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Author Topic: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat  (Read 5714 times)

flack

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Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« on: November 19, 2014, 05:43:04 pm »

About to start the build of my Waveney Lifeboat and decided its time to bite the bullet and go brushless. The instructions recommend Graupner 900 BB's which apart from being impossible to get hold of would, I feel, overpower the model.

After a fair amount of research and some very wide ranging advice I seem to have arrived at the following options:

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=29565 for the speed controller

http://www.marionvillemodels.com/rc-shop/overlander-3548-900kv-tornado-thumper-v2-brushless-motor-rc-outrunner_1762.html for the motor

It will be fitted with a pair of M4 2022/3 brass props from the Prop Shop.

Would appreciate any advice or experiences about this set up please.

Cheers Shaun
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nick_75au

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #1 on: November 20, 2014, 08:07:09 am »

The speed 900 is about 550 Kv so the motor you selected is faster and more powerful than the 900, ;)


For a 50 mm prop and style of hull you want to aim for about 4000-5000 RPM 


One missing bit of information is what voltage you intend to run on?
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flack

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #2 on: November 20, 2014, 09:47:44 am »

Hey Nick,

Thanks for the reply and your observations. I'm hoping to run the boat on 12v gel lead acids mainly because I have a few but also because I need the weight for ballast.

Cheers Shaun
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nick_75au

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #3 on: November 21, 2014, 10:57:39 am »

OK,


The issue you run into with displacement models is finding low enough Kv to achieve the RPM required, not an insurmountable problem, just a bit of homework :-))


For 12 volt and 5000 RPM 415 Kv is indicated though anything up to the 550 Kv of the speed 900 will be OK, most motors in this range are capable of much more than a 50 mm prop will ever demand, so you end up with a much larger motor than necessary (but will probably be about the same dimensions as the 900   ok2 though in the case of your vessel this probably is of minimal consequence, + 20%

Nick
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Calimero

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #4 on: November 21, 2014, 02:22:39 pm »

Here's the motor finder on HK : http://www.hobbyking.com/hobbyking/store/brushless_motor_rc_data.asp


There's a nice 480kv Helidrive outrunner that could match. Way overpowered, hence should run cool with "tiny" 50mm props.


http://www.hobbyking.com/hobbyking/store/__21869__Turnigy_HeliDrive_SK3_Competition_Series_4962_480kv_700_90_size_heli_.html


I'd go with car ESCs. Get some 60/80/100A car ESCs with nice heatsinks and fans and you can ditch all the water cooling.
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nick_75au

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flack

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #6 on: November 25, 2014, 01:54:24 pm »

Thanks for all the advice, I eventually opted to go for 2 x Turnigy D3542/5 1250KV Brushless outrunners with the following spec:

Battery: 2~4 Cell /7.4~14.8V
RPM: 1250kv
Max current: 46A
No load current: 3A
Max power: 665W
Internal resistance: 0.021 ohm
Weight: 130g (including connectors)
Diameter of shaft: 5mm
Dimensions: 35x42m
Prop size: 7.4V/11x7 14.8V/9x6
Max thrust: 1350g

Slightly less watts than the speed 900's but I thought they would overpower the boat anyway.

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nemesis

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #7 on: November 25, 2014, 07:35:21 pm »

Hi, Please remember that the white line on the hull is not the waterline. It is placed where it is so it does not get dirty when lying afloat. That was what I was told by Robbie Maiden, the "Scout" cox when I was building mine. Nemesis
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flack

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #8 on: November 25, 2014, 08:51:14 pm »

Thanks for that info Bill, I was only reading something on a build blog somewhere about the plans showing the waterline in the wrong place, this seems to explain the discrepancy.

Cheers Shaun
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nick_75au

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #9 on: November 25, 2014, 10:32:15 pm »

So you opted with an even higher kv. That same motor I use in my ski boat on 11.1 volts and a smaller 40 mm prop. I think it's a bit ambiguous to expect the motor to work well .

Nick
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flack

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #10 on: November 26, 2014, 08:58:12 am »

I'm really confused about what the various specification values mean then Nick, I opted for the motors based upon the Wattage which is about a quarter less than the speed 900's and thought this would be a good compromise.

How does the value Kv fit into this?

Shaun
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nick_75au

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #11 on: November 26, 2014, 10:50:28 am »

Kv is RPM per volt so a 1000 Kv motor spins at 12000 rpm at 12 volts, (1000 x 12) 500 kv spins at 6000 rpm at 12 volt, the motors you selected are 15000 rpm at 12 volt.


Sorry if I came across a bit sharp :embarrassed:  I assumed you knew what Kv was.
Have a read of this article I wrote a while ago, will help to explain the numbers :-)

http://www.rcgroups.com/forums/showatt.php?attachmentid=4261513&d=1315289844
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flack

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #12 on: November 26, 2014, 08:41:29 pm »

Thanks Nick,

I understand the Kv being RPM per volt relationship.

From your very good article this is defined as the "no load speed" of the motor, I assume this means the motor is spinning in air and once operating in water (i.e. under load) will be different, almost certainly lower?

Please excuse my ignorance as I am very new to the electric motor, be it brushed or brushless as all my previous modelling experience, quite a few years ago, has been with I/C motors

Shaun.
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nick_75au

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #13 on: November 30, 2014, 04:51:32 am »

Yes its no load speed, the motors power will influence how much the load causes the RPM to drop, a higher amp (watt) capacity motor of the same KV wont drop RPM as much as a lower amp motor, (if that makes sense?)



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flack

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Re: Metcalf Mouldings - "The Scout" Waveney Class Lifeboat
« Reply #14 on: November 30, 2014, 09:03:26 pm »

Thanks Nick. By the time I've studied your article a few more dozen times I might start to get my head round it - trouble is something else will come up!
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