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-   -   Help - New to brushless (https://www.rcuniverse.com/forum/speed-electric-109/9824501-help-new-brushless.html)

Quigs84 06-24-2010 01:47 PM

Help - New to brushless
Help - New to brushless

i have a boat with a brushed motor in at the momonet(750 TYPE-means nothing to me)

i want to change it to a brushless one but i don't understand them, what i mean is which is more powerfull etc. they all say Kv, is the high the number more powerfull?

the boat is 1050mm long and390mm wide.

i would like to make the boat go a fair bit faster, i have a few 8.4v Ni-Mh battery that i would like to use if i could. (too Save Buying new ones

Thanks for the help

nick_75au 06-25-2010 12:53 AM

RE: Help - New to brushless
Kv is RPM per volt, so a 1000 Kv motor will spin at 1000 RPM at 1 volt, 12000 Rpm at 12 volt, in general higher Kv means more power hungry the motor is as well.

A rough guide ~28mm diameter in or outrunner is fairly close to equivalent in power, a ~36 mm will be a better choice as it will run cooler. You can see in the table the Watts (power each motor can handle)

link has some specs on 750 motors

top one has a Kv of 415 bottom one has Kv of 1250 so that gives you an equivalent to look for.

you have a couple of ways top approach the selection of a motor as well, you could get a lower Kv motor and feed it with lots of voltage ie 2 to 3 8.4 volt batteries in series which is efficient.
or high Kv which will need a number of batteries in parallel to give reasonable runtime.


Quigs84 06-25-2010 01:30 AM

RE: Help - New to brushless
thanks that is very help full.

so what sort of rpm do you want to echeive?

i read some where 30,000rpm.

if above is correct.

i could use a 3500Kv motor using a 8.4v power supply (or two 8.4v in parrellel)

or a 1800Kv motor using 16.8v (two 8.4v batterries in series)


or and any info on what sort of prop would also be grateful

nick_75au 06-25-2010 02:51 AM

RE: Help - New to brushless
Im not an FE guy but they do run at 30-35000 RPM on surface drive, If your boat is not surface drive I would drop that figure down somewhat.

Your figures are spot on. Some more details of the boat would help in sorting out a good set-up. Props would be around the 35-45mm size, but some experimenting with the inexpensive plastic carbon props will help to find best one.


Quigs84 06-25-2010 06:07 AM

RE: Help - New to brushless
i'm pretty sure it is surface drive.

here is a link to the one i got.

i have already changed the radio gear as the factory stuff is crap.

i will also be changing the prop shaft and rudder aswell.

link to rudder. http://cgi.ebay.co.uk/RUDDER-21-45-C...item53e20fdb80

not sure if right size though?

nick_75au 06-25-2010 07:00 AM

RE: Help - New to brushless
Definitely a FE type set-up, Ill leave it for the FE guys to help with the setup, there are quite a few of these NQD conversions done a search through these and RC groups will reveal what others have achieved.


SV27dayton 06-25-2010 12:52 PM

RE: Help - New to brushless
I think we need to know what the hull is and what your expectations are. that's a decent size hull. So if you want to go fast you will need some power to do it. I would not suggest staying at 8.4v if you want to go more than 15-20mph. Doing so at such a low voltage would require lots of amp which stresses a system. Can you give us more details on what the hull is, what you are wanting it to do, and what you thought you were going to spend.:D

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