T-Pain Is On A Boat: Re: [Electric Boats] Why gear reduction?

Wednesday, February 24, 2016

Re: [Electric Boats] Why gear reduction?

A motor,s power is directly proportionate to rpm. To make power you have to make rpm. But propellers work best the slowest you can turn them for the desired result Hence the need for reduction

To achieve the same amount of power at low rpm with a direct drive, the motor has to be huge in comparison. A geared system can be 1/4 of the weight of a direct drive system.

James Lambden

Electroprop

6 Harbor Way #226

Santa Barbara, CA

93109

www.electroprop.com

On Feb 24, 2016, at 6:33 PM, 'Jason (Electric Boats) Taylor' jt.yahoo@jtaylor.ca [electricboats] <electricboats@yahoogroups.com> wrote:

Short answer: Heat,efficiency and cost.

Heat
The motor I use, the ME0913 is open-frame air cooled. The motor has a cooling fan on it, connected to the output shaft on the opposite side. At lower RPM, it doesn't move enough air to cool it under heavy load.

Efficiency
The motor and controller are both more efficient at higher RPMs. The motor may have full torque at lower RPM, but those Nm cost more per watt. As a sailor, I'm a cheap bastard.

Cost
See my earlier post about the magnomatics pseudo direct drive gear motor. That would be able to directly drive a prop shaft at very low rpm. There is also the ST37 from Solomon Tech that does something similar but with machined planetary gears. These are very expensive compared to belt and pulley or even industrial fixed gear reducers. There was a vendor here, Advanced Marine Electric Propulsion, who had a direct drive offering but he used lower voltage systems which meant higher amperage on the battery bank. This means bigger, more expensive cabling, bigger battery banks in order to deal with the increased demand efficiently (peukert effect). Again, I'm a cheap bastard. I don't want to spend money where it doesn't directly benefit my sailing. But I also hate spending money more than once on something, like replacing a burnt, overloaded motor.

Cheers,

/Jason

--
Jason Taylor
v:514-815-8204
e:j@jtaylor.ca

On Feb 22, 2016, at 9:36 AM, johnrhines@gmail.com [electricboats] <electricboats@yahoogroups.com> wrote:

Jason,

Thanks for the detailed reply, but I'm am still confused. Let me ask the question a different way. You said to get a cruise speed of 4.7 kts with optimal conditions, your prop spins at 650rpm. With a gear reduction of 2.55, that means your motor is turning 1657rpm. Since electric motors have full torque at all rpms, what is the problem with spinning the motor at 650rpm and eliminating the gear reduction? Is it just a measure of efficiency? How would you calculate the load on a motor at 650rpm versus 1657rpm.

Thanks,
John