Experimenting with brushless out runners and planetary gear reduction units

[Monahan Steam Models]

Sunnysky motors worth a look for low KV. lots of poles too, so very smooth running at low RPM.

https://sunnyskyusa.com/collections/v-motors

Thank you for sharing this link. Very helpful. Had never heard of Sunnysky until now.

[Sinksalot]

I have 2 of these motors in my big Engel Type IX. Even running at just 6v they got power.

https://engel-modellbau.eu/shop/en/Electric-Motors/E-Motors-Brushless/RO-POWER-TORQUE-NAVY-500-kV.html

[Monahan Steam Models]

3,000 rpm sounds about perfect for a boat that size.

[Monahan Steam Models]

After 25 year in dry dock I got the little harbor tug back on the water today. Refitted with the 22mm 2300kv brushless out runner and 4.75 planetary gear reduction unit. Was originally planning on using a 7.4 2s LiPo battery but only had 11.1 3s LiPo packs of suitable ballast weight on hand. Regardless I got everything set up and trimmed at waterline.

The boat is only 25” long with a 7 1/4” beam and maybe weighs a few pounds. The prop is a 2” diameter with a 3” steam pitch as the boat was originally steam powered. At a 2300kv motor running on the low side number of a 11.1v on a 3s pack with a gear reduction of 4.75:1 that prop shaft is spinning at 5,375 rpm’s unloaded. On the water the thing can cut the Red Sea better than Moses.

After backing off the throttle to a maximum of 45% total, running behavior became more scale running like. This equates to an optimal motor kv size running in this 22mm size with a 4.75:1 reduction to be 1035kv or a final out put of 2,400 rpm.

Still a lower rpm than a low let’s say 350kv motor hooked up direct dive spinning on the same 11.1 voltage.

[Subculture]

Should think you'd be planing with that RPM and pitch.

[Monahan Steam Models]

At 2,400 rpm the little tug still definitely moves a bit faster then scale speed, but not nearly as bad as it did running at the full 5,375 rpm. The plan was to use a 7.4v 2s LiPo pack but even that would have produced an unloaded final rpm of 3,580. Still way too much.

At the start of the tug’s refit, I was not sure what to expect from this propeller and running gear combination. The propeller was purchased and fitted years ago while the boat was not operational. Now with a couple brief test runs in the pond I can see that a maximum rpm of below 2,400 is what I need to aim for with this build.

[Subculture]

Usually a model prop designed for electric will have a pitch somewhere between 1-1.25 times diameter. that gives good efficiency without bogging the motor down. A prop for reciprocating steam would be designed for lower rpm but large torque.

I usually factor in about 50% efficiency for model props, as I find they slip a lot being so small compared with fullsize. I use the following formula which gives a rough ballpark in mph-

RPM x pitch in mm x 0.00002

Imperial

RPM x pitch in inches x 0.0005

In your case that gives about 3.6mph at 2400RPM.

[Monahan Steam Models]

Great info again Andy thank you.

That’s a neat formula and I’d say that it is pretty spot on to what I witnessed during the test runs yesterday at the reduced rpm of roughly 2,400.

[rwtdiver]

Nick,

I am Hi-jacking the blog again! Just wanted to show you my new boat build!


Dumas "Lord Nelson Victory"

It's all wood construction 29" with a 10" beam! For my first boat I am going to go electric, and then start saving for a complete tug build with STEAM POWER!

Nick! Thank you for all your advice and help! I am really looking forward to the build!!

Rob
"Firemen can stand the heat"

[Schmitty1944]

This is a great topic. Finding good reduction units has become quite difficult in an era where most of the hobby suppliers are geared (pun) towards speed for airplanes and cars. Many subs can benefit from a reduction unit in more scale speed, longer run time, better low end throttle response, greater efficiency. I recently discovered that you can still buy some very nice quality gear reduction units from Mpi: https://mpihobby.com/collections/motor-parts they offer offset gear reduction boxes and planetary gear reduction. Most require some adapting to fit brushless motors which tend to have larger shafts.

In my case I was working on an old Subtech Marlin with an updated WTC featuring a brushless motor with a belt reduction unit. That survived for about 10 minutes of run time. The belt was shedding its teeth and the pinion sprocket which was plastic was also looking not long for this world. Runs great now with a nice gear reduction although I think it is still too much power.

Jason

[Subculture]

Did you have the belts tensioned right? Else the parts must have been moulded in icing sugar or something. Plastic or resin pulleys should be more than up to the job, The most important thing is that they run concentric.

Ron Perrott said he stopped making his piston tanks with belt drives because many modellers didn't tension them correctly leading to all manner of problems.

[Monahan Steam Models]

Nick,

I am Hi-jacking the blog again! Just wanted to show you my new boat build!


Dumas "Lord Nelson Victory"

It's all wood construction 29" with a 10" beam! For my first boat I am going to go electric, and then start saving for a complete tug build with STEAM POWER!

Nick! Thank you for all your advice and help! I am really looking forward to the build!!

Rob
"Firemen can stand the heat"

Rob

That should be a good one to start with.

[Schmitty1944]

"Did you have the belts tensioned right? Else the parts must have been moulded in icing sugar or something. Plastic or resin pulleys should be more than up to the job, The most important thing is that they run concentric."

Yes, although to be fair there was a problem with the belt reduction unit before the last fatal run. The pinion pulley was plastic and attached to the brushless motors shaft with a grub screw. The plastic around the grub screw was cracking apart and I made a brass collar around the plastic with a hole in it for the grub screw to keep it all together, At that time I also tensioned the belt, fairly tight with just a little slack. That part held together but after running for a while it was loosing power because the belt was jumping teeth. I do think a belt drive can work. In this case I believe the motor had too much torque for the plastic pinion pulley/sprocket. I suspect it would inevitably fail. Maybe if the speed control featured a soft start would help?

Jason

[Subculture]

Shouldn't be any slack in the belt. A good way to check you have the right tension is to twist the belt, should be able to twist 90 degrees from its normal position, but if you can twist more, it's too slack, much less and you may have it it a bit too tight.

[Monahan Steam Models]

Found a 22mm OD 1806 brushless motor built using the same parts as the original 2300kv unit in a 1400kv version.



Ordered a couple and they should be here this week. Planning to swap the shaft and pinion from the 2300kv motor into the 1400kv motor to make them work with the planetary gear reduction. Although the kv rating is still not ideal, it’s better. Also ordered a properly sized 2s LiPo battery pack to replace the 3s pack that I had on hand at the time. This should yield 7.4v * 1400kv / 4.75reduction = 2,181rpm with zero transmitter throw reduction.

Using Andy’s handy formula this brings the speed at full throttle down from 3.6mph (while using a 55% transmitter reduction with the current motor and 3s battery) to 3.3mph running 100% with out any transmitter reduction.

During my research of 22mm planetary gear reduction units I believe I had found the original source who manufactured them. On this site was a detailed complete list of all ratios available in this design. And there was many. Regrettably I did not bookmark the link as now it would be handy to be able to pair a certain kv motor to an available gear reduction unit to arrive at an ideal rpm for a given motor/ reduction combination. I’m still searching to find it again and will post a link when I do.