Myles, thanks for some very interesting insights.
So, the biggy is prop design. It looks like the one thing you can make a significant difference in.
What are the essential variables? Pitch and size clearly matter. How does one allow for them? Is weight a biggy? Seems like a lighter weight plastic would be good.
Another question: is the ideal prop design for driving the boat the same as the ideal design for being spun as the boat is drawn through the water by sail forces?
As we are going to be doing more sailing and little motoring and that at low speed, I'd rather have -- if you're forced to choose -- a prop that generates electricity efficiently.
The average speed under sail is probably going to be three to four knots.
Supposedly those cylinders the enclose props in in some designs improve driving efficiency. I imagine it forces more the water to the rear instead of pushing some of it off to the side. But intuitively it doesn't seem to me that this would help the prop spin faster when generating electricity.
How does one get help with prop design? Are there professional designers or resources/tables, etc?
md
--- In electricboats@
>
> Two points:
>
> 1) AT 100% efficiency, dropping speed by � knot with prop/motor as
> generator for 12 hours wouldn�t equate to yielding ability to travel at �
> knot later with electric. It�d be better than that due to the nominally
> square relationship between power and speed. So, say you were sailing at
> 4knots, and dropped the 100%eff prop/gen into gear, reducing speed to say
> 3.5knots, the energy difference is on the order of 40% (SWAG). So if a 100%
> perfect efficiency, using that energy later for propulsion would be
> equivalent to perhaps 2+knots for 12hrs. But we don� live in a perfect
> world, so:
>
> 2) Reality shows that at best, prop efficiency will be on the order of
> 70%. Expect more like 60% if you do good design. That�s where the big
> losses are�not the motor, not the electronics and not charging the battery.
> And you have to take the product of all these efficiencies to get the total
> efficiency going IN and going OUT. Hence, realistically, you can�t
> realistically expect better than 50% energy efficiency in charging via prop
> from boat kinetic energy to electrical energy stored. Reversing this, you
> really shouldn�t expect better than 55-60% efficiency from electrical energy
> to boat motion, better at slower speeds. But it gets worse---the prop spins
> so much slower when driven by the water versus driving the water. So you
> need a good regen controller or other scheme to boost the voltage to charge
> the batteries due to the lower speed of the prop.
>
> Justifying doing this depends on the situation. Clearly if substantial wind
> is there NOW and more than you can use, capturing some of that energy for
> later makes sense. There are likely many situations where this is not
> justified.
>
>
>
> -MT
>
>
>
> From: electricboats@
> On Behalf Of constancedraper
> Sent: Wednesday, June 10, 2009 7:01 AM
> To: electricboats@
> Subject: [Electric Boats] Re: 29' CAL Looking to switch to EV
>
>
>
>
>
>
>
>
> Hmmm, I wonder what the answer is? On the face of it, there has to be a way
> to capture more of that kinetic energy. I've heard others say that turning
> the prop to generate electricity cuts speed by a half knot or more.
>
> If you had a 100% efficient system (impossible, of course) that would mean
> that at the end of 12 hours of sailing you'd have accumulated enough in the
> batteries to drive the boat 12 hours at a half knot. Maybe 4 hours at one
> knot.
>
> A 50% efficient system should give you half that.
>
> The electric motor/generator is one of the most efficient machines around,
> so that's not the problem. Evidently, as you've suggested, the prop design
> is crucial. This is a subject I know almost nothing about.
>
> I understand that electric motors have great torque; I'm guessing that it
> requires more torque to turn them than than the relatively slow moving water
> flowing past provides. Seems like there needs to be some way to reduce the
> load so the prop spins easier.
>
> md
>
> --- In electricboats@
> <mailto:electricboa
> <aweekdaysailor@
> >
> > --- In electricboats@
> <mailto:electricboa
> <drmark.draper@
> > >
> > > Randy,
> > >
> > > Don't overlook the possibility of recharging your batteries by capturing
> the energy of the prop's turning while you're under sail.
> > >
> > > md
> >
> > Actually - do. It doesn't work. On a good day, you'll maybe get another 10
> minutes of propulsion out of it.
> >
> > Unless...you can actually fit a 20" prop - I am stuck with 13" max. But
> most boats in this size range will have that problem. The shaft-angle would
> have to be too steep to hang a (much) bigger prop.
> >
> > -Keith
> >
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
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