Brushless motor testing (for scale boats, not FE) - RC Groups
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Oct 06, 2009, 04:09 AM
NeverAgainVolunteerYourse lf
nick_75au's Avatar

Brushless motor testing (for scale boats, not FE)

Going to do some brushless motor tests, aiming towards the scale end of the spectrum.

I built a 4 foot by 8 inch barge and I decided it was too narrow for my Springer's, made a prop tube and glued it in using a 4 mm thread for the shaft.

I have just purchased this motor

Required Voltage: 11.1v
Suggested Battery Capacity: 1300mAh +
Suggested Prop: 10x4.7, 11x3.8 or 11x4.7
Max current draw: 13A (with 11x4.7 prop)
Max thrust: 900g+
No Load Current: 1.2A
Shaft: 4M (perfectly suited to Towerpor/GWS SF Slow Fly props)
Weight: 78g
Diameter: 41mm
Overall length: 77mm
Kv: 700rpm/v

Initial test results on 6 volts which is more appropriate for scale
No load (spinning bare shaft)
on 6 volts 4200 RPM
40 mm Raboesch 3 blade prop AKA springer prop
3700 RPM
Amps 3.3 max, about 3 average
18 watts

More to follow, I have a 55mm prop 4 blade as well, same brand, initial testing shows about 5-6 amps on 6 volts and mega thrust but its dark now
Last edited by nick_75au; Apr 23, 2011 at 05:49 PM. Reason: Changed Title slightly
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Oct 06, 2009, 11:44 AM
3 Blades to the Wind
Shaun Hendricks's Avatar
You're underpropped for that motor so it's just idling along at 3.3 amps max. That 55mm prop should pull closer to it's 13amp max. I'm curious what it will actually pull.

I want to do some tests with my 380 outrunner to see if there's a consistent % of motor diameter to prop diameter ratio across outrunners.

I gotta build that test tank!
Oct 06, 2009, 05:17 PM
NeverAgainVolunteerYourse lf
nick_75au's Avatar
I know its under propped, but interesting to note my 22-20 motor draws similar current with the same size prop, I have to pull it out and do some proper measurements on the test rig, Im interested in whether the smaller diameter motor drops much revs when under load.
This motor is definitely a good one for the scale boats.

Ok its a 55mm 4 blade prop, on 6volts
2200 rpm
10 amp peak about 8 amps average
about 50 watts average,
very high thrust though, probably 2-3 times that of the 40 mm 3 blade,I need to set up some form of thrust measurement.

This taxed my 4 amp gel cell and voltage fell off fairly quickly, NiMH or large capacity gel would be more appropriate for this current draw.
still under the motors stated limits
Oct 06, 2009, 06:21 PM
NeverAgainVolunteerYourse lf
nick_75au's Avatar

A brushed motor for comparison

One of my springers has a brushed generic 540 motor, its probably close to a 55 turn crawler motor, Im not sure, its from Jaycar electronics, local version of Radioshack, it was designed for 12000 rpm at 12 volts so 6000 rpm at 6 volts, just about right for a Springer.

standard 40 mm Raboesch 3 blade prop
no load RPM 6500 rpm turning the prop shaft
0.9 amps no load
loaded 4500 rpm
3.7 amps
about 22 watts

Oct 07, 2009, 12:32 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
Okay, so the 55mm pulls 10A max.

I'm guessing somewhere in the neighborhood of a 58-59mm prop would pull the max 13A and still be operational on the motor. If you assume the lower number for a safety purpose, that would be about a 142% in diameter increase on a single stator, outrunner motor like this. Not bad at all!

*IF* the percentage remains true for all single stator outrunners in a similar KV class (at least as a ballpark number) then my little 380 at 27mm in diameter would top out at a 38mm prop.

Hmmm, with some more refining and testing we might be able to come up with formulas for driving prop and motor combinations that we can stuff in a sticky and finally answer all of those 'power train' questions we always have.

I'm no math wiz, so this is really rough and probably wrong (someone who is a math wiz/ engineer please fix!) but it might look something like this:

D = Motor Diameter
K = 1000 divided by RPM's per volt of motor (targets 1,000 kv as 100% ratio. Anything higher works as a reduction of ratio, anything lower, an increase)
E = Motor Efficiency % (Brushed vs. Brushless will in come here, also higher voltage drop of torque)
S = Stator Ratio (Inrunner will likely always be 1, Outrunner will be boost ratio of stators, ie: >1 but more stators are not mulitplyable by stator count, only a rough increase of torque)
A = Imperically Tested adjustment factor. This number controls the final ratio. It may be determined to be a constant among all motors or a variable depending on motor type/size/class. It's the 'catch all' number that corrects for practical testing. In a perfect world, it would be 1, in reality, it will likely be less than 1, possibly a lot less - maybe more?

Prop Size Max = D(E(K+S)/A)

I know I'm probably forgetting things or may have stuff out of order. So, if you see something wrong with it, fix it! Again, this is only rough and would need to be proven out with testing. I'd love to get a better rule of thumb for prop size than we currently have. I don't care if it's right, I'd just like to see a start!
Oct 07, 2009, 12:50 PM
Registered User
I've been told that, you want your motor to run at approx 70-75% of the unloaded speed(or in this case about 2900rpm). hth
Oct 07, 2009, 05:04 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
That would be "cruise" speed (I think), but you likely design for WOT (Wide Open Throttle). You'll get yourself into trouble if you design for 70-75% (ie, a more powerful motor) and you accidentally 'ask' for 100%. Systems (like ESC's and drivetrains) will start to break on you.

Fast Electrics are all about WOT. Scale usually has massive power headroom and designed with overkill. Most scale boats will go 2x, 3x, 4x the 'scale speed' of their 1:1 boat.

In the case of Springers, they aren't really 'scale' so they are generally designed for WOT levels of power, even if it turns them into submarines...
Oct 08, 2009, 01:03 AM
NeverAgainVolunteerYourse lf
nick_75au's Avatar
KA22-20 motor 40 mm Raboesch 3 blade prop

KV: 1020 rpm/volt
Current rating: 26 amps continuous
Weight: 86 grams
Battery: 3 cell lipo
Shaft Size: 3.0mm dia, Length-12mm
Dimensions: 28mm dia,Length39mm

6 volts
5000 RPM
6.2 amps but pulled 4.5 Ah gel to 5.7 volts

3s lipo 12.2 volts under load
9900 rpm
22 amps


I have run this motor in my Springer tug for a couple of years now, measured lower amps in the past but with a clamp multimeter(accuracy?), and just transferred it to my crash boat hull. I ran a 35 mm carbon 2 blade prop and got 12 amps draw on 3s lipo on the watt meter but no rpm figure

I have put a 75mm prop on this motor in the past, see

I have no Idea on amps and the ESC could not drive the motor at low power so I guess that out runners are fairly tolerant of mismatches, this was a little bit ridiculous though
Last edited by nick_75au; Oct 08, 2009 at 06:42 AM. Reason: Added some anecdotal results
Oct 08, 2009, 07:03 AM
NeverAgainVolunteerYourse lf
nick_75au's Avatar
I understand that for brushed approximately 75% of no load RPM under full load is most efficient, around 60% gives best power (watts) I'm not sure if brushless is the same.

I need to figure out how to measure thrust as well, not sure how to translate this into a useful piece of data. I understand for planes a thrust equal to weight give exciting performance. How this works for boats is anyone's guess.

My thought was to rig a set of kitchen scales on the ground with a weight see diagram
any thoughts

Oct 08, 2009, 12:24 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
I started with designs similar to that one but quickly found out it would generate flawed results, similar to Mythbusters style flaws. It would put you in the ball park but would be in error.

For small test tanks, it's almost impossible to get semi-accurate results on surface drives. Sub-surface is difficult but not impossible.

To counter the issues that 'rectangular' tanks create, I designed this tank to allow both static and 'fluid' tests. It still has issues but it's the best I could come up with and still keep it to a reasonable bench-top design. The motor would be above the pulley and be belt drive to run the prop. The water level could be lowered to mimick a surface drive and have a good 'backsplash' to deal with the rooster tail! Subsurface, just raise the water level.
Oct 08, 2009, 05:15 PM
Registered User
I measured the thrust on my big tug by driving a stake into the edge of the lake and hooking the tug to it by means of a digital fish weighing scale. Hit the throttle and watch the scale. Lot easier than building something. Course being a lazy ol' southern boy it just comes natchal'.
Oct 08, 2009, 07:03 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
Looking more for a rig to test a lot of motor and prop combinations. Doing that with an actual boat would be harsh realm on the poor boat!

Plus, a tank lets you use a stable power supply that doesn't fade (like batteries).
Oct 08, 2009, 11:03 PM
NeverAgainVolunteerYourse lf
nick_75au's Avatar
I already had the tank built a few months ago, its about 10 foot long and 4 foot wide and at bench top hight. I could add the curved ends fairly simply with baffles, so far it hasn't been necessary, I think?

A stable power supply is handy but it pays to test with the intended batteries as well, as in my tests with the 22-20 motor I never thought the batteries sagged in voltage(springer uses 2 in parallell but test was with one only), If I had used a power supply I would never have seen that

Oct 09, 2009, 07:21 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
Yeah, testing with batteries is good for drive train tests, using the components you intend to use.

When doing performance testing (props, motors, etc.) you probably want a very stable supply so your tests can be run with consistent, repeatable results.

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