Basically a gear box (or belts, or chains, etc.) trades speed for torque, but the power throughput stays the same. So faster or slower depends on specific attributes of your motor and/or prop.
From the prop side, less resistance will allow it to spin easier so a a ratio where the prop spins faster than the motor (torque advantage to the prop) will spin the motor sooner at low speeds. This will limit the amount of power generated at any given speed
On the motor side, the faster that you spin the motor, the more energy it will generate. So a ratio where the motor spins faster than the prop (torque advantage to the motor) will make more electricity for each turn of the prop, but too much resistance will stop the prop completely when the flowing water doesn't overcome the gear-multiplied resistance from the motor or cause more prop slip when it does turn.
So less resistance starts the process sooner but makes less energy and more resistance makes more energy but requires more speed (force) to get started. The optimum answer (gear ratio) will be different for each prop, motor and hull. This is one of those places where logic can justify answers in either direction, but experimenting in the real world can point to the practical answer for a given setup.
I'm not aware of anyone who has already done a controlled experiment to try to find that optimum regen gear ratio for even one drive system. My guess is that it will be different than the optimum drive gear ratio. So your suggestion of a two speed "gearbox" might turn out to be worth the trouble.
Eric
--- In electricboats@yahoogroups.com, "Glenn Dennis" <vega1184@...> wrote:
>
> Myles
>
> Thanks for the explanation.
>
> But which would be more efficient, turning the motor faster or slower with more resistance?
>
> Glenn
>
> >
> > From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> > On Behalf Of Glenn Dennis
> > Sent: Wednesday, December 01, 2010 10:31 AM
> > To: electricboats@yahoogroups.com
> > Subject: [Electric Boats] Re: more regen conversation
> >
> > All
> >
> > I was disappointed to learn that regen would not "completely solve all
> > problems" ;-) But as I read the last post I wondered, has anyone tried a
> > different ratio for regen sort of a two speed trans, one for propulsion and
> > one for regeneration. Wouldn't be that hard to fab up, but would it work?
> > I'm inclined to think not, in that there is only so much force being exerted
> > against the prop by the water flow.
> >
> > You could get the motor to turn faster with a small resistance or the motor
> > to turn slower with large resistance, which would produce more power?
> >
> > Thoughts?
> >
> > Glenn
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