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Author Topic: Which motors  (Read 1793 times)

Lifeboat 70-001

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Which motors
« on: January 22, 2018, 10:33:54 PM »

need some help please, got a 6 foot boat which is a displacement hull so not a speed boat, twin props and thinking of putting in two 500 kv outrunners but not sure what Amp Esc to use and would like to use the gel cell battery's because they are part of the ballast. Maybe this isn't the right set up and if not then what size brushless to use. Thanks.
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T33cno

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Re: Which motors
« Reply #1 on: January 22, 2018, 10:42:30 PM »

Firstly in case you don't know? 500kv = 500 RPM per volt so 12 volts would be 6000rpm.
I have absolutely no idea how these work in terms of Torque.
The motor should state its maximum amps in the spec and you want an ESC at least 10% higher but I think most go 20%
Hope that helps a little.


EDIT
Re batteries, I don't think Gel or PB are good at delivering higher currents so need to be looking at Nicad or LiPo
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Dave Cook

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Re: Which motors
« Reply #2 on: January 23, 2018, 06:52:08 AM »

Lipo all the way
Dave  :-)) :-)) :-)) :-))
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kinmel

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Re: Which motors
« Reply #3 on: January 23, 2018, 07:00:22 AM »

Can you give us a link to motors please, we need more information.
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chas

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Re: Which motors
« Reply #4 on: January 23, 2018, 08:01:36 AM »

Also a better idea of the hull shape, a long thin warship hull is easier to move than a displacement cabin cruiser.

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canabus

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Re: Which motors
« Reply #5 on: January 23, 2018, 11:55:57 AM »

Hi Lifeboat
Check out the Hobbyking web site on brushless motors e.g. 4250-410, 4250-500, G60-500 and the 5055 series.
These have the specs, current and ESC requirements.
These motors are about 1300 watts(about 2HP)each.
If you use car ESCs like the HK60a-SL or HK100A you can run 3S(11.1volts) or 4S(14.8 volts).
A program card is required, but it's very easy to use.
I can post my setup on the ESCs if you require!!!
You require two ESCs(one for each motor) also a Y connector to link together for one channel(disconnect one of the red leads on one ESC).
Batteries you require are Lipo ones to handle the current draw, sorry the others will not cut the mustard.
5800mah 40C will work OK.
Canabus
Battery weight is between 400 and 550 grams each.
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chas

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Re: Which motors
« Reply #6 on: January 23, 2018, 02:07:31 PM »

We might need a reality check here, this is a displacement boat, which seldom need much power, it depends on what it is. As an example, my 4 foot warship runs on 7.2 volts, and less than 2 amps, say about 12 watts. That makes it go well over scale speed.  If the model were one of the displacement lifeboats, that would be another matter.
  500 kV out runners should be fine for a big prop, but this also depends on the detail.
 Lastly, if the motors are low consumption, say less than 5 amps, they will work just fine on lead acid batteries of 10 ah or more.
  The devil is in the detail here, and until we know more, it's all speculation.
Chas

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Netleyned

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Re: Which motors
« Reply #7 on: January 23, 2018, 02:34:11 PM »

A six foot displacement lifeboat methinks.
A couple of T12 from Mobile marine, with
big props and Gel Cells to use as power/
ballast.
Why o Why brushless?


Ned
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chas

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Re: Which motors
« Reply #8 on: January 23, 2018, 03:09:31 PM »

Ned, if it does turn out to be a lifeboat, I absolutely agree. The T12s are superb for that sort of thing.
Chas

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Lifeboat 70-001

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Re: Which motors
« Reply #9 on: January 23, 2018, 04:28:56 PM »

Thanks to you all for the info. Bit more on the build, like I have said six foot long, 18" beam, 7" draft, all up weight 150lb / 68kg, 90mm props. All ready tried a couple of blower type motors with problems ie blowing fuses (action Esc)  and yes I tried different combinations with Esc's, motors (4500rpm) by changing them around , on there own, with mixer, different motors etc. Or perhaps there is a problem I'm missing with it keep blowing fuses or Esc's faulty. So I thought why not brushless because some tug owners are using them now with good results but sounds like it might be an expensive option using brushless ecpecially on the battery side of things so to get decent running time.
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chas

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Re: Which motors
« Reply #10 on: January 23, 2018, 04:49:41 PM »

Firstly, I doubt the ESC s are faulty, blowing fuses is probably down to not knowing how many amps the motors are drawing when the boat is in the water. If it were my model, I would,
1) connect a motor straight to the battery, with an ammeter in the circuit, and check the consumption, in the water.
2) check again with a smaller prop, or one with a finer pitch.
3) estimate if the model is fast enough, or too fast with this set up.
4) armed with some facts, start to make sound decisions about how to proceed.
5) check everything is running freely, no dodgy couplings or binding shafts.


What type of model is it? Tug? Lifeboat? Cabin cruiser?...or something else.
I wouldn't recommend just getting stuff and seeing if it works, that's the way to frustration.
Chas.

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McGherkin

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Re: Which motors
« Reply #11 on: January 23, 2018, 05:09:13 PM »

http://www.rcgroups.com/forums/showatt.php?attachmentid=3402635

Have a good read of this, it should help you plenty. Judging by the graph in this guide it suggests that 4500rpm is rather fast for a 90mm prop.

Out of interest, what are the scale, top speed and horsepower of the full scale boat?
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destroyer42

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Re: Which motors
« Reply #12 on: January 23, 2018, 05:32:23 PM »

Hi Lifeboat,
I have a 5 foot Arun lifeboat boat running on 2 Emax BL 4030/10 turn 850 kv  brushless motors turning 80mm 4 bladed propshop props with 60 amp Hawk speed controllers and 2 12v gell cells batteries for ballast and power, it runs great at scale speed. I can run it on 24v as well, but a little to fast.

Destroyer42
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Lifeboat 70-001

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Re: Which motors
« Reply #13 on: January 23, 2018, 07:43:11 PM »

The full size boat is 70', 12 knots, 570hp, my boat is 12th scale. The motors I've got have 4500 rpm but I know it might be a bit much for 90mm props but that is max and you don't drive a sports car flat out all the time.
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nemesis

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Re: Which motors
« Reply #14 on: January 23, 2018, 09:31:05 PM »

Prop, down the pitch or gear down 3 to 1 if keeping the 90mm prop. nemsis
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McGherkin

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Re: Which motors
« Reply #15 on: January 23, 2018, 10:44:47 PM »

Hi Lifeboat,
I have a 5 foot Arun lifeboat boat running on 2 Emax BL 4030/10 turn 850 kv  brushless motors turning 80mm 4 bladed propshop props with 60 amp Hawk speed controllers and 2 12v gell cells batteries for ballast and power, it runs great at scale speed. I can run it on 24v as well, but a little to fast.

Destroyer42
850kv x 24v = 20400RPM!!! I wouldn't recommend running that unloaded!


Lifeboat, feel free to skip the following man-maths but it should allow you to see how I got to where I am.






Assuming you wish to run at 12v using your existing gel cells, and a target RPM of 4500ish, that gives us a required 375kv (4500/12)
We also want to know how much of a load the boat is going to produce. Take the HP of the real thing, and divide it by the cube of the scale. In this case, that's 570hp/12^3.
That gives us 0.33ish hp, but we want the answer in watts, so multiply that by 746, giving us 246 watts. No scale drive setup is perfect, so we want to give ourselves a 25% safety margin, so 246x1.25=307w, give or take. There are two motors generating that power, so let's say for simplicity, 155w per motor.

All we need to do now, is find a motor which is around 375kv, can handle 12v (a.k.a. 3S in the LiPo world, and what you'll see most motors rated to), and can handle 150w of power. (at 12v, that's 12.5 amps)





A quick search of Hobbyking later, and we have a winner.
https://hobbyking.com/en_us/ack-3515cp-400kv-cw-brushless-motor.html?___store=en_gb

This motor would work at 12v, it is man enough for the job and gives you the 4500rpm you want. They recommend a 45a ESC, but it's just as easy to find a 60a one.
https://hobbyking.com/en_us/turnigy-marine-60a-bec-waterproof-speed-controller-with-water-cooling.html

You probably wouldn't even need to watercool it as most of the time it'll be running at 1/4 of its rating.

And the bonus with this setup, if you got all this and found out that it was a bit too fast or slow, you could go up to 16.8v or down to 7.4v and it would be fine!
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Lifeboat 70-001

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Re: Which motors
« Reply #16 on: January 23, 2018, 11:36:46 PM »

Thanks for the info one and all, lots of maths Mcgherkin but understand but had a look at the motor your on about but has it got any tork because I thought the bigger the can the more tork a motor has got.
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McGherkin

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Re: Which motors
« Reply #17 on: January 23, 2018, 11:46:23 PM »

Yes and no, it can get a bit complicated as motors can be wound differently - the more poles a motor has, the less RPM it has but the more torque it can transmit. So you can have two motors of the same size, one spins very quickly but has very little torque, the other turns more slowly but has more torque. This will probably have quite a high number of poles for its size.
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RST

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Re: Which motors
« Reply #18 on: January 24, 2018, 01:26:06 AM »

With all due respect Mr Lifeboat, for all the requests for what type of boat it is, I don't see any answer and that's critical to what advice you should take.  You still haven't said what kind of model it is -that is not helping anyone at all - all the info I see here is speculative and pretty much useless unless you can give more info or a pic please!!!!
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McGherkin

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Re: Which motors
« Reply #19 on: January 24, 2018, 02:15:28 AM »

With all due respect Mr Lifeboat, for all the requests for what type of boat it is, I don't see any answer and that's critical to what advice you should take.  You still haven't said what kind of model it is -that is not helping anyone at all - all the info I see here is speculative and pretty much useless unless you can give more info or a pic please!!!!


Itís 70 feet long with an 18 foot beam, has 570hp, a top speed of 12 knots and his name is lifeboat.


I think we can safely deduce that the OP is building a Clyde class lifeboat  O0
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