May 22, 2011, 11:58 PM
Registered User
Discussion

# 30 000 rpm, the magic number.

I know now that one should aim to get a motor that spins at 30 000 rpm's for optimal power and speed. But that is unloaded right? What is the optimal loaded rpms? I want to look more into this as some motors will loose more rpms in the water than others. When you take a 2835 and a 2860 both at 2200kv, surely the 2860 will have a higher rpm in the water? Does this mean when getting a smaller motor like a 2835 that you should rather look at a 2500kv motor to get the same speed as the 2860?

 May 23, 2011, 12:03 AM I feel the need for speed! The 2835 2500kv wont be able to spin as large a prop therefore speed drops anyways. A motor can only put out so many watts(power) so a smaller one with higher kv still cant compete with a larger/lower kv motor in top speed. The larger motor can turn more prop. If you have a 4 cylinder car and give it a 6th or a 7th gear, it doesnt have enough power to rev out all the way and will "bog" down................
 May 23, 2011, 12:42 AM Registered User So what exactly is the optimum RPM for loaded regardless of prop size? I assume there would be a point where the cavitation would cause barely any increase in forward momentum. I have noticed lots of the mind blowing speed boats are actually in the 40-50k range unloaded.
May 23, 2011, 01:38 AM
Quote:
 Originally Posted by SARCFAN I know now that one should aim to get a motor that spins at 30 000 rpm's for optimal power and speed. But that is unloaded right? What is the optimal loaded rpms? I want to look more into this as some motors will loose more rpms in the water than others. When you take a 2835 and a 2860 both at 2200kv, surely the 2860 will have a higher rpm in the water? Does this mean when getting a smaller motor like a 2835 that you should rather look at a 2500kv motor to get the same speed as the 2860? Please share
Look at this chart I have posted 4,987,543 times. Like you say, around 30k is good for sport setups and the higher red + some, for saw setups.

I have always said, lower kv motors with more volts and bigger prop is best. Try this method and just 'gear' up or in our case, prop up. Get the best, highest 'C' rated battery you can afford and try to get an onboard datalogger to measure your amps. Of course, check your temps with a proper guage of somekind and don't say, cool to touch or warm or my finger didn't burn. Do it right and you will be rewarded with a great running setup.

Biggest kv I have in a f/e is around 1600kv. I just give it more volts and pick prop accordingly. Torque is your friend in f/e and always remember that.

Pete
Last edited by 10x8; Dec 12, 2014 at 12:45 AM.
 May 23, 2011, 01:51 AM Cookie And, you'll typically drop 2-4k from unloaded.
May 23, 2011, 02:11 AM
Registered User
Quote:
 Originally Posted by 10x8 Look at this chart I have posted 4,987,543 times. Like you say, around 30k is good for sport setups and the higher red + some, for saw setups. I have always said, lower kv motors with more volts and bigger prop is best. Try this method and just 'gear' up or in our case, prop up. Get the best, highest 'C' rated battery you can afford and try to get an onboard datalogger to measure your amps. Of course, check your temps with a proper guage of somekind and don't say, cool to touch or warm or my finger didn't burn. Do it right and you will be rewarded with a great running setup. Biggest kv I have in a f/e is around 1600kv. I just give it more volts and pick prop accordingly. Torque is your friend in f/e and always remember that. Pete
Lol.

Thanks for the chart, again
May 23, 2011, 02:37 AM
Quote:
 Originally Posted by SARCFAN Lol. Thanks for the chart, again

Volts are your friend too, even with the almighty spark.

Just remember, the point of brushless setups is to be efficient.

Pete
May 23, 2011, 03:08 AM
Suspended Account
Quote:
 Originally Posted by 10x8 Look at this chart I have posted 4,987,543 times. Like you say, around 30k is good for sport setups and the higher red + some, for saw setups. I have always said, lower kv motors with more volts and bigger prop is best. Try this method and just 'gear' up or in our case, prop up. Get the best, highest 'C' rated battery you can afford and try to get an onboard datalogger to measure your amps. Of course, check your temps with a proper guage of somekind and don't say, cool to touch or warm or my finger didn't burn. Do it right and you will be rewarded with a great running setup. Biggest kv I have in a f/e is around 1600kv. I just give it more volts and pick prop accordingly. Torque is your friend in f/e and always remember that. Pete

WOW my boat will be doing great then on 4s 2000kv 2074 29,600 rpm I great now i cant wait to run it.

Michael.

Just hit the Sweet spot.
May 23, 2011, 03:24 AM
Registered User
Quote:
 Originally Posted by .Electric Rules. WOW my boat will be doing great then on 4s 2000kv 2074 29,600 rpm I great now i cant wait to run it. Michael. Just hit the Sweet spot.
2074? It would barely move Just joking, I know you meant to say 4074.
 May 23, 2011, 03:48 AM Suspended Account opps my mistake.
May 23, 2011, 08:32 PM
Registered User
Quote:
 Originally Posted by deadspeedv So what exactly is the optimum RPM for loaded regardless of prop size? I assume there would be a point where the cavitation would cause barely any increase in forward momentum. I have noticed lots of the mind blowing speed boats are actually in the 40-50k range unloaded.
There is no magic "perfect" rpm to aim for. As long as you prop according to motor size, kv and batteries you'll be fine.
 May 24, 2011, 05:12 AM Registered User Oi you got another chart for prop size to motor size?
May 24, 2011, 05:23 AM
Registered User