On 06/03/10 04:18, Ron wrote:
> Hope I'm not showing myself to be too simple minded, but whats different about an electric motor and an ICE turning a prop in the water?? If the final drive is 2500 rpm the prop should show the same results, regardless of what is suppling the power.
> A little help please :)
>
It will probably take less KW of power to spin that prop at 2500rpm
with electric than ICE.
My cruiser is pottering a long quite happily with about half the
electric horsepower it had under ICE. I'm still using the same prop it
had when it had a petrol engine.
I've come to the conclusion that the ICE engine (a petrol 2 stroke in
my case) was being run harder than absolutely necessary to get enough
torque to spin the prop. Electric motors (or or less, gross
simplification here) produce the same torque no matter what - rotation
speed is dependent (more or less) on voltage. The electric motor will
pull what ever amps it requires and no more to spin the prop at your
chosen speed. The ICE on the other hand has a more complex relationship
between speed, torque and throttle.
I could probably make my system more efficient by changing the prop to
a bigger slower spinning one. I'd then have to put a reduction gear on
the motor (i'm currently direct drive) so that the motor spun fast
enough to properly cool (and so I can run at higher voltage (spin
faster) but with less amps). High amps is where all the loses are.
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