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Old Jan 07, 2013, 09:17 AM
Ragbagger's Avatar
St Simons Island Georgia
Joined Jan 2010
49 Posts
Engine Temp and Prop Size

I am ready to close up the ER of my Billings Andrea Gail. I have gotten back to the project after a long time off. I have powered the hull with 2 x Speed 400 on a 4:1 gear turning a Prop Shop 2"x 2.2" LH 4 Blade(described as being good for models to large models using 550 geared motors). Needless to say I made extensive modifications in the depth of the hull, rudder ect.

I burned up one Speed 400 6V on the first run using a 3:1 gear. I then installed 2 Speed 400 @ 7.2v and changed the gear to 4:1, and installed some cooling sinks. The 7.2v motors on the 4:1 gear run much cooler and everything seems fine. After running on my pond for 3 hrs continously my motor temps are 113 -114 degrees at 70 degree ambient temp according to my electronic scanner. The temps were scanned on the cooling sinks. The brush part of the motors only showed 70 degrees. Good hull speed, great handling, beautiful scale look at scale speed, and I could not be more pleased.

I looked extensively and I cannot find any firm data about motor operating temps. Just stuff like "if it feels too warm" or "too hot to touch". If 113-114 degrees is too hot, I may have to de-prop. The motors did not feel "too hot" at all.

Water cooling is out. I tried to fabricate a cooling system and failed twice. Engines are too close together for manufactured jackets.

I do have a PC cooling fan that mouints on top and drop temps significatly. I will need that later to deal higher ambient temps we have here , 80-90 degrees.

I have two questions---------

1. What are normal numeric operating temps of the Graupner Series motors and if I am ok?

2. If my engine temps are still a little high, I need to De-prop, what option is best?

De-prop otions are:

1. Go to same prop with 3 blades. With real boats, You don't get too much decrease in thrust by changing the number blades, but you do get different
performance curve.

2. Go t 1.7" x 1.9" 3 or 4 Blade prop. At 4 blades it would be a little smaller and a little less picth.

I attached some pics for your reference.

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Last edited by Ragbagger; Jan 07, 2013 at 09:22 AM.
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Old Jan 07, 2013, 09:41 AM
Vita ex Machinis
patmat2350's Avatar
Motor City
Joined Dec 2004
12,899 Posts
Personally, I wouldn't obsess over numbers... rule of thumb is if you can put your thumb on the can and not say "ouch", it's thumbs up.

Sounds like you have a working solution there, can't wait to see the rest of the build.
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Old Jan 07, 2013, 11:10 AM
Straight up is all I ask
Fairfield Ct USA
Joined Apr 2002
2,328 Posts
I would say your pitch speed with that setup is 15-20 mph . On a 5mph hull . A 2.2pitch at 7000/8000 rpm . Check it out on a tach and see what rpm is unloaded . I flew geared speed 400's for years and am running a pair of those 2x2.2 props in my dumas osprey . I am using a slightly hotter brushless sp400 for a 20 mph top speed .
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Old Jan 07, 2013, 09:54 PM
Registered User
United States, PA, Slatington
Joined Feb 2007
416 Posts
My guess is your at or slightly over the max load for this setup. If this was my model my first idea would be to increase the gear ratio to 5 or 6 :1 by changing the spur gear to a larger one and or decrease pinion gear. Second choice would be find a prop with less pitch and if you have to small diameter. The last choice would be to replace the motor with a direct drive 555 on 7.2 to 12v. Below is my truckabe Tug with a 555 2580 motor turning a 2 inch 4 blade prop on 14.4v (4S). Here is the link for the motor and a picture of the install and prop. Good Luck and keep us posted.

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Old Jan 08, 2013, 02:06 PM
"day ain't over yet-"
der kapitan's Avatar
Western N.Y. winemaking country
Joined Jun 2005
9,801 Posts
There was a guy in our club who built beautiful models, but his drive systems were always screwed up.

One memorable installation was a pair of 540 motors turning 1-3/4" props, and geared 1:3---. He wondered why the motors overheated, and his batteries barely got him past the buoy gate---.

It doesn't exactly take rocket science---.
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Old Jan 08, 2013, 02:24 PM
Straight up is all I ask
Fairfield Ct USA
Joined Apr 2002
2,328 Posts
Ragbagger what are you running for a battery ? That makes a big difference .
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Old Jan 08, 2013, 05:58 PM
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Tollytime's Avatar
United States, MI, Macomb
Joined Nov 2006
1,328 Posts
Hmm, I'm not sure what is hot on a brushed motor, but brushless motors should stay below 180F. 130F to 140F is ideal, but a hard run may require a cool time. Since brushed motors have more friction, they do tend to get hot when used for long periods at high rpm's.

Have you thought about a more powerfull brushless option with no gearing at all?
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Last edited by Tollytime; Jan 08, 2013 at 06:05 PM.
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