T-Pain Is On A Boat: Re: [Electric Boats] Abusive and dangerous Re-Epower technical specification

Monday, July 27, 2009

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

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
>
>
>

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