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, 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 <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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