Hot motor

I wonder if anyone can offer any advice on a hot running motor? I'm finishing my granddaughter's Atlantis (see 'Wot paint woes' previous thread). It's around 600mm long. It's running a 30mm three blade brass prop (replacing the enormous two blade plastic job it came with). I originally fitted an MFA 385 5 pole motor, rated from 4v - 15v, powered by a 7.2v NiMH pack. The decision was taken to limit performance, as my granddaughter is only six years old. In sea trials on my garden pond, it's clear that performance is really very sluggish, with 'full astern' giving very little braking effect. I've therefore swapped the 385 for an MFA 380 3 pole, rated from 3v - 7.2v. Performance is much better, but the motor quickly runs hot on very partial throttle, producing visible smoke. This happens on the bench, so I don't think it's the effort of swinging the 30mm prop in water. No hot running problems with the 385 at all. I also have a 380 swinging a 25mm brass prop in my eldest granddaughter's Aeronaut Diva. This runs quite warm, but no smoke! I've attached a pic of the power train. The propshaft is a 3mm Cap Maquettes item - and the universal coupling is also Cap Maquettes. Nothing's binding (apart from the grease in the stern tube!) and everything is lined up nicely - I used an accurate, solid coupling when setting up shaft and motor bracket. Interestingly, the 380 blew the 10A supply fuse when I first fired it up. I'm thinking I might have to revert to the 385, so any advice would be warmly welcomed! Pun intended......

Hi Grumps,

I would start by taking out the grease, you don't need it and it would cause a lot of friction for the poor little motor.

Regards Ray

Ray is right, the grease could be causing the shaft to bind, I have experienced that problem myself and it is surprising just how much resistance it can cause.

It's worth having a look a Glynn Guest's article ob the subject of drivelines:

**LINK**

Just a bit of grease on the bearings is all you really need, a washer on the prop end will effectively stop more than a few drops of water getting into the tube.

Colin

The first thing to try is to disconnect the UV joint and run the motor disconnected. If it still runs hot there is a problem with the motor/ESC set up. If it runs cool there is a problem with the transmission, grease being the favourite culprit.

I once converted a Revel Type VII U-Boat to RC and used two servo motors for propulsion. It was recommended to me to grease the 10" tubes. The motors wouldn't even turn! Cleaned the grease out and used light oil and everything worked perfectly.

Thanks Ray, Colin and Richard. I'll try running the motor with the U/J disconnected. I'm guessing the resistance offered by the grease is indeed the culprit - it's quite noticeable, compared to the dry tube (from memory). It's going to be a bugger to clean, as I'll have to remove the rudder to pull the shaft - a job I was hoping not to have to do! I'm guessing this is also the a cause of the warm motor on my eldest granddaughter's Diva. However, the Diva is running with a 2mm shaft, so the area offered to the grease is much smaller. Incidentally, the '3mm' Cap Maquettes prop fitted to the problem Atlantis is actually a 4mm shaft, turned down to 3mm at the ends - so a lot of area to stick to the grease! Guessing I should really change the motor (again) too, given that this one has got cooked! Ho-hum, live and learn. I was hoping to deliver it to her in a week or so for her birthday too! Thanks again.

A 380 should spin a 30mm prop no problem. It will/might get warm or even quite warm,but that’s it.

Grease in my book a no-no for shafts. Smear, drip of oil nowand then.

I have several boats with speed 400’s and S35 props that run ok. The 385 is a wonderful motor and quite feisty on 12v. A 385 on a 3s Lipo will boost power quite a bit.

Ashley

Sorry...I forgot to quote the rule-of-thumb caveat. “A motor that’s correctly loaded shouldn’t get hot”.

I would follow that by saying at its maximum rated power any motor is likeLy to get warm or even very warm of course.

Ventilation in a boat can also be overlooked. A closed box is basically an oven, and if there’s no ventilation, a moderately warm motor will soon heat up the inside of the boat and it will get warm, making the motor even warmer and so on.

Interestingly my sub, which uses a low power 540 on only a 7.2v stick, gets REALLY warm inside despite using no more than about 1/3 throttle most of the time, and the motor gets into the very warm” category.

The 385’s on say my Devastation, on 7.2vor 8.4v driving three blade brass 40mm props rarely get even just warm, as although undoubtably overloaded, just don’t pull the current to heat things up. Great little motors.

Computer fans that run off a usb outlet (so that’s 5v, same as a spare channel on your receiver) are cheap and move air about nicely.

Ashley

Thanks Ashley - and everyone. I shall work out just how to flush the grease from the stern tube and report back! I'm thinking maybe squirting WD40 in at the motor end and blowing through with an air line.....

....no no no...megga messy!!!! simply use a pull-though of some description....scrap of cloth on the end of a thin wire/string...thick string superglued to a thin wire rod...etc

You dont need to get ALL the grease out, just sufficient that it does not bind on the shaft. Even running a bamboo BBQ skewer in out/up and up/down may remove enough.

Ashley

Thanks Ashley. Makes sense. Though probably not as much fun!

Just wish I hadn't put it in in the first place....