Hi Bill,
You're correct. It all depends on the vessel, prop, motor, etc. The trick, just like an ICE, is to match the available power from the motor to the desired prop shaft RPM. On every inboard electric boat drive system that I know of with a gear ratio, the motor turns faster than the prop shaft, just like ICEs.
The Propulsion Marine systems use gearboxes that come in a variety of ratios, they use different gear ratios to match their drives to the customer's boat. If the ratio is too high, the load on the motor is low and the motor will hit max RPM without pulling max amps. Alterenatively, with the ratio too low, the load on the motor is high and you'll hit max amps before you get to max RPM.
On my 10,000 lb, 30' x 8.75' ketch, the installed gear raio is 2.55 with a 13x15.5 4-blade prop. The boat achieves 6kts at 5100W at 1900 motor RPM. I believe that the available gear ratios range from 1.25 to 5.6:1
Most modern PMAC controllers have a setting for regen torque. Too high and it will stall the prop at sailing speeds and you'll get nothing but drag. Too low and you'll hit max your "collection" RPM (depends on the boat, prop, speed, etc.) with a lower amp output, but you'll get something more often.
This does exactly the same thing as changing the drive ratio, a reversed ratio will likely stall the prop (not enough torque to turn the motor) and you'll get nothing.
If you're really interested in maximizing regen in your own boat, you'll need to buy the right equipment from the controller manufacturer (Sevon has a windows PC interface or a calibrator handset) to re-program the controller and tweak the regen torque settings until you maximize your amps at your regular sailing speeds.
Two weeks ago, using the stock programming from Propulsion Marine, my boat averaged about 0.7A at 5.5kts with momentary peaks of 1.5A, that's 37W average and peaks of 75W. These readings came from the E-Xpert Pro HV monitor attached to the battery pack so that is current going back into the battery. I doubt that any full keel boat with the prop in an aperture can do much better with my sized prop and boat speed. I'm actually impressed that I am able to get any regen at all.
--- In electricboats@yahoogroups.com, Bill Heintz <bheintz@...> wrote:
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> When using choosing a reduction drive system what is a "good" Motor : Shaft Ratio?
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> The SolidNav Explorer used a 2.222:1 Ratio I believe.
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> I presume a lot depends on the propeller size, pitch, # of blades, vessel displacement, etc.
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> If you planned to travel an extended distance offshore, would it make sense to "optimize" this ratio the other way around such that the motor turned faster than the propeller for better regen?
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