The challenge is getting the best compromise.
For any electric motor, of any flavour, increasing the torque it has to deliver will increase the current, which in turn will increase the losses due to that current (primarily power lost to the resistance of the winding, cables, controller an internal battery resistance).
Losses due to resistance aren't linear, they are proportional to the square of the current. In simple terms this means that running a motor at half the rpm but twice the torque, rather than use a 2:1 reduction drive, will just about double the current and quadruple the power lost from the total system circuit resistance.
In contrast, some types of reduction drive have fairly constant losses over a wide range of drive torque. Toothed belts, for example, have most of their loss from flexing the belts around the pulleys, which isn't torque dependent. A 25mm wide 5M HTD or GT2 belt may only lose around 15 watts or so at typical boat reduction ratios and rpm, although the losses increase fairly steeply with the thicker belts with broader teeth. To this has to be added the bearing and seal frictional loss, which can be surprisingly high (if, like me, you're working with low power).
Gears tend to have a fairly strongly torque related tooth frictional loss, which may be a couple of % or so.
Overall, the loss in a reduction drive is almost always less than the loss from running the motor at a higher current, so it usually pays to use one. Having said that, it is well worth looking hard at whether this is the best option for your requirement, as sometimes it can make sense to just use a significantly over-size motor and under-run it to keep the resistive losses low. This approach doesn't scale well, but for low power applications it can work efficiently.
Jeremy
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
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> aren't belt drives inherently inefficient if one compares that method of connection to a directly coupled shaft?
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> AND...
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> if electric motors produce the same torque at any rpm then why would one wish to gear an electric motor either up or down? wouldn't one simply want a slower revving more efficient propeller and then run a motor rated to run at 2400 rpm or more at 1000 rpm instead? i understand the cooling might be an issue with some motors due to the fans being an integral part of the motor but surely a secondary dedicated fan system could make up the difference for VERY little wattage
>
> again, am i missing something.... BLDCs do run at constant torque do they not?
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