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Old Apr 06, 2011, 11:18 AM
Registered User
South Africa
Joined Dec 2010
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Discussion
Motor for Tug boat

Hey all

Im building a rc tug but I have no idea what type of motor I should use to power it.
The tug doesnt have to be fast,(want to have scale speed) but it must have quite a bit of power.
I'll be using a 3 blade prop with a shallow pitch (robbe 50mm 3 blade prop). What rpm should the motor have to use a prop like that effectivly? The motor must be direct drive.
Tug is +-70cm long and quite heavy.

Thanks
Philipp
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Old Apr 06, 2011, 07:09 PM
Old wreck in Milwaukee
Prins Willem's Avatar
United States, WI, West Allis
Joined May 2007
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I have always had good luck with Dumas Pittman motors #2204 12volt or 2005 6volt. They turn slow so your tug won't plane, have good torque, and low amp draw (less than 2 amps). All my tugs are direct drive. Never had a heat problem even when running in weeds. These motors are not cheap but my oldest one is from 1979.

http://www.dumasproducts.com/index.php?cPath=66_67
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Old Apr 07, 2011, 10:24 AM
Straight up is all I ask
Fairfield Ct USA
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Wow $75 for the dumas pittman I guess they re evaluated their stock . I got to go catch my breath .......
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Old Apr 16, 2011, 01:05 PM
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South Africa
Joined Dec 2010
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Hello everyone
I've found 2 suitable motors.
The one is a 65t rc crawler and the other is a 80t rc crawler motor.
The 65t no load rpm at 7.2v is 7000rpm
the 80t no load rpm at 7.2v is 5500rpm
So which one would be better suited for my tug? (im assuming the 80t because of lower rpm?)
What would the ideal max prop rpm be for a scale tug?
Thanks
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Old Apr 16, 2011, 06:11 PM
NeverAgainVolunteerYourse lf
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Australia, QLD, Regents Park
Joined Mar 2007
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I would probably go the 80 T motor the RPM is closer to what is optimum for your requirements.
Nick
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Old Apr 16, 2011, 06:19 PM
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If the two motors share the same core,armature,magnets,air gap and timing then more turns means it is wound for higher voltage.You would not be getting more torque or the effect of a reduction gear.

You can go wildly wrong http://www.rcgroups.com/forums/showthread.php?t=1242653
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Old Apr 16, 2011, 06:22 PM
NeverAgainVolunteerYourse lf
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True, but the prop needs less torque at lower RPM and the motor will be spinning at closer to peak efficiency

Nick
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Old Apr 17, 2011, 12:13 AM
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United States, PA, Slatington
Joined Feb 2007
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I would recommend looking at this motor direct drive on 12v.

http://www.surpluscenter.com/item.as...tname=electric
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Old Apr 17, 2011, 01:30 AM
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Quote:
Originally Posted by nick_75au View Post
True, but the prop needs less torque at lower RPM and the motor will be spinning at closer to peak efficiency

Nick
The peak efficiency and rpm would be the same for both winds.
The 80t motor would need 80/65 times the voltage of 65 t motor,for the same job.
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Old Apr 17, 2011, 03:27 AM
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proteque's Avatar
Aukra, Norway
Joined Aug 2010
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I have until now used the Aquacraft 550 low rpm motor. I am very happy about it so far. Running with 7.2V at direct drive
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Old Apr 17, 2011, 05:44 AM
NeverAgainVolunteerYourse lf
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Should have looked at the OP more carefully
You will want to spin that prop at around 4500-5000 rpm at full throttle, this RPM will be about 75 % of no load speed of the motor under Ideal circumstances

So let me get this correct Lambsy
so at 7.2 volts the 80 turn crawler motor is going to be better, 6 volts for the 55 turn motor for the same performance?

I know for a fact that a 40 mm prop at 6 volts and 6000 rpm is about 25 watts input power

Nick
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Old Apr 17, 2011, 07:32 AM
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Canada, ON, Peterborough
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6v will draw more amperage, simple as that,
watts = v x A
say 10 amps x 7.2 volts = 72 watts, 72 watts/6 volts = 12 amps.
Thats all things being equal for same performance. Up the volts drop the amps.
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Old Apr 17, 2011, 11:58 AM
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South Africa
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Thanks for all the replies
The 80t motors rpm at no load is 5500 and at
max efficiency 4450 rpm at 7.2v
I'm using a 50mm prop so the rpm should be ok for the tug right?
Philipp
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Old Apr 18, 2011, 12:06 AM
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Quote:
Originally Posted by siberianhusky View Post
6v will draw more amperage, simple as that,
watts = v x A
say 10 amps x 7.2 volts = 72 watts, 72 watts/6 volts = 12 amps.
Thats all things being equal for same performance. Up the volts drop the amps.
On the same prop a motor will draw less amps at the lower voltage, because the RPM is lower and the prop doesn't load up the motor as much so watts(power) is lower.

Nick
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