James/Jason,
A little more info here guys otherwise the quoted statements below are misleading for others I think. As are the heat v motor speed issues mentioned earlier in the thread and the reduction in battery capacity due to load if applicable.
> A motor,s power is directly proportionate to rpm. To make power you have to make rpm.
True but it has to be looked at in context, quantifying numbers and the application. Let's take similar motors that many of us use, be that a Mars/Motoenery/Etek/Perm/Heinzmann/Lynch/AGNI or Saietta, all of a similar design, all of a similar rpm per volt constant of around 50 RPM, and similar torque per amp, most suitable for voltages up to say 84V and suitable for a small a displacement sailing yacht of around 8m to 10m and say up to 8,000 laden displacement - as an example. Design differences in the motors above dissipate heat differently due to the materials chosen in their construction, the power used and the motor frictional losses due to the rpm chosen.
Now let's decide on the power we need at our cruise speed in calm conditions, which due to the application is where we may spend most of the time if motoring and not electric sailing or sailing. Let's say that power is 800 Watts measured at the battery. In my case that would be 4 knots. Different for other boats. Consider an intermittent use at 1,500 Watts for up to an hour and say 2kW short term. We can now have any battery bank voltage we wish, starting at 24V up to 84V in 12V multiples if using monoblocs in series. For ease let's look at a 24V direct drive system with two batteries of 220Ah and a 48V geared system with 2:1 reduction with twice the number of batteries but the same Ah. Either of these systems could have up to 7 batteries in series at that Ah, assuming the chosen controller will accept the input voltage, but for simplicity we'll stick with 2 versus 4 and AGM or better for safety. The peak voltage measured at the motor for the direct system being around 22V and 46V for the geared system, percentage throttle demand set in the controller to limit shaft speed to 1,000 rpm for both (in case of using higher voltage battery strings).
>But propellers work best the slowest you can turn them for the desired result Hence the need for reduction.
True to an extent. Larger diameter is better but in dealing with a small displacement sailing yacht as above there may or may not be a need for reduction depending on the propeller that can be fitted and the rpm the shaft needs to turn at to achieve ones desired goals.
>To achieve the same amount of power at low rpm with a direct drive, the motor has to be huge in comparison.
This is wrong in the case of the above. We are dealing with light motors that are small and weigh around 12Kg.
>A geared system can be 1/4 of the weight of a direct drive system.
'Can' is the operative word here. It depends on what power output you need or want and type of motor chosen. Many people in my view choose more system power than they need, that they never use and more range than they need or use, particularly with sailing boats.
Back to the 24V direct drive v the 48V geared drive example above. The lightest system is the 24V direct drive. Less batteries if you choose, no reduction ratio and therefore less weight and a simpler and likely cheaper system.
But what about overall system efficiency. For this we have to assume props absorbing the same amount of power and likely around 12 " diameter, both systems to have same max shaft RPM. In this case the systems in the example shown can operate in the 88 to 90% motor efficiency range at cruise but the direct drive will likely be more efficient at cruise as no reduction ratio, providing that the combination of voltage and current at the motor results in those efficiencies. I'm in no doubt that the greatest efficiency gains are to be had in the prop design, but again it comes down to what you want. Maybe you want range, maybe you want a cheaper and simpler system with less batteries and weight and are prepared to sacrifice a little prop efficiency for a non turning motor under sail with a folding prop for less drag. If we stay with a fixed prop in this example the 48V system will be noisier than the 24V system due to the motor turning at twice the speed when freewheeling, plus further noise from the reduction ratio and more cost.
Lastly the Peukert effect comparison between the two systems. Let's assume at the same cruise speed in this example the direct drive system measures around 44 Amps at the motor and 32 Amps at the battery with of course the reduction ratio system being half that in the example, so 22 Amps at the motor and 16 Amps at the battery. Both motors are running in the high efficiency range in this case as they are within that area of the efficiency map, but there is a different Peukert effect on the batteries assuming the same exponent. For example:
220 Ah 20 hr rate battery capacity reduction at 25C @ 16 Amps = 200 Ah/50%= 100 Ah.
220 Ah 20 hr rate battery capacity reduction at 25C @ 32 Amps = 168 Ah/50%= 84 Ah.
Clearly the 48V has twice as many batteries with useable kWh of 48 x 100 = 4.8 kWh and the 24V 24 x 84 = 2kWh. But of course either system can choose to use multiples from 2 to 7 for the 24V system or from 4 to 7 in the 48V system. Personally in such a situation I would choose the higher capital initial cost of Lithium for the longer term gain in cost per nm. The ratio being that AGM will likely cost 1.5 times the cost of the Lithium over time. The additional benefit of course with Lithiums being even less weight and less volume and little if any Peukert effect.
In conclusion it all comes down to what you actually need and/or want for your particular application. In the example shown the least expensive, lightest and quietest system is the direct drive it seems.
John R.
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