in the cells (I know it is not good for them) I cant remember exactly how much as it was years ago that I ran 5000mAh, the 6600mAh I use now have about 7% left after a run for what its worth. The boat is very much an oval boat not a SAW boat and as such I have never run the 110 to find out how fast it is, if I had to guess I would say it was about 50mph on 5000s and about 55 now on 6600.

[ray schrauwen]

Time for me to get a Data logger esc...

[tlandauer]

Time for me to get an electrical engineering tutor.

[flraptor07]

Time for me to get an electrical engineering tutor.

I feel your pain.......

[T.S.Davis]

I wonder if on a twin cat setup where the props are behind nice sharp transoms you get more predictable water. Think nice smooth sheet of water leaving the bottom of the boat. So your prop always has a blade in the water. Less slap slap slappy effect on squirrely water in the tunnel. So fewer spikes? Just a thought.

I SO wish I was an electrical engineer. My old man tried to teach me. It was hopeless. At one point he was a thermographer for Amway Corp. Worked in a ginORmous plant here in MI. They used thermal imaging to predict motor failures. They had many thousands to document. So cool. Way retired now.

[jkr]

Measuring tools always helps to understand how a setup works

[olwarbirds]

Travis, tks for starting this...just what you and I had been discussing.... I understand needing to keep rpm and amp draw at a limit that allows the completion of a heat race and having mah left in lipos at a safe level, but thats heat racing. All my SAW boats are running 45,000 and up. I've been told that this is "disturbing, wrong, misleading" ETC...I like the OP had got tired enough of it , that I had stopped posting and abandoned my threads.

I dont think that a first boat should be a screamer and safety is always a factor. With the amount of knowledge available on this forum, I have seen it used by newbs to build some very nicely done fast boats. Instead of discouraging this trend of higher rpm setups, why not help in getting it done the right way ! This trend is not going away, the rpm's are going to go higher.

We need to explain the differences sport, heat and saw. My sport setups, I like to run on the edge of equipment and hull limitations. This helps me in setting up my heat boats, is this wrong ? works for me ! Yes I have burnt up a few inferior esc's and most of those were my fault. No burnt motors , we use Neu/Leo/TP . We been running these 2000kv and up motors on 6S some for over 2 years now. So we got our monies worth if they blowup now !

So please like OP pointed show us these failures caused by running 40k and up. All I have seen so far is 1 and alot of posting from " FE engineers" with some good points and info....old dog who is not staying on the porch

[kfxguy]

Travis, tks for starting this...just what you and I had been discussing.... I understand needing to keep rpm and amp draw at a limit that allows the completion of a heat race and having mah left in lipos at a safe level, but thats heat racing. All my SAW boats are running 45,000 and up. I've been told that this is "disturbing, wrong, misleading" ETC...I like the OP had got tired enough of it , that I had stopped posting and abandoned my threads.

I dont think that a first boat should be a screamer and safety is always a factor. With the amount of knowledge available on this forum, I have seen it used by newbs to build some very nicely done fast boats. Instead of discouraging this trend of higher rpm setups, why not help in getting it done the right way ! This trend is not going away, the rpm's are going to go higher.

We need to explain the differences sport, heat and saw. My sport setups, I like to run on the edge of equipment and hull limitations. This helps me in setting up my heat boats, is this wrong ? works for me ! Yes I have burnt up a few inferior esc's and most of those were my fault. No burnt motors , we use Neu/Leo/TP . We been running these 2000kv and up motors on 6S some for over 2 years now. So we got our monies worth if they blowup now !

So please like OP pointed show us these failures caused by running 40k and up. All I have seen so far is 1 and alot of posting from " FE engineers" with some good points and info....old dog who is not staying on the porch

Right on friend! No matter what rpm your running if it ain't setup right, it can/will fail. I myself feel like a rpm isn't a one and only cause of failure. I have been pushing my setups hard and everything seems to be Ok. In my video of my mini rivercat I was running a 2650kv 1512 castle motor and a water cooled mamba mister 2 esc on 4s. That 44,520 rpm (unloaded with full pack and not counting voltage drop...so more like 40k rpm) and noting even got warm. My son was run inning it back and forth repeatedly. I know that was only one running session, but hey...it was successful! I have been running 6s in my rivercat on a 2200kv 1515 castle. That's 55,440 rpm. (Same stipulations as above). It has a x642 prop. I think my failure will come from too many amps if I use too much prop.