Output power question

[cugino]

Hello,

i have some exprerinece with FE Boating, but i am not sure, if i understand this correctly.

Lets have this hypotetical setup (torn from real world):

Motor: KV 2000rpm/V with eff. 80% at any Amp draw,
Power source: 20V under any load (not real world either)

Then no load RPM will by aprox. 2000KV (times) 20V = 40000rpm and
under load it will be 2000KV times 20V times (eff.) 0.8 = 32000 rpm

So it means, that 80% of energy is transformed to rotary motion and 20% is transformed to heat (bearing friction, winding Ri).

Am i correct?

I am asking you, because of DMAcalc http://brantuas.com/ezcalc/dma1.asp gives me another numbers (considering other variables, of course). I develop my own FE Calc, it should be more accurate!!!

Second question:

Do you have comparison of efficiency. of Turnigy 3656 motors, BJ25 3630-1500, Feigao motors (same size and KV)?

Simple question, expecting simple answer.

[Ub Hauled]

In the 1st statement you are correct, well, partially...
there is the motor only, but I believe the calculator gives you the math on the prop as well, so there would be a "prop slip" involved there and don't forget the battery impedance.

As far as the second question, the only way to get an accurate measurement of efficiency from these motors is finding people that has measured it and have an average.. since these motors are usually different then each other. The numbers that the factory give you are not too accurate usually. People use 80% Eff for the Feigao motors.
One more thing, I think you'll find that the calculator you are using is for airplanes, if you are trying to use it for boats you'll find that either FEcalc or the calculators on OSE are better suited for a starting point.

[Flying Scotsman]

Also, I suggest you state what hull, power, speed expectations, run time, before you get hung up on the motor specs, which as Jan has mentioned really stress the motor and ESC under boat conditions

Douggie

[cugino]

Thank you guys for you reply.

In my calculator i will try to involve all varibles that R more or less imporatant for calculations. ()

Developing calculator is quite hard for me. I have MSc of IT and masters degree of Economics, but it is useless in this situation. :-) I have built lot of impressive boat with various setups for other people - my business. :-)
My favorite built was MHZ micro Mystic with two NEUs and HaM Agitotor with two lehners 1950 with 6S Kokam Lipo that was able to reach 150kmh.
So, you can see I have some experinece with FE boats.

But if you help me understand, how this things work exactly, result will satisfactory.

So if the relation of U*rpm*eff. is really correct, i will use it in my calculator and put feedback soon.

[cugino]

Also, I suggest you state what hull, power, speed expectations, run time, before you get hung up on the motor specs, which as Jan has mentioned really stress the motor and ESC under boat conditions

Douggie

I am not currently building any boat. I only want to know, how these things works in general.