How to measure what motor I need.

[Florian HebekerlParticipant @florianhebekerl32192]

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.

[Florian HebekerlParticipant @florianhebekerl32192]

[ashley needhamParticipant @ashleyneedham69188]

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.

Ashley

[Florian HebekerlParticipant @florianhebekerl32192]

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 needhamParticipant @ashleyneedham69188]

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!

Ashley

[Chris EParticipant @chrise]

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 FraryParticipant @malcolmfrary95515]

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 EParticipant @chrise]

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 MilbournParticipant @davemilbourn48782]

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 3Participant @raywood3]

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 EParticipant @chrise]

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

[Florian HebekerlParticipant @florianhebekerl32192]

Hi everybody, this forum is alive while I sleep because of the time difference. 7am Vancouver time now.
Thank you all for your input. First of all, I made a mistake mentioning there is two wires on that ordered motor cause it is three but I didn’t see that in the picture very well. Also I didn’t know until now that brushless is always three and brushed is two.
second, I am learning every day, I didn’t know that esc not always have a reverse so, before even getting the one I ordered, I need to order one again with reverse. This will give the opportunity to also build a speed boat with the spare parts I am collecting by accident.
My ship is going to be 2.10 meter long , 32 cm wide and it might be that the underwater part is not scale. I will upload pictures later today. The weight of the ship I can only guess but I would think it is going to be about 30kg when ready and the ship is fairly slim for a bulk carrier but still scale above the waterline. I do want to order two bow thrusters (front and back). As a ballast I will use about 15lbs of lead pallets. All the hardware is very light ( even the batteries ) And the hull is made out of tiny strips of wood, wood filler, fibreglass and than paint and I only got the idea of building a ship because I had too much wood from projects around the house.
I have seen rc tug boats having no issues pulling big ships on YouTube so, I do believe I might have ordered a too big of a motor but honestly all the rc motors look small to me. I am living close to a few lakes (all within a five minute drive), and they are actively used for real motor boats (Vernon BC Canada) so the water is less calm as it is on a pond ?! So for my spelling mistakes, I am a german living in Canada,LoL.
Thank you all again, you helped me a lot.
Kind regards Florian

[Florian HebekerlParticipant @florianhebekerl32192]

[Dave Cooper 6Participant @davecooper6]

30 Kg – wow, that's going to take some shifting and transporting…

I expect you are planning a purpose-made launching trolley /model slipway ? (Most probably you are in a different age bracket to me !).

I suppose the ballast could be added after the model is in the water ? Just thinking outside the box now.

Good luck with the project – let us see some photos please,

Dave C

[Florian HebekerlParticipant @florianhebekerl32192]

Thank you Dave,

The exact needed ballast will be measured when the hull is completely finished. My neighbor has a big pool where we will put in the ballast and see if it lays perfectly in the water.
Pictures will come today, just need to figure out how the upload works. I saw there is a thread for that too.

Florian

[Chris EParticipant @chrise]

At 2.1m x 0.32m I calculate around 5kg for every cm below the water line. Your 30kg is thus around 6cm under water which sounds very little.

How have you estimated 30kg? I await the plan with interest.

[Florian HebekerlParticipant @florianhebekerl32192]

[Florian HebekerlParticipant @florianhebekerl32192]

I didn’t calculate but guessed since it is very light still. I can pick it up with two fingers .

[Chris EParticipant @chrise]

OK That is a very shallow underwater version of a bulk carrier so it will not have the weight problems that I envisaged with a full scale hull but it will still be heavy.

[ashley needhamParticipant @ashleyneedham69188]

Florian. A) What size is your 1300Kv motor? 35 or 28mm (or something else?). B) do you have room for a larger prop if required.

A 28mm one would probably be ok on your 42mm prop i think (but maybe not ideal) depending on spec, however a 35mm one definitely would do.

It is easy to change the motors to different spec ones in the same size. Lower Kv versions would give more grunt for prop swinging.

Others will disagree but, Personally speaking, I would fit what you have and once you in a position to test the boat in the water use a wattmeter and see how things are. The wattmeter will show you if the motor is straining or not. You have already spent the money it would be a shame to waste it and not know if the equipment would do after all.

i am not talking about “the optimum” here, just seeing if the stuff will do.

Ashley

 

Edited By ashley needham on 27/07/2020 08:56:10

[Charles OatesParticipant @charlesoates31738]

Sometimes I feel sorry for posters needing advice, so many of us go about things differently. The methods each of us propose aren't wrong, just different. Anyway, here is my way.

This is a long narrow heavy model, it doesn't need much oomph to keep it going, but plenty to start and stop it. In other words, a version of torque.

For me this starts with the propeller, a large multi blade one moves a greater volume of water, which is needed to start and stop the model. The speed of the prop should be slow compared with , for example a fast cabin cruiser.. once the model has started it won't need much to keep is going nicely. It's all about overcoming inertia.

This boils down to needing a very low kv motor if using brushless, a gearbox / belt drive to drop the revs, or a brushed motor of the big slogger type. If starting from scratch, I'm certain Dave's suggestion in his post would do the job perfectly, not expensive either..

If you are going ahead with brushless, you are aiming for a propeller rpm of under 200 rpm to 2 or 3 thousand, so choose a motor etc accordingly.

In my view, the propeller should be around 70 mm minimum, three or four blade .

 

Good luck with the build, and keep the ballast very low and we'll spread in that hull, or you'll be sailing upside down.

Chas

 

Edited By Charles Oates on 27/07/2020 10:24:06

[Florian HebekerlParticipant @florianhebekerl32192]

Okay, needed two days to evaluate what you guys are all writing.

1). I will be able and also plan on putting the ballast in during the first floating test. In full length I keep a space for that. So I don’t worry too much about that.

2). I checked and I ordered a 1300kv 28mm size brushless motor. I stilldont know the difference between the sizes but I will get there reading into it. The motors are dirt cheap so I can always change motors for other specs. From the spare ones I will just build a speedboat for kids in our family.

3) prop size I am having the space to go up to 90mm. So 45 mm from its core. (Just saying in case I tell it wrong).

Question I have :

Do I spray paint after I fibreglassed the hull and sanded it smooth with four coats of resin or, do I do that in between ?

And how are you guys making little tiny windows in the bridge for example ? I have a dremel but didn’t figure out yet if I should router windows in , drill the windows in and than saw or drill and than sand. I noticed while practicing, that it is very hard to do that super precise.

Thanks for all your answers. And it is okay to also criticize if I do something absolutely wrong cause I do want to learn and eventually know what I am doing . Enjoy your day.

[Chris FellowsParticipant @chrisfellows72943]

Hi – whilst brushless motors are very powerful for their size I think I'd use the 28mm 1300kv in a much smaller speed/sports boat. Asking it to turn a prop of say 60 to 80mm say is a bit much.

If you want to use brushless rather than the brushed with reduction gears or pullies then you need to go for a lower kv rather than the higher that you have done, say 500/600 kv and a bigger diameter, 50 or even 60mm to give plenty of torque.

Chris

Edited By Chris Fellows on 29/07/2020 17:36:31

[Florian HebekerlParticipant @florianhebekerl32192]

Right Chris,

i will go for the lookout on a motor like that. 600kv to 60mm size. Is size adding torque too?

will update when I found a motor to order and will also go on the lookout for different size props so I don’t need to wait ones I get everything delivered. Delivery here, always takes two weeks doesn’t even matter if parts are coming from Canada or the other end of the world.

[Chris FellowsParticipant @chrisfellows72943]

Hi Florian

Had a look at Overlander Batteries motors (as an example) and they do a 6354/16 200kv which is an even lower kv which on 14.8 volts which is the minimum stated will give 2960 rpm which is around the speed required. Based on this even 500 to 600kV will give too high a speed and will require a reduction drive as Chas recommended.

Yes, for greater torque from a brushless motor you need an outrunner (as opposed to an inrunner), a larger diameter and a lower kV. From the kV you can calculate the rpm – for the above it's 200kV x 14..8v = 2960rpm.

Chris

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