Electric boat conversion - the propellor

This posting is rather a cheat since we merely kept the propellor on our boat as it was prior to the conversion(!), but I've included it since in researching our conversion I came across several articles and other comments concerned about the best propellor to use for an electric drive such as ours.

A common recommendation is to use a larger prop design with greater grip on the water. I personally cannot see the argument for this since the overall physics process is that you set a certain power input and then the electric motor will accelerate until "power out" = "power in" (less incredibly small losses - but these cannot be large or the motor would rapidly get blindingly hot and it doesn't!))

The propellor therefore speeds up until this power equation balances (actual revs are irrelevant)... when the boat will be being given a driving power exactly as it was for conventional drive. So, on this basis, if the propellor drove the boat "sensibly" before conversion, it still will in exactly the same way!

I think propellor considerations people make are really based in terms of maximally efficient connection between the prop and the water when the conversion is such one needs maximum range for a given drive battery charge. But this isn't a prime requirement in this case since one can choose to recharge at any time.

Clearly, any increase in coupling efficiency between prop and water would mean less re-charging time - but from my researches gains only seemed to be of the order of 5% at best... really quite trivial compared to the variations one would meet for meeting different river currents, etc. - not really of practical significance.

Incidentally, with our control set to maximum, the motor draws 14kW (rather than the designed 12kW) so it would seem that if anything we are "over-propped" by retaining the original propellor and its size should be reduced for the motor to run as specified!

All-in-all, our system seems to work entirely satisfactorily retaining the original prop!