I've been impressed with Mark Stafford's comments on electric drives. Clearly, he knows what he's talking about, and I don't.
But ignorance has never managed to silence me before, so...
Mark, I'm not sure that Keith's analogy with golf cart wheels is such a great analogy. With carts, you get regen only when you are braking. With a sailboat, you're turning that prop 24/7 so long as you are under sail.
People pay nearly $800 for a 100 watt solar panel and it's only really effective about 8 hours a day.
I do believe that you're right about props being optimized for propulsion -- and at maximum RPM and power output at that. So, how do you optimize a prop for regen?
From the bit of reading I've done, I think you want to maximize diameter. I don't know about pitch.
One outboard site said "Houseboats and cruisers care more about efficiency at displacement speeds, therefore they require a lower pitch to achieve low-end power and the largest diameter their lower unit can handle."
Intuitively, I think I want the prop to be spun readily at low speed, because most of our sailing is going to be done at low speed. (I've read some reports of units not regening until sailing at nearly 4 knots).
Permanent magnets seem the way to go, but if you had wound fields, it would seem that a controller could adjust the resistance to shaft turning to optimize regen? I'd rather miss opportunities to max regen output at high speed in favor of getting some juice out of it when you're barely coasting...
I'm grateful to have stumbled into a forum where so many knowledgeable folk are posting.
Best,
md
--- In electricboats@
>
> Keith, Mark Stafford always appreciating your real-world numbers. Your almost 100watts regen @ 6knots.... what prop size/pitch? 12x8? The other problem with regen: most props are optimized for propulsion, not for braking.
>
> Great traction analogy with golf-cart wheels vs propellers.
>
> Mark Stafford
>
>
> --- In electricboats@
> >
> > Most of the systems in use are in fact derived from golf-cart parts. But we typically use more efficient PM (permanent magnet) motors instead of the older series wound motors common in the used market. So it definitely can be done and I think a few on this board have examples. The drive and controller are only about 1/4 the total cost of a system so it's not really a great place to skimp.
> >
> > As for regeneration - it's one of those things that sounds great in theory but isn't really practical. Consider that all 4 tires on a golf cart are gripping the road with several thousand pounds of stopping friction - vs all that water easily slipping by a very small prop.
> >
> > So golf-carts get great braking, and a few watts of power - on a boat you just get more expensive electronics. I have a system that can regen and I get less than 100watts sailing at 6knots and then only when I carefully feather the controller (though I think the controller can be programmed to do this better). It's not impossible to boost this, but it's a science experiment, not a proven tech.
> >
> > Regards,
> >
> > -Keith
>
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