Electric motor guru's please help

[Mike Bontoft]

I am looking for advice on which motor would be best to use as a starter motor for this engine. I have this Lehner 2280 673KV but it is of course way overkill and too large and heavy. I think I would like to stay with a brushless motor so I do not have to worry about brushes and maintenance. The starter belt gear ratio is 4.4. As set up here I have a 2S 5400 pack and a fairly hefty speed control I had on hand. The engine is 30cc. 500 to 600 cranking RPM makes a quick job starting it.
I am looking to put together the smallest and lightest setup possible within reason. For the 600 RPM cranking I am setting the servo driver to about half throttle.
I could build a small reduction box on the front of the brushless motor to increase torque but if there is something out there small with a very low KV I can direct drive with the torque required that would be great. As a note the engine is very low compression ratio and rotates easily. From very crude measurements pulling less than 20amps with this setup. Thanks

 

[LohringMiller]

Forget the starter, tell us about the 4 cylinder(?) engine.

Lohring Miller

 

[RaceMechaniX]

Mike the Lehner is probably the opposite of what you want. They are 2-pole motors designed for high speeds. A 2280 has no problem because of the long rotor length. A multi-pole outrunner motor would be much better suited and 1/4 the weight. Based on how easy the 2280 kicked over the engine, you can likely use a helicopter outrunner for a 450-600 size heli. Like this: https://www.scorpionsystem.com/catalog/helicopter/HK5_series/hk5_32/HK5_3220_955/

Here is an inexpensive one: https://www.offshoreelectrics.com/proddetail.php?prod=leo-lc500

 

[Mike Bontoft]

Hi Lohring. I hope your well.
The engine is a 1947 design by Edgar Westbury. I built a few years back from castings available through Hemmingway kits. The simplest of the inline 4 cylinder engines. Side valve, 2 main journal crank. A fun project and a sweet running engine. I've been looking for a boat to install it in, decided on the Miss APBA/ Severn runabout which I am in the process of building now. Some pictures.

Tyler and Lohring, Should I within reason then not worry too much about the KV as the starter motor will not be ran wide open? And I guess if its got enough oomph to crank the engine I can adjust the speed control throttle and note the cranking RPM and amp draw. I think I've just answered my own question but it would be nice to get the nod from someone that has more experience of these things.

 

[Daniel's Racing]

They make powerful smaller sized brushed starter motors that are not very big and heavy and are designed as starter motors made for high torque.

Four cylinders are easier to spin over than a single cylinder and fire faster. All cylinders compressions cancel out the cylinder that is 180° from it.

If you always get it to spin first then hit the ignition switch on also makes them fire up so much easier and faster. Did that with all my drag cars and starters did not get fried regularly like most people were doing at that time.

You really would not want a brushless motor made for high rpm mostly I would not think. The higher voltage requirements and cost of it and the speed control and all you won't need with brushed high torque starter motors. They are 100% sealed as well so water will not get in the motor.

Brushed is way to go. 3S lipo and a solenoid to switch it. Simplicity.

This motor in my hand may be small but it is one powerful little son of a gun. The motor I have on my YZ is a even higher torque version of same motor dimensions but one in my hand started it just fine too. I just had to have the baddest. This bike has no charging system and I run it off 4S LiFe only 2400 mah pack. Will get 100 starts before it is low.

The bike has a aftermarket starting system. The flywheel that was dry now runs in oil 1/3 of the way up. I built all the wiring harness to power and switch it. Nice KTM start switch, quality solenoid and a small 2400 4S Life starting battery.

Really cool boat and engine project you have there.

If your doing the brushless motor I think I would want the in runner style.

 

[LohringMiller]

That's a really great project. What machines do you have in your shop these days since you sold the CNC? Daniel is probably right on the starter setup. Finding a small, brushed motor is beyond my expertise.

I ran a Scorpion outrunner in my electric scale boat and it worked very well. With a 40 amp maximum current on the motor Tyler suggested you can use an inexpensive ESC like this one: https://www.offshoreelectrics.com/proddetail.php?prod=ker-seaking-30a-v3 It will power your receiver as well.

I've been trying to learn how to race P limited boats under the new motor rules. I bought a Nextgen rigger from Mark Grim, a Black Pearl sport hydro, and repowered an old Leecraft XT-460. The rigger ran perfectly out of the box, but the other boats have issues. I just rebuilt the Black Pearl after talking to Mark Anderson and Mike Hughes. It may stay on the water now. The Leecraft has also been modified so we'll see how it runs at next weekend's District 8 race.

Lohring Miller

 

[Mike Bontoft]

Thanks for the info guys. Gives me plenty to think about. On these kind of projects I usually end up with lots of surplus until I come up with something I'm completely happy with.
Lohring, The shop is full . No CNC machining center but all is not lost. Onefinity CNC router
I think I have an issue! I may have become a hoarder.

 

[LohringMiller]

Wow, and I thought my shop was bad. I gave my big machines, the mill, lathe, and cutter grinder to Mike Hughes. I still have a baby mill and lathe. I gave the parts for a dyno and all my gas engines to Mark Anderson. I spread my gas boats and other stuff around District 8. The shop is still full of stuff.

Lohring Miller

 

[JfromJAGs]

An email asking to support IntlWaters brought me here. Haven't been here for quite a long time. Reading the names Mike and Lohring brought back some good memories from about 20 years ago when we met at the LA SAWs. Nice to see you guys are still playing RC boats too.

Let's do some math. 2s Lipo (7.5V) at 20A means about 150W is required to start the motor. 600 rpm at 4.4 gear ratio means the starter motor should spin 2640 rpm under load. At 7.5V this is 352 rpm/V, which would lead to a 400-450kV motor.

As Tyler pointed out, a low KV outrunner would be the best way to go. In the range of 150-250W motors I didn't fine one which spins as low as 450kV, but I did find a Surpass Hobby C2838 600kV which I think would be usable. About US$ 25 from ebay. 4mm shaft, weight is about 75g. Combine this with a 30-40A 2-3s brushless ESC, also available from ebay for about US$20.

BR,
Joerg

 

[Daniel's Racing]

How many cc is the engine ? I guess maybe I was thinking it was little bigger than it may be.

 

[Mike Bontoft]

Thank you for the reply Joerg and for adding some clarity to my little project. Yes the times we had at Legg Lake made for some great memories. I have messed with model boats from about 10 years of age so there is no help for me. I definitely think Lohring is a lifer too.
Living in the UK the last ten years I now spend more time building than sailing.
By the way, congratulations on your European SAW records. Very impressive and looks like an awesome venue. Maybe a visit from me in the future.
All the best

 

[Mike Bontoft]

Hi Daniel. Yes I was hoping to have a starter motor a little bit closer to scale. From your hand my engine is the roughly the same size.
I'm going to do some more playing but may have to add more gear ratio than 4.4.
Cheers

 

[LohringMiller]

Joerg, good to hear from you as well. Congratulations on your speed records. Your information from a long time ago allowed Mike and I to set the full sized electric boat water speed record in 2008. It stood until last year when a group from Princeton went 140 using modern equipment. I still love two strokes but gave up IC powered models for electric power. I've even had a Tesla Model 3 performance version for 6 years.

Lohring Miller

 

[Don Ferrette]

This is a really cool thread, that motor is awesome Mike. And Joerg it's nice to see you pop back in.

 

[Daniel's Racing]

Hi Daniel. Yes I was hoping to have a starter motor a little bit closer to scale. From your hand my engine is the roughly the same size.
I'm going to do some more playing but may have to add more gear ratio than 4.4.
Cheers

Didn't think it was 250cc Lol. Not for you to use this exact motor. Just the type of motor.

Just example of how small of a brushed starter motor designed for torque can start a 250cc engine like nothing with only a 2400 mah pack.

Your engine seems to spin over pretty easily. Amazing how multi cylinders crank easier than a single with only one of even the same exact cylinder. The opposing cylinders somewhat cancels out the other one 180° out then throw two others doing that on same crank and there are no lulls once starter gets it moving it is smooth starting.

Not sure what timing advance you run but what I mentioned about not energizing the ignition until after the starter really makes them crank easier and with more rpm when fire comes on it starts easier too.

With better mechanical ratio that would really decrease the size of motor required.

Your ratio is still pretty tall as compared to other starter/crank ratio's of engines.

Flywheel with 153 or 168 teeth uses a starter pinion with 9 or 11 teeth.

Be difficult to get anywhere near those with belt and pulleys without jack shafts and more complicated drive. Some planetary gearing on the snout. Lol.

 

[Mike Bontoft]

Here's hopefully where the hording pays off. Guts of a faulty cordless drill from a few years back that I just couldn't throw away.

 

[JfromJAGs]

Mike, if you can use a higher gear ratio, then an even smaller outrunner would do the job, or the same size would give you more margin if the engine needs more power to start.

I know that you and Lohring set the record for a manned electric boat in 2008, and I also know that it has already been beaten. I also know you moved to the UK. With the motors and controllers available in modern electric cars, a lot should be possible these days, but I stick with model boats. Failures hurt a lot less.

The 10th Munich SAW will take place from September 23 to 26 at the Olympic rowing facility in Munich. Participants and visitors are always welcome.

BR, Joerg

 

[Mike Bontoft]

This outrunner looks to be the ticket. It spins the motor over a little fast at 1400rpm. It could probably be used to bring the boat back to shore in the event the engine quits.

 

[JfromJAGs]

Good idea to use the starter as a backup engine. In that case it's also a good idea to oversize it a bit. What's the target rpm of the motor? Maybe you can use the starter motor with a 2 step switch to regulate the PWM of the ESC. Low step to only start the motor, higher step to use the starter as backup engine.