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How to measure what motor I need.

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Florian Hebekerl25/07/2020 19:04:31
18 forum posts
7 photos

Hi everybody. Very new to this and just started building a very long bulk cargo ship. It is a 1:100 scale and 2.10 meter long. I guess you prefer feet (about 7 feet). So far I ordered a metal prop of 42mm (three blade, 300mm shaft, flysky fs-i6x 10 channel Transmitter with receiver, got four cheap servos 12kg for the cargo Lids to open on the Deck,, will order a better quality servo for the rudder cause it cannot fail on me (I am a bad swimmer). Got 4 3600mah 7.2volt batteries that I will connect parallel. I didn’t go for lipo batteries because my son his rc car batteries (lipo) burned down our kitchen five years ago. So, my problem is : I ordered a 35a esc with brushless 4800kv motor and I just didn’t think of what I was doing. I don’t want a speed boat. Can I run this motor slow so it looks a bit more real or do I need to order a different motor with esc. Many many years ago I was rc flying not boating and I had nitro engines instead. Also , I didn’t build that one. So for this long story for a simple question. Here in Canada they only have two rc boating clubs and they are still very far away from where I am living. Here people are more into rc cars.

ashley needham25/07/2020 19:23:47
6572 forum posts
144 photos

Hi Florian, and welcome to the forum!

The motor you have is most unsuitable, it has a much too high Kv rating for your style of model, as you have surmised. It will not run slow enough for you!

Something around 900Kv to 1300Kv would be more like it, say a 35mm outrunner. I would not like to suggest how powerful, but if you bought something your 35A esc could handle you wouldnt go far wrong.


Florian Hebekerl25/07/2020 19:37:23
18 forum posts
7 photos

Thank you Ashley. Went to eBay right away and ordered one. 1300kv cause I couldn’t find one less fast that also will be delivered within two weeks. I am not sure if I can use the same esc though cause it has three wires and the one I ordered now has two wires. Esc and motor are brushless though.

ashley needham25/07/2020 23:08:57
6572 forum posts
144 photos

Not sure I am with you..if the esc and motor are brushless you should be ok?

Motors for model use will have three wires and so should the esc’s. You may have to wait and see what turns up!


Chris E26/07/2020 08:36:59
98 forum posts

There is a lot to question here.

Firstly a brushless esc has 2 wires that go to the battery and 3 wires that go to the motor. An esc for a brushed motor only has 2 wires that go to the motor.

Secondly if it is a scale bulk cargo ship over 2 metres long it is going to be heavy when ballasted, very heavy.

You have told us the kv of your motor but not its size. You can get 1300kv motors that weigh 40 grammes and you can get them that weigh 500gm.

Thirdly for a heavy displacement model of over 2 metres I would have gone for less kv and more propeller.

Until you tell us more about your model and the motor not much more can be added. So exactly what motor, esc and prop have you ordered and what is the expected finished ballasted weight of the model?

Edited By Chris E 1 on 26/07/2020 08:40:45

Malcolm Frary26/07/2020 10:24:57
892 forum posts

The original 4800KV motor "could" be used, but would need gearing down by about 6:1 to get the prop revs to a sensible level.

A 42mm prop seems small for a 7 foot hull.

To get an idea of how much power is needed, it helps to know the horsepower of the real thing. Converting to watts and dividing by the cube of the scale will put you in the right area. Guessing at a Welland laker, at 1:100, only about 20 watts output is appropriate. They were not designed for performance, rather carrying ability. Call it 30 watts input to give a bit of margin and allow for inefficiencies. On 7.2 volts it should be pulling about 4 Amps.

Chris E26/07/2020 11:48:28
98 forum posts

A model that might weigh 200lbs+ and 20watts power (around 1lb. thrust) doesn't sound enough to me. If it once got moving stopping it might be fun.

Edited By Chris E 1 on 26/07/2020 11:49:37

Dave Milbourn26/07/2020 12:09:33
3998 forum posts
282 photos

If it's anything to go by then my Model Slipway 'Envoy' tug was just over a metre long. It was powered with a 6:1 geared MFA 500 brushed motor using a 12v supply. This gave approx 2500RPM to the 70mm diameter 4-blade cast bronze scale prop. Performance was sedate without being dull. A bulk-carrier would have no need for any sort of 'performance' - all they do is move slowly. Your proposed 1300kv motor will still produce over 9000 RPM on 7.2v and without gearing down would almost certainly be too fast for the size of prop needed for such a big heavy model.

If I were contemplating such a model then I'd be looking at something along the lines of an MFA 800 or 850 brushed motor with 2.1:1 reduction belt drive running on 12v.

Yes, Ashley - I know I'm not supposed to like MFA 850 motors but everything has its place somewhere!

Dave M

Ray Wood 226/07/2020 12:21:36
1979 forum posts
705 photos

Hi All,

I can only work from experience, I don't do the technical approach 😀

I do know that my 20" TID tug driven by a brushed 400 motor turning a 30mm 3 blade prop was a able to move a 14' bulk container ship on 7.2 volts. And could also push it sideways.

A 600 brushed motor would be fine in my view with lead acid SLA's for ballast.

Regards Ray

Chris E26/07/2020 13:46:16
98 forum posts

I'm with Dave M on this. A small tug can move a big load but with all sorts of limitation that you wouldn't want for the prime mover system.

A scale bulk carrier of 2+ metres will weigh 200+lbs unless the underwater shape isn't scale.

Length 210cm x Beam 45cm x Draft say 12cm x allowance for hull shape 0.8 = 91kg or 200+lbs which is a lot of SLA batteries.

Edited By Chris E 1 on 26/07/2020 14:00:00

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