Hi
Don't have time to go into it at the moment, but I suspect it has to do with
the larger, high torque slow speed propeller sizing for EP compared with
smaller faster props for ICE, giving more efficiency at slow speeds.
Like running a tugboat prop at 5 knots compared with running a container
ship prop, designed for 15 knots, at 5 knots.
-----Original Message-----
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
On Behalf Of Eric
Sent: Friday, 19 August 2011 5:33 a.m.
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] power requirements - predicted vs observed
Hi hanermo,
Interesting point and I agree with each point, but it doesn't address the
question. The Gerr formulas are stated to cacluate the propeller shaft HP
required to push a hull. So it's supposed to be the HP applied to the
propeller AFTER all of the parasitic drag and driveline efficiency losses.
So regardless of the power source, the number is supposed to be how much
power needs to be supplied to the prop for a given speed. Everything that
you mentioned were losses before the power made it to the propeller shaft.
So all of this is just derailing the conversation. Accurate info, but not
what we're discussing right now.
Let me try to simplify the topic to keep this conversation on track. The
Gerr power formulas claim to quantify how much power the propeller needs to
push a hull at a given speed. These numbers have been verified through
years of observations with ICE engines and are accepted to be accurate.
However, through observation, one can see that an electric drive can push a
hull at the same speed with much less power applied to the propeller.
My boat at 3kts uses about 17% of the power via electric compared to what
Gerr predicts. That is so far off, that I think that Gerr is predicting
something other than what I am measuring. By 5.5kts, my observations are
42% of what Gerr predicts. Gerr must be using some assumptions that are not
accurate for my boat.
My simple question was if Bill S. had a single data point to validate this
question. What is the observed energy load on Barbara Ann's electric drive
at 7.2kts?
It is physics, not magic, so I know that there is an explanation.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "\"hanermo\" - CNC 6-axis Designs"
<gcode.fi@...> wrote:
>
> To me this is obvious. I am quite certain but might be wrong. (No, not
> really).
>
> 1. The ICE system is quite inefficient....
>
> 2. At low speeds, the efficiency is much lower, as small ICE systems
> are optimised for top speed running....
>
> 3. The efficiency numbers are also consistent - practically all are
> within 10%-15% as Eric said....
>
> 4. Given that the trends are all very close, these also poitn out to
> the well-known fact that most hulls are very close to each other in
efficiency....
>
> 5. The power of the early 200-400 ton trading ships, in the early
> 1900s, was about 80-150 hp.
> When the age of sail transitioned to steam.
>
> 6. As a conclusion LWL is the best way to increase effiency and reduce
> overall costs and speed made good (if marina cost is not taken into
> account)....
>
> 7. Bill Southworth -
> As you have excellent data, measurement systems and experience, could
> we see that is the actual consumption of power at 0.8 and 0.9 hull
> speed (hull speed) with the 100 kW conversion ?...
>
> Best,
> hanermo
>
> > 3kts = 2.97kW or 3.98hp
> > 4kts = 4.37kW or 5.86hp
> > 5kts = 6.81kW or 9.13hp
> > 5.5kts = 8.74kW or 11.72hp
> >
> > 3kts = 9.54A at 52.4V = 500W or 0.67hp 3.9kts = 19.2A at 52V = 999W
> > or 1.33hp 4.4kts = 28.9A at 51.5V = 1488W or 1.98hp 5kts = 48.5A at
> > 50.7V = 2459W or 3.28hp 5.7kts = 78.8A at 49.6V = 3908W or 5.21hp
> >
> >
> > What do you think? Feel free to share this info with the rest of the
> > brain trust behind your boat. It does make one wonder...
> >
>
------------------------------------
Yahoo! Groups Links
No comments:
Post a Comment