Myles
Thanks for the explanation.
But which would be more efficient, turning the motor faster or slower with more resistance?
Glenn
--- In electricboats@yahoogroups.com, "Myles Twete" <matwete@...> wrote:
>
> OOPS.I meant to say: Since the power is proportional to the speed cubed, the
> prop driving 10knots of current will transfer substantially MORE power than
> if it is "driven" by the water with the 5knots of water stream current in
> regen (assuming efficiencies were the same both ways, which they aren't).
>
> -mt
>
>
>
>
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of Myles Twete
> Sent: Wednesday, December 01, 2010 11:07 AM
> To: electricboats@yahoogroups.com
> Subject: RE: [Electric Boats] Re: more regen conversation
>
>
>
>
>
> Probably the best you could do is get the same prop and conversion
> efficiency as your system does in delivering electrical energy into boat
> kinetic energy. So if you happen to have a damn good prop that gets 65-70%
> propulsion efficiency and assuming the other electric-mechanical efficiency
> is a decent 80%, we see perhaps 50% energy loss in the electric-to-kinetic
> energy conversion.
>
> Reversing this, you'd similarly expect AT BEST, a 50% return energy
> efficiency of water energy delivered to the propeller in a generation mode
> to your battery bank. Now, as a drive, the prop sends out a water stream
> that is significantly faster than the free flow under the boat. Therefore,
> cruising at say, 5knots, the water flowing thru the prop might be 8knots.
> Since the power is proportional to the speed cubed, the prop driving 10knots
> of current will transfer substantially less power than if it is "driven" by
> the water with the 5knots of water stream current in regen (assuming
> efficiencies were the same both ways, which they aren't).
>
>
>
> So say that it takes 2kw from your batteries to cruise at 5kts.
>
> And say that the water shoots out thru the prop at nom. 10kts to do this.
>
> It's arguable that the best you could get in regen at 5kts with the same
> prop is about 2kw*(5/10)^1/3, or 125watts.
>
> This is optimistic depending on the prop and the boat.
>
> But with regen we can take the boat largely out of the equation because it's
> just a matter of stream flow and prop and prop loading. Someone might want
> to explore this for a few typical props used on your sailboats.
>
>
>
>
>
> -Myles Twete
>
>
>
> 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
> --- In electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com> , Sally Reuther <smreuther@>
> wrote:
> >
> > Eric,
> >
> > Thanks for adding this clear explanation concerning regen. I've found the
> > people who are asking us about regen under sail seem to think of this as
> > "free power" and that it will solve all of the charging problems. What we
> > know is that it's a trade off and you really need to make some choices if
> > you want to have regen work for you at all. Most folks want to be able to
> > motor as fast as possible so they make sure the gearing on their electric
> > systems is such that they can get optimum propulsion. If this is 5 knots
> on
> > a boat that can only sail (because of displacement ratios) 6.5 knots, then
> > the window for regen is that 1.5 knots of speed. The regen does not kick
> in
> > until you get past the point where your motor is set to motor at top
> speed.
> > I'm a sailor and I know that being able to sail at my boats hull speed for
> > any length of time is dependent on many factors and usually doesn't happen
> -
> > especially if I am only out sailing for the day and tacking a lot. To be
> > able to make regen work, the top motoring speed needs to be set lower, and
> > you need to be able to sail for a sustained period of time at that higher,
> > constant speed to get anything back into the system. Even then, as you
> > pointed out, the numbers are not huge. I don't understand the math
> involved
> > - I'm the artist and idea person, but I do understand the basic theory as
> I
> > explained it above. Regen is a fantastic idea, it's just not the solution
> > to our charging problems we would love to have.
> >
> > Sally
> >
> > On Mon, Nov 29, 2010 at 6:52 PM, Eric <ewdysar@> wrote:
> >
> > >
> > >
> > > Larry,
> > >
> > > As the most visible "regen naysayer" here, the link that you provided
> > > completely supports my previous premise.
> > >
> > > According to the Whoosh site "... And since we average 5-6 kts, that's
> over
> > > 100 amp/hrs/day - for us quite possibly 150 amp/hrs/day - that is
> available
> > > for running the radar at night, making water with offshore water
> quality,
> > > feeding the SSB radio, and keeping the iPod charged..." So what does
> this
> > > mean? They get 100-150Ah at 12V in 24 hours, that's 1200-1800Wh in 24
> hours
> > > or 50-75W at 5-6kts. They do say that the harvested water power is
> > > sufficicient for house loads, a statement that I wholeheartedly agree
> with.
> > >
> > > My statement is (and always has been) that regen at speeds around 5kts
> is
> > > not a viable source of charging for a traction battery bank. The speed
> is
> > > important because most displacement auxilliary sailboats between 27-32
> feet
> > > (optimal for a 5kW electric drive) have a hard time hitting sustained
> runs
> > > above 6 kts. For this size boat, average sailing speeds of about 5.5kts
> is
> > > more realistic.
> > >
> > > So let's say that you use 4kWh of energy out of your 10kWh battery bank,
> > > that's about 10-12nm at 4kts in most of our boats. How far would you
> have to
> > > sail using the system described by Whoosh to bring your batteries back
> to
> > > full charge, assuming that you are using no electrical power in the mean
> > > time? Generating 75W at 6kts, that's 12.5Wh/nm. So 4kWh will take 320nm
> of
> > > sailing at 6kts and will take over 53 hours. Slow down to 5kts under
> sail
> > > and the recharge distance is up to 400nm and will take 80 hours. If you
> have
> > > a 12V house load of only 2A, the recharge times and distances double.
> > >
> > > Ignoring house loads, the ratio of drive distance (at 4kts) to regen
> > > distance (at 5kts) is about 1 to 40. Motor one mile at 4kts (15 minutes)
> and
> > > you need to sail 40 miles at 5kts (8 hours) to get the charge back. If
> you
> > > use a radio, autopilot or lights, the ratio gets worse.
> > >
> > > So regen works, but is not as effective at recharging your traction
> > > batteries as some drive vendors would like you to believe.
> > >
> > > Fair winds,
> > > Eric
> > > Marina del Rey
> > >
> > > --- In electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com> <electricboats%40yahoogroups.com>,
> > > "Larry Doyle" <ldoyle@> wrote:
> > > >
> > > > Here's to those NaySayers that say regen doesn't work. Here's some
> boats
> > > that are doing it using towed gens as well as the free wheeling the
> prop/dc
> > > motor in regen mode.
> > > >
> > > > www.svsarah.com/Whoosh/WhooshPacificPrepTowGenerator.html
> > > >
> > >
> > >
> > >
> >
>
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