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What is the right RPM for my boat - This can not be right

Place for general submarine conversation

Postby 75-1045824342 » Fri Apr 18, 2003 6:48 pm

Hi Subs,

Im building a 1:96 Ohio class, almost 2 meters long, and it has a 7 blade prop.

If you download the following AVI of my motorsection pre-test you will notis an extreamly high RPM (I think it is):
(About 3,5 Mb)

- And I would just like to know if this is normal, as I have not seen any RC Submarine with my setup.

My setup are as follows:
- 12 V battery power
- Im not sure about the motor but it should be a speed 550!! (Voltage ??)
- 3:1 belt reduction unit.
- Home made Motor regulator.

Im interrested in knowing what others use as motor, as mine might not be correct - apparently.

Best Regards
Kim Mortensen

Postby 68-1046821426 » Fri Apr 18, 2003 7:49 pm

I tried getting to your site but Tripod wouldn't allow it to download. Why are they so difficult? :;):

Mike Day SC#2502

Postby Dolphin » Sat Apr 19, 2003 12:41 am

Hello Kim,

A subjective interpretation of what is high, low, or correct RPM is somewhat irrelevant here. The model prop rpm will never match the 1 to 1 scale prototype, the pitch and surface area of course are radically different in size, and the density of the water (allowing for that we are discussing here fresh water instead of salt water) are a relative constant. Basically written, you can scale down everything but the water.

Perhaps a better measure would be to find out (with your 3 to 1 gear [or belt]) ratio what amperage does your Graupner motor draw? A 3 to 1 ratio sounds correct here. Take a amp meter and test what the motor draws while the propellor is immersed in water. The stern end in a swimming pool or bath tub. Considering the size of a 1/96 Ohio, then you better consider a pool!

The less draw, the cooler this motor works (the motor will get warm anyway) and the longer the running time will be from the same given battery charge. A large motor, like 1 to 2 amps might be acceptable for a 550 Graupner, lower the better. Careful alignment relationship of the motor, shaft, water tight seals, and bearings can effect this. It varies much, if it was a beautiful Maxon motor, I would be concerned about 1 to 2 amps being maybe too high. So it depends. Graupner motors are fine for the price, though I am sure everyone has their favorites.

Build her good and straight forward water tight first, then refine her later as you grow in experience as to what is important and what is not. Try not to succumb to analysis paralysis. It is very easy to do, I have! Hope this helps.

Steve Reichmuth

Postby 75-1045824342 » Sat Apr 19, 2003 3:44 am

Hi Steve,

Im not trying to scale everything down, but I realy thing that the screw will cavitate in the water - Its rotating faster than my kitchen blender (-:

I have started a FTP server so it can be downloaded from there (Dont know why im having so many trouble with tripods) but as its from my local machine, the link will only be valod as long as I can keep windows afloat (-:

My motor might be a johnson 550. and I can tell you that it draws about 4-5 Amps on my desk above water!!

You say that graupner motors are the one to go for... Ill have a look at it.

Kim Mortensen

Postby 75-1045824342 » Sun Apr 20, 2003 8:36 am

Hi All,

Well I have split apart one of those handu car tire compressors, and removed the motor, and I found that it was a much better RPM and power consumption, only 0.9 Amp in free run at 12V.

I mounted it in my sub, and powered it up, and it now had about 4 amps when fairly loaded (by hand). And I thing the water will load it that much, so Now Im happy about the speed of the prop.

I Thing the other motors I used was not for 12 V... No wonder it was on the breach of taking off from my desk...

(Anyway, if anyone would like the AVI, please email, As I cant seem to get the downloading to work..)

Best regards
Kim Mortensen

Postby Carcharadon » Tue Apr 22, 2003 5:47 am

Kim, just wanted to toss out a couple of ideas for what it's worth. I would guess that as a rough approximation of the motor RPM versus motor heat, you could run the prop in water and hold the motor. If the motor gets too hot to hold I would take this to mean that the RPM is too high for the prop pitch. Also if you've gone to the trouble to take apart an air compressor motor you might want to consider the motor in a bilge pump. You could probably remove the bilge pump impeller and somehow replace it with the shaft to turn the prop. Although I haven't done this I would think that a bilge pump motor would be more suitable than any air compressor motor. Also it's designed to run in water so your motor could be outside of the watertight chamber.
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Postby 75-1045824342 » Tue Apr 22, 2003 8:38 am

Is it possible to provide me with a picture of such a blich pump, and possibly where to get it (-:

Well the first motor would get hot even when running in free air!! So I didnt hesitate to remove it from the hull. The You go through the process of taking all items apart that might have a motor insede that would fit.

The Compressor motor (The cheep compressors you to pump the wheels of your car) was a high tourque motor with great RPM, If this will not do the trick, nothing will (-:

But as the motor type is unavaliable, im still looking for motors that might replace the compressor motor.

Best Regards
Kim Mortensen

Postby Bob the Builder » Tue Apr 22, 2003 8:47 am

I know that this has been beaten over the head before, but you may wish to consider the cooling fan from a car's radiator.

I'm using such a motor. It's about 3" long, 2.5" diameter and has a 5/16" shaft. My initial test revealed current draw at 1A free running, 4A under full load (that's as much friction as I could apply with my bare hands... I'd imagine that propelling my sub will require far less torque), and 14A fully stalled. This little sucker will virtually screw off the lug nuts from your car! My cost: $4.00CAD. It's a 10 pole motor and runs ultra smooth forwards and reverse. Mine also came with the capacitors installed to reduce radio interference. I don't know if that's standard practice or if I just lucked out.

I've got pics of the motor posted on my site at The Nautilus Drydocks. Just click on the "electronics" button at the top of the page and scroll down to see the motor (pic is on the right hand side).

If you have any questions, I'd be happy to help out. :cool:
Bob Martin,
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Postby Carcharadon » Tue Apr 22, 2003 10:20 am

Kim, any marina should carry a variety of bildge pumps


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Postby 75-1045824342 » Wed Apr 23, 2003 6:51 am

The Radiator fan motor seems to be right, Ill look at the option as soon as I go to the scrap heap.

Thanks for the help...

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