Propeller sizes for Brushless motors

[Tim CooperParticipant @timcooper90034]

Is there a general rule for prop sizes for brushless motors.

I know for brushed motors we tend to keep the prop and the can diameter about the same. ie 28mm can, 30 mm prop.

The only brushless motor I have used so far is in my Eezebilt PT Boat, that is a Leopard 2826/17 and I used a 30mm dia 3 blade prop.

Tim

Edited By Tim Cooper on 19/06/2018 17:00:46

[Tim CooperParticipant @timcooper90034]

[harry smith 1Participant @harrysmith1]

Hi Tim

About he same for brushless motors depending on the power and rpm of the motor.

For the more powerful higher kv motors generally 2 blade props.

Harry Smith

[ashley needhamParticipant @ashleyneedham69188]

Tim. Can sizes do not mean so much with brushless. However the Kv rating is the clue, a lower Kv means more torque and thus a bigger prop can be swung.

The power of the motor also needs to be considered, in relation to how much you need, and then consider how fast the prop needs to go (|Kv and battery combination)

I have a 28mm motor of 1210 Kv and 80Watts turning a 45mm prop no probs.

Ashley

[Tim CooperParticipant @timcooper90034]

Thanks Harry and Ashley for the replies.

Tim

[Charles OatesParticipant @charlesoates31738]

I'm just experimenting with props on my asrl. The motor is a 28 mm 1400 kV out runner. I've tried a 30 mm and 35 mm 2 blade and a 40 mm 3 blade. The boat runs much better with the 40 mm 3 blade than any of the others, and the run time is good. Nothing gets hot, just slightly warm. Speed looks just right for scale looking performance.

I hope that helps.

Chas

[ashley needhamParticipant @ashleyneedham69188]

A wattmeter is what's required, to see if the motor is being overloaded. It is the final arbiter.

It is important to bear in mind the power of these things. The Farman had its 28mm motor changed and the Kv were not too dissimilar, but one was more than twice the power of the other.

You need to know three things when looking at other peoples usage….size, Kv and power rating.

Ashley

[Charles OatesParticipant @charlesoates31738]

That's a very good point Ashley, I measured mine at 8.5 amps being held in the water, battery is 9.6 volts NiMH. … About 80 watts. When running normally that's probably about 75 watts.

Learning the numbers and testing things is a big help.

Chas

[ashley needhamParticipant @ashleyneedham69188]

Chas. Looking at the motor specs is the thing. Unfortunately most only come with suggested air prop sizes, however I think a gauge on your requirement can be obtained by knowing what sort of prop size you need on the boat and the likely revs this would do.

Power obviously needs to be "adequate", and these things are deceptively powerful. Your 80w (measured, so possibly rated in watts a bit either side) little brushless is probably better than a normal revvy speed 600 (I know there are loads of types), just for a rough comparison.

I would think a 28mm brushless of one Kv and power or another would suit any application instead of a speed 400 or 540 of any type.

Ashley

[Dave MilbournParticipant @davemilbourn48782]

I would think a 28mm brushless of one Kv and power or another would suit any application instead of a speed 400 or 540 of any type.

That matches my experience with my little Huntsman and Huntress designs, both of which would previously been fitted with a 500/600 motor.

DM

[harry smith 1Participant @harrysmith1]

Hi All

This is a few I have listed in my go to book.

NTM 2836-750kv 265 watts.

Turnigy 2836-1000kv 300Watts.

A28XL-890kv 330Watts.

Prop Drive V2 2830A-1000kv 370Watts.

T2836-1200kv 340Watts.

Sk3-2836-1040kv 335Watts.

A28XL-1100kv 270Watts.

As you all can see the variation quite great !!!

Jumping up in kv in 28mm brushless you can get into the 750 Watt area.

e.g. Turnigy X2845-1980kv 792Watts.

All specs are from the Hobbyking Chinese Watt Doctor !!!

Harry Smith

[Byron Rees…(Ron)Participant @byronrees-ron]

Hi all,

I'm lucky to have a couple of friendly fast electric boffins in my club and have followed some of their BL/Prop recommendations which have proved sound.

They maintain that a brushless motor likes to spin at its stated Kv rated revs to get the most efficiency out of it. I and currently working on small (A3 paper sized) scale models which are using motors like 1806 x 1500kv. These are running off 7.4 lipos and work best with small Hydro 2 bladed props, My latest, an Air Sea Rescue Lanch at 16 inches long uses that setup with Graupner K series 25mm 2 bladers and they go like the clappers, no heat and 90 minutes running.

I want to try a pair of 4 bladed 22mm coarse pitch props on my new 16 inch Severn Lifeboat, same motors, if I can find the props without a mortgage!

Cheers…………Ron.

[Charles OatesParticipant @charlesoates31738]

Hi Ashley and all, I did try an old speed 600 I had in the bits box, and wasn't happy with it, the brushless is much better. There are so many variables with models that hard rules are difficult to make. Size and weight, hull type, and mostly, the builders idea of what fast means. To me for my asrl, ( 39 inches built light) it means performing like the original. To some an RAF crash tender should go like a fast electric racer. Each to there own as long as we enjoy it.

If we all keep posting our set ups with model size motor, prop and wattage, we will at least have some starting points for reference. I should probably add weight to that list, my scratch built crash tender is also built light, half the weight of the aerokits model. It makes a huge difference, accelerates and stops instantly just as the original was reported to do. That's the next one to get a brushless transplant for the old 540.

Chas

[Martin Field 1Participant @martinfield1]

Harry, that means you're talking just over 100 Amps! So what ESC would you use with that?

I am currently building a 152VO type outboard kneeler from plans. OK, I'm never going to Germany again, to race (I used to work there, but no passport now), but I wanted it to at least look good. I have a Hobbyking CNC outboard (sans motor) which I have to set up the sit of the boat. I have an ebay brushless coming, but it says little about it, so I don't have a clue about watts and amps. Is it really necessary to have a watt meter? I seem to have all sorts of testing gear to get to a "brushless conclusion". I still don't have any LiPos and haven't a clue where to get this stuff for a reasonable sum. Some of it seems outrageously expensive. Even a Chinese prop is 14 quid!

The motor is a 2600 Kv 2845 size.

As a brushed alternative I have an ex aeroplane speed 400 I could try. Is that likely to be worth the trouble? I only want the boat to go like a real one.

Martin

[harry smith 1Participant @harrysmith1]

Hi All

The Hobbyking F1 outboard had a 28mm 3000kv motor with a 32mm 2 blade prop.

They recommended a 35 Amp ESC, but, I used a 45 A ESC with a 3S 2650mah Lipo 30C.

I used a 9.45KG servo to get the boat to turn.

In a 24" shallow vee boat it stands up on the prop in a straight line !!!

Goes like A BAT ON OF HELL !!!!

Harry

[Martin Field 1Participant @martinfield1]

Harry, thanks, that was quick!

I can find the HobbyKing o/b still, albeit at several times the price I paid for it, but I can't find a suitable motor on their site any more. In fact their listing of an outboard without a motor in it has gone 404, Not found.

But if watts equals amps times volts, your statement of 792 watts, divided by 7.4 gives just over 100 Amps. Even with 3S it means 70 amps, so how come your one works OK with a 45Amp ESC? This all very confusing.

Cheers,

Martin

[harry smith 1Participant @harrysmith1]

Hi Martin

I think the ESC's are to fully cover the power of the motor.

I had no problems with the 45Amp ESC on the motor.

I did purchase a 2836-3700kv which was rated by the Chinese Watt doctor at 700Watts, but, still using a 45Amp ESC !!!

At 3S=41,070 rpm and 4S= 54.760rpm unloaded !!!!

A 3000kv on 3S is really powerful !!!

Harry

[Martin Field 1Participant @martinfield1]

The Chinese Watt doctor? Well, my old school physics always said watts=amps x volts. Ergo, if you know it's 700 watts and you're using even 4s LiPos, you must be talking 47 amps. Most people go twice what their amperage is for an ESC. On your previous sample of 792 watts with a 3S battery, that's 71.35Amps.

And I thought I was learning about brushlesses! HELP!!!

All I know is that the motor I've ordered is 2845 2600Kv on 2S. It's an inrunner. It will go in an outboard from Hobbyking. What would the watts be likely to be?

Cheers,

Martin

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