Rpm?

[gumby]

I dont know much but i am guessing the more RPM the faster the boat will go with the correct prop...
I might buy that mini scat cat from BBY and put that mystery motor and esc in it but i was thinking why do so many people put in 3000RPM when they have 5000RPM for around the same price...

[lonewolf]

One of the brushless motor specs is KV that is how many rpms you get for each volt.
To use this figure multiply it by the amount of cells(1.2 nihm) you plan on useing ex: 12 nihm X 1.2 volts =14.4 volts you then X that by the kv of the motor ex: 14.4 x 2000kv =28,800 rpms unloaded. A nice sport rpm would be 25-30,000 rpm (correct me if im wrong) I hope this helps some in your mission of learning , it is a long journey but be patient you will learn to use the force....

[Sneasle]

Ok, lets go over this using a certain set of parameters:

Setup 1:
3000kv motor
30amp esc
2s li-po (7.4 volt)
same receiver, servo's, boat hull, drive train, prop.

Setup 2:
5000kv motor
30amp esc
2s li-po (7.4 volt)
same receiver, servo's,boat hull, drive train, prop.



This all being said, boat 1 will probably run at about 18-20 amps @ full throttle. Boat number 2 on the other hand, will probably run at 30+amps.

Yes, a motor with a higher kv will spin faster on the same pack, but it will require more overall power to do it, and in an enclosed area like a boat, this isn't really the best thing :)

If you want faster, use the 3000kv and go to a 3s battery. A higher kv means you will have a 'hotter' setup in many respects.

[Mich. Maniac]

These are true statements except one thing. If running LiPo at least I know remember your full charge pack will actually be at 8.4V. At least it will read this. After it warms up it may stay around 7.4V for majority of the run until your MAh is depleted. I was thinking the same way you were when I first started. I was gonna run a 6XL Feigao on 6S LiPo in a 36" V hull! boy that woulda been fun for about .02 seconds.

[NorthernBoater]

The reason everyone chooses 30,000 RPM is because this is a happy median with run time and speed. You can run a setup that will do 50,000 RPM but you will over heat everything or possibly destroy everything in about 10-20 seconds.

[Sneasle]

These are true statements except one thing. If running LiPo at least I know remember your full charge pack will actually be at 8.4V. At least it will read this. After it warms up it may stay around 7.4V for majority of the run until your MAh is depleted. I was thinking the same way you were when I first started. I was gonna run a 6XL Feigao on 6S LiPo in a 36" V hull! boy that woulda been fun for about .02 seconds.

Li-po is 3.7v nominal, nimh is 1.2v nominal. Yes, a fully charged lip is ~8ish volts, but a fully charged nimh is also above ~8v, so i didn't see the need to state that.

The point was, all things being equal, going to a 'hotter' motor will give a certain set of results, while bumping up to a 3s will cause a different set of results, each are good and bad in their own way.

[ED66677]

this thread will help you to choose the appropriate motor according to the hull size, you will see that basically the proper method is to decrease KV and increase cell count as the hull size increases!
The first value of the KV pretty much always give a no load rpm around 40000 and the second one gives a value around 25000rpm!
And it is exact for me : 29", 8XL with 2084KV, 4S, prop is 40-42mm!

[Dr. Jet]

I have tried both the 3000 Kv motor and the 5000 Kv motor you mention. An important factor in selecting a motor is the torque constant, or Kt. Kv and Kt are both important factors in selecting a motor. The 5000 rpm/V motor you are seeing just does not have the torque to spin a readily available propeller. You would probably need something like a 20 mm prop. I have yet to see one available in that size. The 3000 rpm/V motor will turn propellers in the 27~30 mm range, which means you have a selection of readily available prop from which to choose.

Many boaters choose an appropriate Kt motor without really knowing it, when they select a motor can length, i.e. an "S" can versus an "L" can versus an "XL" can versus an "XXL" can. These motor can lengths are directly proportional to the torque of the motor. In other words a 3000 rpm/V XXL can motor has a magnitude more torque than a 3000 rpm/V S can motor, and the longer motor can turn a much larger prop.

The rationale that many boaters use to choose can length is the motor's ability to absorb and dissipate heat, but again this really is an issue of torque and the ability to spin a particular prop size.

Back top the little Chinese outrunners that are the root of your question, I made a little gearbox for the 5000 rpm/V motor (which by the way is a much shorter can than the 3000 rpm/V motor), and saw almost identical performance and current draw as the 3000 rpm/V motor. The additional complexity was not worth the effort.