Check my math please

[Chenige]

Seeing if I understand the math.

20" mono electrified, 15T speed gems 2 (4006 kv), X435 prop, 7.2V 3800mAh NiMH battery, 35 max amp Astroflight ESC

Ran 50 seconds til battery almost gone. Ran wet, not as fast as expected. Charger put 3520 mAh back in

Max rpm= 4006 x 7.2 = 28,800 rpm

run time = 50/60= 60(1000)/3520 x (motor draw rate)

Therefore motor draw rate = 20.5 amps/min

Watts = (motor draw rate) x V x mAH = 20.5 x 7.2 x 3520 = 520 watts

Motor draw = watts / volts = 520/7.2= 72 amps

First is my math correct ?

Second, how can it draw 72 amps on a max 35 amp ESC?

At the end of run ESC and motor were cool. Battery was slightly warm.

Third, how do I increase run time? I am going to try an x432 prop. And then move the cg back to get more air under it.

Thanks

[Jacked1]

3520mAh in 50s would be 253amps!
(3.52Ah/(50/3600)) capacity/(time in hours)
I am guessing your batteries were not fully charged.
When I had NiMh batts sometimes they would lose a great deal of charge in a couple days.

[Make-a-Wake]

That motor will draw in the 15-25 amp range at most in your setup, batts either werent charged or are bad.

[Chenige]

Confirmed. Ran again for over 10 minutes on the same charged battery but with x432.

Motor came out very hot. ESC barely warm and battery hot but no problem holding onto it.

Thanks. I will redo the math once I get the battery charged again and get the mAh. Still want to make sure I am doing the math correctly.

[rc2266]

Try another battery

[Make-a-Wake]

I am of the opinion that a 15T is too hot of a motor.............a 19T on 6 cells seems about right to me...............

[Chenige]

It seems like I need a bit more prop speed. Kv for 19T is 3175 which would give 23000 rpm. Isn't this a bit low?

[Jacked1]

You can also use this for your calculations.
http://www.offshoreelectrics.com/conversion.htm
This is also the "Speed Calc" tab at the top of foum page.

[chriser0]

i think the battery was dead. i always check my battery's before a run with a multimeter. get a harbor freight one (free in ads) and check the amps. a 3200mah battery should show 3.2Amps when fully charged.

[Make-a-Wake]

It seems like I need a bit more prop speed. Kv for 19T is 3175 which would give 23000 rpm. Isn't this a bit low?

Your 15T motor came in VERY hot as you said, a 540 brushed motor isnt capable of putting out 30k rpms in a boat in my opinion............

[Chenige]

what kv would you recommend for a 540 brushless motor with a 3S lipo? Or am I better off with 2S?

I really do not mean more prop speed but a bit faster boat.

[7500RPM]

My son and I, ran those hulls for better than 7 years and we used 17 T Speed Gems with home made hardware and used X632 props, never had a problem with overheating...Awesome combination...