Two points:
1) AT 100% efficiency, dropping speed by ½ knot with prop/motor as
generator for 12 hours wouldn't equate to yielding ability to travel at ½
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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