Hi Eric
Now had time to give some thought, and calculations, to the subject.
The propellers could account for the difference.
Using as an example (As that is what I have real world numbers for) A tug
propped with a propeller designed for a high speed, low torque engine
compared with the same tug with its proper propeller for the powerplant and
speed does account for most of your observed efficiency difference.
An ICE engine and prop combination designed for high revolutions can be just
churning up lots of water and wasting power at slow speeds. While a suitably
propped and slow revving engine can get the same speed with much less
horsepower.
Of course, like the tug, you could get much the same effect by gearing the
ICE down and using a bigger prop.
-----Original Message-----
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
On Behalf Of Eric
Sent: Saturday, 20 August 2011 4:57 a.m.
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] power requirements - predicted vs observed
Hi Arby,
I'm with you, "Empirical data definitely rules!"
Like some have said, torque provides acceleration, and power provides
constant speed against a given load. We can see this because changing the
reduction ratio from 2:1 to 4:1 doubles the output torque at half the
rotational speed, resulting in the same power. So gears can increase your
torque (acceleration) but you can't mechanically or magically increase the
power (constant speed).
I'm impressed with everyone's contributions to this discussion. I know that
it is easy to get sidetracked. But we're doing pretty well to stay on
topic.
Collectively, we have the chance to change the perception of what electric
power can do in the mainstream marine world. Everybody here has a good idea
of what electric can do, gained mostly through observations, trial and
error. If we're lucky, we might end up with a better way to objectively
predict electric traction requirements for a wider variety of vessels.
Keep up the good work everyone.
Eric
--- In electricboats@yahoogroups.com, Arby bernt <arbybernt@...> wrote:
>
> Nice discussion.I'll be glad to work on a shaft measurement package.
> With just a couple of inches of shaft and clearance, it should be
> possible to place two strain gages (not gauges), and telemetry to resolve
a set of forces. Record the rpm, power, torque and thrust, and the system
will be characterized. There is an existing device designed for measuring
power output on an automotive drive shaft, sort of a real time dyno for
gearheads to tweak drag racing home-builts. Empirical data definitely
rules. My great-grandfather, Milton Humason,ran the telescope on Mt. Wilson,
and took the spectra-scope data on the accelerating expansion of the
universe. With enough members, a measurement system can be used in many
different boats, and the data now assumed can be verified. I'll cover the
cost of the device(s).
> Be Well,
> Arby
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