Watts question

[JPriami]

I have a simple question and just wanted to make sure I was thinking this is possible or maybe i need to do some different figuring.

Lets say you have a motor thats 1400 watts on 5S and you wanted to know what the watts per lipo cell were if you wanted to run 4S or 6S can you just divide the 1400 by 5 to get your per lipo cell watts?

[ManuelW]

Hello,

short answer: NO

longer answer: Where do you know from that your motor uses 1400watts on 5s lipo? Do you have an Eagle Tree System or the UniLog from SM-Modellbau?
To explain it as easy as possible: Power = rpm x torque = voltage x current. So with 5S you have a certain voltage (eg 18Volt) and a current of 77.8 Amps to get a power output of 1400watts. If you have now 6cell instead of 5 cell you will have more rpm resulting in more power but to spin a prop at higher rpm also needs more torque --> will increase current and also the power. So as a handrule the effect is not linear (eg double the cells --> double the power) but more quadratic or even above (eg double the cells --> 4 times the power output).
With 6S instead of 5S this may result in 40% more power, ~2000W at about 93amps. I's say quadratic behaviour is under perfect conditions, if you make mistakes eg with the setting of the drive or the boat running too wet it might be even more than that.

These are just some approximate estimations as a "guideline" and the Pro's might forgive me my easy explanation but I didn't want to scare the people with Inner Resistance and physics behind it.

Best regards,
Manuel

[JPriami]

Thanks,
I was just trying to rough Calc a setup based on specs of a motor. Said 5s lipo, 80amps, and 1400 watts.
This is not a built boat so I can't eagle tree it. It's one I'm planning out. So I'm taking motor specs. And rough calcing up the weight of the boats individual componets installed. To Calc weight to HP ratio to see what would be the best route to go first.

[ManuelW]

Well but I see a big problem here how people understand these specs. When it says 80amps and 1400W this only says what the motor should be able to handle. So these numbers have NOTHING to do, with what the motor really pulls out. This is just an information where the limit of the motor will be.
So with 5S and a nice setup you might be for example at 50amps current on the straight and everything will be fine. But a wrong prop on it or run a wrong trimming the motor may pull 200amps and will be destroyed within a short time!

Best regards,
Manuel