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Author Topic: does High RPM = High speed?  (Read 4814 times)

Lord Bungle

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does High RPM = High speed?
« on: December 20, 2010, 11:51:34 pm »

I have managed to confuse myself again (not that hard)
I was looking at the graupner speed 600 as they are 1/2 price, these are going in my twin engined Huntsman (approx 34 inches)
it has 540s in it at the moment but these are starting to smoke a bit (probably due to the 12 volt battery I am running them off)
the boat doesn't really run right (another semi built off ebay job) anyway.

then I found the list of details at http://homepage.ntlworld.com/edward.matthews/ddmbc_files/motors/graupner.htm
and have managed to totally confuse myself.
If this is to be believed a speed 600 rated at 7.2 volts has a no load rpm of 18,200
but the speed 600 rated at 8.4 volt has a no load rpm of 15,500 so turns the prop slower, but at a higher Max Efficiency,

these both turn faster than the graupner 700bb turbo that some have said I should use in my mfa spear.

so what is more important on a hull that you wish to have up on the plane High RPM or High Torque?


the spear didn't plane with a very high torque but low rpm motor or am I just making it to complicated?
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Shipmate60

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Re: does High RPM = High speed?
« Reply #1 on: December 21, 2010, 12:34:04 am »

Just to confuse you even more it is down to the prop too.
1 large motor with a large prop can have a huntsman planing at about 9000 rpm but with a 60mm prop.
Once you have decided on the motors the fun can start with props.
That is whi it is usual to start with a known set up for a popular boat and see what voltage /speed you prefer.
One thing to bear in mind is that graupner motors will take a higher voltage than stated and give far more performance but at the cost of consumption.

Bob
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Lord Bungle

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Re: does High RPM = High speed?
« Reply #2 on: December 21, 2010, 12:45:56 am »

was thinking 40 or 45mm brass props (havent messured them yet but they are on the boat) and was thinkin 12 volt on the moter as you say these units can take a bit more, but what ratting ? the 7.2 volt seems to have a higher rpm than the 8.4volt one.
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Shipmate60

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Re: does High RPM = High speed?
« Reply #3 on: December 21, 2010, 12:50:09 am »

The faster revving motors need a 2 blade racing prop as some of the 3 blade props can cavitate at higher revs.
The best idea is to try it!!

Bob

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Lord Bungle

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Re: does High RPM = High speed?
« Reply #4 on: December 21, 2010, 12:56:07 am »

well as they are 1/2 price I was thinking I may buy 2 of each try them (hopefully get a discount on the post)  {-)
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brianB6

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Re: does High RPM = High speed?
« Reply #5 on: December 21, 2010, 02:40:46 am »

I used a Sagami S3560 in my 30" Thornycroft ASR (a beautyfull 550 size racing motor, unfortunately no longer made) that has high revs and torque running on 7.2 volts, which did nothing with 3 bladed props but started to sing with a Graupner X35 mm.
You should try out various sizes and pitches in cheaper plastic props before lashing out on brass etc.
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nick_75au

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Re: does High RPM = High speed?
« Reply #6 on: December 21, 2010, 04:04:23 am »

The 7.2 volt motor will draw much more current compared to the 8.4 motor, the faster you try to spin the prop the more current is required to achieve the RPM.

 7.2 =2500 Kv

8.4 = 1800Kv

so at 12 volts

7.2 =30000 RPM
8.4 = 21000 rpm 

lower RPM requires less torque and therefore less amps.

what ever motor you choose the prop RPM for 45 mm would be about 10-15000

The old Raboecsh chart recommended max 7800 for 45mm (as mentioned
 can be exceeded)
Both motors would be better off with a gear reduction of some sort around 2:1 -3:1 to get the best efficiency and achieve the best RPM for the prop.

The 540s are smoking because thy are trying to spin the prop at RPM that exceeds the torque the motor can output, I would say that the motors are spinning at less than half their no load RPM. so even though they may be high speed motors the load is too great for them to be able to achieve it.

For best efficiency the motor should, under load be about 75% of no load RPM

Nick
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rathikrishna

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Re: does High RPM = High speed?
« Reply #7 on: December 21, 2010, 06:08:11 am »

Great...it is a valuable advices...helpful...thank you...
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Lord Bungle

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Re: does High RPM = High speed?
« Reply #8 on: December 22, 2010, 08:11:23 pm »

I used a Sagami S3560 in my 30" Thornycroft ASR (a beautyfull 550 size racing motor, unfortunately no longer made) that has high revs and torque running on 7.2 volts, which did nothing with 3 bladed props but started to sing with a Graupner X35 mm.
You should try out various sizes and pitches in cheaper plastic props before lashing out on brass etc.

I braved the snow today to get to the shed and messured the props, No I bought the boat as a semi finished/ready built/altered model (I have a funny impression it may have been a single prop i/c powered model not to long ago)
any way I measured the props they are 45mm brass 4 bladed items, so may change these for 2 blade plastic ones.
this will leave me with so nice props and some motors in my parts bin, oh well I may have to build so hulls myself soon  :-))
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triumphjon

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Re: does High RPM = High speed?
« Reply #9 on: December 22, 2010, 09:03:10 pm »

the full sized huntsman was fitted with a pair of 16x17 " three bladed props that was handed ie left and right rotation , rough equivelent s about the 35mm diameter mark . my 28 is currently running a pair of 30 mm 3 bladed plastic graupner props , driven by a pair of 6 volt speed 400s on 7.2 volts , but owing to a pair of faulty speed controllers im unable to improve my performance yet ! having looked at a spear last summer it too was running on 600 motors from a 12 volt nicad pack if youd like i can contact him to see what set up hes using ?
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Lord Bungle

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Re: does High RPM = High speed?
« Reply #10 on: December 22, 2010, 10:31:49 pm »

that would be a great help mate,  The Huntsman will be running 2x speed 600 in either 7.2 or 8.4 (they were 1/2 price so I got 2 of each  ;) ) and will see how well they run with 12 volts running through them (12 volt each motor) this will hopefully give them some poke :)
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Subculture

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Re: does High RPM = High speed?
« Reply #11 on: December 23, 2010, 07:24:16 am »

I think that you will find you have too much prop for those motors- you will cook them like the 540's. You need at least 2:1 reduction as advised earlier.

If you want directd rive brushless outrunner is the way to go, and cheaper than brushed too if you know where to shop. Does mean new controllers though.
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malcolmfrary

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Re: does High RPM = High speed?
« Reply #12 on: December 24, 2010, 09:36:28 am »

For prop sizes, the usual rule of thumb is that the prop diameter shall not exceed the motor can diameter unless there is gearing involved which effectively increases the can diameter.  To get a faster boat, counter intuitively, a smaller prop is usually better.  This is because the smaller prop spins faster, and this causes a faster moving column of water, and the boat can never go faster forward that that column being shoved backwards.  Of course, there is also such a thing as "too small", and with the right diameter, "too much pitch" and "too many blades".
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Lord Bungle

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Re: does High RPM = High speed?
« Reply #13 on: December 24, 2010, 10:48:37 am »

thank you malcom I shall use that as a good rule of thumb :) keep the prop smaller than the motor can  :-)) I think even I will be able to remember that  :}
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Subculture

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Re: does High RPM = High speed?
« Reply #14 on: December 24, 2010, 11:10:56 am »

That is generally the case, however it depends greatly on how the motor is wound.

A 540 wound for high torque will pull a prop double it's diameter without complaint.

Note that none of the Graupner range will do this.

I have a small brushless outrunner motor I'm fitting in a submarine currently under construction. This has a can diameter of just 28mm, yet it pulls a 50mm two blade prop around at about 10000RPM (under load) with ease.
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rathikrishna

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Re: does High RPM = High speed?
« Reply #15 on: December 24, 2010, 01:57:52 pm »

Friends..i am willing to share my experience here...all we know that , if we fed a motor to a battery pack, in direct, surely it will drain more amps at will...then if we have a speed controller based on a simple electronic circuit, it will be a different story..i am adding this short cut of my postings and video here...try once..and feel the difference...
http://www.modelboatmayhem.co.uk/forum/index.php?topic=27882.0

http://www.youtube.com/watch?v=SS3gkDgQSz4....if we can control the draining amps, our motor will work for long time, because all we know most of the time the motor revs at high as a waste...by making lots of cavitation in water...so my study shows after a particular speed the prop never make any good results but bad...so if we control the motor amps the life of the motor also increased...this way of thinking helps more related to a full displacement craft, as it makes more drag to foreward....
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