I suspect the ultimate answer is going to be a generator, similar to those already available for pedal power...

Generators

Amazon's 300w jobbie

One set of plans

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.....think in terms of three miles an hour.

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Interesting concept.......I'll havta think about that.

Typical belt drives (of today) would be more easily driven
than a chain.....considering the speed....

Some in-depth study would have to be done to get alt speed / output
to correspond to required usage.....If only charging battery,
a slightly higher voltage (13.5) is required, and this is determined
by voltage regulator...usually now, contained in alt.

If you add lights, or whatever in the system, speed is important
to determine output, AND overcome usage, AND have enough remaining to charge battery.

Punky - idle speed, BTW, is not indicative of alternator speed,
unless you know the pulley ratio of engine-to-alt pulley.......
and in the diesel system, that alt is probably 130 amps or so,
and has to run lights, fan, radio, yadayadayada.......
so even at idle, the alt is really spinning, up to maybe 7000 RPM at cruise.

OK, going back in my box.............

/

Just splash guards for the alternator rig... They're really not that well protected even in a vehicle. Could easily live in a "bread box" above the axle. The diff's output shaft could well point vertical.

J

I think a serpentine type belt (cogs) would be ideal because who wants to listen to a chain? A canvas curtain or a mud flap should be adequate to keep the alt. dry and allow for ventilation. OK auto heads, tell me how many revs a drive shaft would make for each revolution of a wheel. I would think I would go for an axle from a light rear wheel drive car like a Chevy Nova. I calculated that a wheel with a seven foot circumference would make 754 revs per mile. Correct me if I'm wrong, math isn't my strong suit,:p at three miles per hour, the wheel is turning at 37.7 RPM.

Another aspect of this is that under hard pulling conditions, the use of a simple on/off switch would leave the alternator idling and creating virtually no drag. Right?

You need to know the diff ratio of the axle you're planning on using. If you have options, go for a lower number ratio. I think the 2.73's and 3.08's were the most common, but the axle code (on one of the tubes I think) will tell you for sure what's in there...

From there, it's "simple math" to calculate the impact of the wheel diameter and needed pulleys to achieve whatever gen shaft RPM you need.

Do you have the axle now? If so, what's the code on the tube say?

Another dumb question: Does it matter which way the alternator turns?

The drive shaft RPM depends on the differential ratio. For every 1 rotation of the wheel, you'd get 3.73 rotations of the drive shaft if it's a 3.73 rear end.

The rest of your numbers look right, and with the alternator off (ie no power to the field coil), there should be very little drag.