Monday, July 27, 2009

Re: [Electric Boats] Abusive and dangerous Re-Epower technical specification

 

We need to be careful lumping all ICE's into the same bucket. I don't believe diesels are rated the same way as gas (in fact, the same debate rages on powerboat re-powering threads - you can apparently use a diesel of about 60% the HP of a gas and get the same performance - the comment is that diesels are "shaft rated" vs "bench rated").

I can tell you that in my experience, my ~5HP electrical setup gives me about the same power as my (ancient) 8HP diesel did but for 1/15th the amount of time.

So...the writers' point about TTL (time to live) in challenging conditions holds true. Hence my contention that everyone keep the "1 gallon" rule in mind - if you wouldn't do the trip with 1-gallon in an ICE - don't do it with an electric.

That's why I keep my boat confined to the SF Bay and won't go offshore anymore.

-Keith

electricboats@yahoogroups.com, James Sizemore <james@...> wrote:
>
> I was curious so I did a search to try and figure out where the 3x
> rule came from that so many EV quote and I think it is here:
>
> http://books.google.com/books?id=XcFbEX-de4kC&pg=PA147&lpg=PA147&dq=electric+hp+vs+combustion+hp&source=bl&ots=opKbD7vC6T&sig=s4MBEFtt-EW7-lttFCVzrzQfw8o&hl=en&ei=yXZtStT1LI2utgegr_iIDA&sa=X&oi=book_result&ct=result&resnum=1
>
> Seem to be a well written book I might have to buy it. And the
> chapter listed it talking about exactly the subject at hand!
>
> On Jul 27, 2009, at 4:37 AM, James Sizemore wrote:
>
> > I don't know a thing about the E-pod 3000. So I am not defending there
> > analysis or figures, but you do have to understand that combustion
> > motors and electrical motors HP is not rated the same way. Combustion
> > motors HP is rated at maxim RPM with out load. Electric motors are
> > rated at contuse power under load. So the two "HP" are measuring two
> > different things. So no combustion HP does not equal electoral HP.
> > The rule of thumb I see listed the most is electrical HP equals 3 X
> > combustion HP. So using that rule 5.5 electrical HP = 16.5
> > combustion HP. So you would be right that they are fudging a bit.
> > But again comparing the two is not simple!
> >
> > Here is a small quote from the really long wikipedia page on HP (http://en.wikipedia.org/wiki/Horsepower
> > ) please take special note of the last sentence about "objective
> > performance" HP rating for combustion engines are really a marketing
> > number and don't translate well to the real world work! Electrical
> > HP rating are more useful to determine real world work!
> >
> > "Measurement
> > The power of an engine may be measured or estimated at several points
> > in the transmission of the power from its generation to its
> > application. A number of names are used for the power developed at
> > various stages in this process, but none is a clear indicator of
> > either the measurement system or definition used.
> > In the case of an engine dynamometer, power is measured at the
> > engine's flywheel (i.e., at the crankshaft output). With a chassis
> > dynamometer or "rolling road", power output is measured at the driving
> > wheels. This accounts for the significant power loss through the drive
> > train. As an example, an early-production BL Mini 850 cc (51.9 cu in)
> > engine produced about 34 bhp (25.4 kW) at the flywheel, of which only
> > 18.2 bhp (13.6 kW) reached the driving wheels.[citation needed]
> > In general:
> > Nominal is derived from the size of the engine and the piston speed
> > and is only accurate at a pressure of 7 lbf/in².[11]
> > Indicated or gross horsepower (theoretical capability of the engine)
> > minus frictional losses within the engine (bearing drag, rod and
> > crankshaft windage losses, oil film drag, etc.), equals
> > Brake / net / crankshaft horsepower (power delivered directly to and
> > measured at the engine's crankshaft)
> > minus frictional losses in the transmission (bearings, gears, oil
> > drag, windage, etc.), equals
> > Shaft horsepower (power delivered to and measured at the output shaft
> > of the transmission, when present in the system)
> > minus frictional losses in the universal joint/s, differential, wheel
> > bearings, tire and chain, (if present), equals
> > Effective, True (thp) or commonly referred to as wheel horsepower
> > (whp)
> > All the above assumes that no power inflation factors have been
> > applied to any of the readings.
> > Engine designers use expressions other than horsepower to denote
> > objective targets or performance, such as BMEP (Brake Mean Effective
> > pressure). This is a coefficient of theoretical brake horsepower and
> > cylinder pressures during combustion."
> >
> > On Jul 27, 2009, at 3:20 AM, bmhfercot wrote:
> >
> > > Present time I prefer to pass more time to build my Tiki46 than
> > > going to the forums but I can't keep the silence concerning the
> > > abusive adverts of Re-Epower.
> > > From its website I copied the technical specifications of the E-pod
> > > 3000 system :
> > >
> > > Technical Specifications of the E-POD 3000 System
> > > Operates on 36-48 volts DC (48 volts recommended)
> > > AMPERAGE: Peak 200
> > > Max continuous 100
> > > Recommended Cruise......
> > > 40-60 amps @ 48 volts
> > > (Performance = to 20 hp outboard / 25 hp inboard)
> > >
> > > In reading that I can't know what is the max continuous power at the
> > > shaft. I have to estimate it : absorbed power 48Vx100A=4800W - a
> > > minimum of 15% of losses in the engines and its controller = 4080W
> > > or 5,5Hp. So by which miracle a 5,5Hp electric engine can do the job
> > > of 25 hp inboard!!!!!!
> > >
> > > I know the advantages of an electric engine are to deliver a good
> > > torque at low rpm and to have a quick reply when you request the
> > > full power and can even during a very short time deliver a peak
> > > power as example to stop the boat (if the controller accept that).
> > > So it's easier to handle the boat with precision when docking. To
> > > have a good reply with a such little engine on heavy boats you need
> > > a large propeller with a little pitch.
> > > All that seems marvelous only if you have good conditions with no
> > > wind and or no current.
> > > As soon as you have to motor against the waves and the wind, the
> > > torque at low rpm and the quick reply don't change anything and
> > > you'll never get the push of a 25 hp inboard engine with a 5.5hp
> > > electric engine.
> > > If you try to reach a shelter in a gale against the wind and waves
> > > in a long rocky entrance, you'll push the throttle at the maximum to
> > > get the maximum continuous power of 5,5hp at the shaft (in fact the
> > > propeller has an efficiency of 50% and the useful power to push the
> > > boat is only of 2,25hp). As you draw a high amperage, the real
> > > capacity of your battery bank will be lower than if you draw a
> > > little amperage. Your speed is very low, so you have no hull speed
> > > inertia and each gust or wave will stop the boat. As the speed is
> > > very low, the angle of attack of the propeller blades increases, so
> > > the propeller stalls reducing its efficiency. The engine and
> > > controller warm more and more and so the useful power is going
> > > down.........
> > > The boat don't progress and in less of a half hour (if you have a
> > > good battery bank) you loose the control of the boat due to lack of
> > > electric energy ( if the controller or engine don't shut down
> > > before). The boat wrecks on the rocks in hoping you don't loose your
> > > life!!!!!
> > >
> > > So I can say IT'S CRIMINAL to affirm to the customers that an
> > > electric engine of 5.5hp can does the job of a 25hp inboard
> > > engine!!!!!!!
> > >
> > > An hp is an hp, there is no difference if the power is produced by
> > > electricity, or by humans, or by animals or by petrol or by nuclear,
> > > so and so. Behind the power delivers on the shaft the difference
> > > depends of the efficiency of the propeller and if the propeller is
> > > well adapted with the hull. This kind of comparison is only
> > > available about facilities to handle a boat in an harbour but it's
> > > not possible at full throttle. With the same propeller efficiency,
> > > with the same boats, the bigger power engine will produce the bigger
> > > propulsion power.
> > >
> > > Bertrand
> > >
> > >
> > >
> >
> > [Non-text portions of this message have been removed]
> >
> >
> >
>
>
>
> [Non-text portions of this message have been removed]
>

__._,_.___
Recent Activity
Visit Your Group
Yahoo! News

Get it all here

Breaking news to

entertainment news

Drive Traffic

Sponsored Search

can help increase

your site traffic.

Check out the

Y! Groups blog

Stay up to speed

on all things Groups!

.

__,_._,___

No comments:

Post a Comment