Partially true, yes, but not the whole story. Certainly the I²R losses are proportional to the square of the torque - i.e. doubling the torque on a given motor means quadruple the I²R loss - but the other motor losses either have no power/rpm/torque dependence (frictional losses in bearings and seals, for example, tend to be fairly constant) or else are strongly rpm (and hence voltage) dependent (eddy current loss in the cores, switching losses in the controller and windage loss from the rotor and any cooling fan).
The key to getting good efficiency is to pick a motor carefully to fit your requirement, which means knowing pretty much how much power you need to drive the hull, plus knowing the rpm of the most efficient prop you can fit, and then run that motor in the best efficiency region most of the time.
I guess few people would want to try and optimise things to the extent I have (and I freely admit it has soaked up a lot of time), so maybe using a rough rule of thumb for sizing a motor etc would be fine for the majority.
Jeremy
--- In electricboats@yahoogroups.com, "Steve" <sstuller@...> wrote:
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> The efficiency is a function of the torque. Even if you cut the voltage in half and the torque(amp draw) remains the same the efficiency doesn't change. Thanks. Steve S.
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