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Author Topic: Beginners question motor prop choice  (Read 1809 times)

Stuartsmall21

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Beginners question motor prop choice
« on: July 14, 2013, 08:56:21 pm »

Hello


I've just started cutting the frames and keel for a 40 inch model of a WW1 coastal motor boat which is preserved at Duxford. I've been thinking about which motor to choose. I'd like to see the model plane and the original was lightweight with a petrol V12. The prop on the original is a two bladed version, and scaled down should be 50mm.


What motor would you recommend? My first model boat had a graupner 500 and I was disappointed with the performance.


Thanks
Stuart
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imsinking

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Re: Beginners question motor prop choice
« Reply #1 on: July 14, 2013, 09:05:13 pm »

It would depend as to wether your driving DIRECT or thru a gearbox , an 850 motor would struggle with a 50mm prop direct (and the prop pitch would come into the equation too) thru a gear box you'd need a high reving motor to counter the reduction in rev's @ say 2:1 but it should manage a 50mm prop , the torque is increased dramatically thru a gearbox  %%  NOW YOUR CONFUSED  O0  . . .
Bill  :-X
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Stavros

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Re: Beginners question motor prop choice
« Reply #2 on: July 14, 2013, 10:33:05 pm »

It would depend as to wether your driving DIRECT or thru a gearbox , an 850 motor would struggle with a 50mm prop direct (and the prop pitch would come into the equation too) thru a gear box you'd need a high reving motor to counter the reduction in rev's @ say 2:1 but it should manage a 50mm prop , the torque is increased dramatically thru a gearbox  %%  NOW YOUR CONFUSED  O0  . . .
Bill  :-X

Sorry to say this but in fact you are wrong in stating an 850 would struggle with a 50mm prop directly .....it will most defo,but it will pull high amps and will run HOT without water cooling.

Personally I would run a 900 motor water cooling it 55mm 2 bladed prop on 2 stick packs giving 24v....ACTion Elsctronics
I know this will have it planing with plenty of speed and should run for about 45mins if driven flat out and obviously longer if pootling around


Dave
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Stuartsmall21

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Re: Beginners question motor prop choice
« Reply #3 on: July 17, 2013, 09:18:46 pm »

Thanks for the advice! I've been looking at gearboxes, Cornwall Model Boats sell one which allows two 600 size motors to drive a single shaft. Has anyone else tried this method and found it successful?


Thanks
Stuart
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pompebled

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Re: Beginners question motor prop choice
« Reply #4 on: July 17, 2013, 10:03:54 pm »

Hi Stuart,

If brushless is an option, take a look at the motorisation in my M.A.S. build:
http://www.modelboatmayhem.co.uk/forum/index.php/topic,34033.0.html

The setup is much lighter than the brushed motor I originally had planned.
As you can see in the running pics, I have more than enough power, running on 4S.

Should that prove too fast for your liking, run on 3S, much more scale like (but less fun...!).

Frankly, a brushed motor in a 40" hull isn't the best solution, unless you go for a fan motor as my original idea was.
This fan motor will turn the prop easily.
The 800 and 900 motors are rather inefficient (up to 30% of your power is lost, transferred into heat) and require watercooling.
Gears are very noisy in general.
The twin 600 motor on a single shaft is an outdated concept from the times cheap motors had to propell too large boats on a budget...

Do you have some pictures of your build, please?

I checked the type youre building, it's a stepped hull!
This requires a lightweight build in order to run as the original, meaning no heavy NiMH and brushed motors, at least in my book...
Check out the running pics in my thread.

Regards, Jan.
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Stuartsmall21

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Re: Beginners question motor prop choice
« Reply #5 on: July 20, 2013, 10:29:14 pm »

Hi Jan
Thanks for the advice. I am tempted to agree, brushless (even though I know nothing about it) seems to be the sensible, smaller option. I've only just laid the keel on the building board and started to glue in the stringers. As soon as she looks boat-like I'll put some pictures up.
 
Thanks
Stuart
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pompebled

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Re: Beginners question motor prop choice
« Reply #6 on: July 20, 2013, 10:54:03 pm »

Hi Stuart,

Something to consider in your build; the reason my M.A.S. is semi scale, isn't just that I went from two to one propshaft, but I also changed the position of the prop, from right at the edge of the transom to further under the hull.

Reason for this is the amount of air, drawn under the hull by the steps; a scale size prop will be too small to be not affected by the water air mixture it'll sit in, once the boat picks up speed.

As you can read in my topic, a scale size prop suffers massively from cavitation, effectively slowing down the boat while the revs go up...

Only a much larger prop keeps sufficient bite to keep propelling the boat despite the air drawn under the hull.

I went from a Ø 40 mm two blade to a Ø 60 mm two blade, high pitch prop.
Tomorrow I'll test with a lower piched 55 and 60 mm prop to see what I can get away with...

Regards, Jan.
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pompebled

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Re: Beginners question motor prop choice
« Reply #7 on: July 21, 2013, 08:47:39 pm »

Tomorrow I'll test with a lower pitched 55 and 60 mm prop to see what I can get away with...
Well, that didn't work out as planned, the response from the boat wasn't what I expected and the topspeed wasn't to my liking.
Going back to the high pitched Ø 60 mmX-prop made the boat run as I wanted; scale(ish) at half throttle and blistering fast at full throttle, skimming through the turns like an airboat...

So Stuart: Think Big! (at least for the prop).

Regards, Jan.
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