Brushless mtor help

[ED66677]

Why would a 1500 KV be great in one hull and not so great in another? Or better yet, I have a 32" cat that came with a 2045KV and have been told by many that, that is the wrong motor for that hull... why?

there's a good thread here where ReddyWatts put a Hull Size/Number cells/Motor Kv/Prop Size/Type ESC chart, very interresting.
Basically a 2045KV doesn't tell enough to say it'll work or not, if it's at least a Feigao XL (or equivalent) yes, others have done t successfully, if its a L size or even worst a S size it won't!

[ozzie-crawl]

Sport running - Kv times volts = 20,000 - 25,000 rpm
Oval racing - Kv times volts = 25,000 - 30,000 rpm
SAW racing - Kv times volts = 35,000 - 45,000 rpm


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i have heard a few times that these rpm guide lines come from the fact that props were and still generally are made for nitro/gas boats
a are designed to work most effectively at these rpm
hopefully someone like fluid may clear this up, if its true it mite give people a bit more of an idea why to stick with in these given rpms

[Fluid]

The chart is based on experience with various motors. High-Kv motors draw a lot of amps and require a lot of rpm to keep from overheating everything. They are not appropriate for sport running if the boater wants much run time - prop characteristics have little to do with it.

At the other end of the spectrum is SAW racing. Here racers expect to burn things up in their quest for records - official or GPS. Low-Kv motors won't get it here - but too high rpm will be inefficient. Most SAW records have been set with on-the-water rpm between 35,000 and 45,000.

An x-series prop can work very well from 10,000 rpm up to 40,000 rpm. A v-series prop is pretty inefficient below 25,000 rpm - but loves to run at 40,000 rpm.


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[tiqueman]

This is fantastic info people. So, now that Im beginning to understand this a bit more.

For a 540 size motor for instance, how does the S, L and XL compare. Is the XL, being a longer can going to be more efficient? More torque? Better running as it had more area to dissipate heat? Also, does it take more voltage to power a XL can over a S?

EDIT: Im still reading up on this. Basically, the higher the KV, the smaller the hull and lower the voltage... 540 size cans XL are more suitable in a 32" hull than a S or L would be, S & L both having higher ranges of KV. Am I on the money here???

[ED66677]

the XL has more torque than the L, that also has more torque than the S. At a given rpm, more torque equals more power. the torque is dependent of the rotor lengh and diameter
the voltage is something else, you can power a S or a XL with the same voltage, Feigao told me the max voltage is 36V for the 540 motor range, but the rpm limit is 50000rpm with stock bearings so for a given KV you know what is the max voltage you can use.
for a 32" hull you would need at least a 8XL under 4S, a 9XL would certainly do it too with a slightly bigger prop, these motor can deliver up to 1200-1300Watt, that's what I'm pulling on my 29" DF copy when I hit the throttle and go WOT, once it is on plane it is around 800-900Watt but a XL is the very minimum you need in a 32" hull, a KB45 or a Feigao 580L would be better but with higher voltage (6S) in order to keep amp draw reasonnable and increase available power.