Hi Eric
I used the tug as example because I have real world numbers for it.
You were asking if a 6 times difference in horsepower for the same speed was
possible.
I am just suggesting that, from the figures I have, that sort of difference
is possible. And suggesting one of the probable reasons.
With the tug you can pour all 6400kW into a small diameter, high revving,
high pitch prop and only get three knots. (Churning up lots of water and
cavitating) A bigger diameter prop suitable for the vessel gives 3 knots at
less than 900kW. (4300kW tows a 40000 ton ship behind at 3 knots).
An ICE engine setup in a boat is generally overpowered. Normally a standard
prop is fitted because efficiency is not top of the list when you have HP to
spare. Whereas those fitting EP put a lot of attention into the right props
and general efficiency.
Looking at the numbers behind Gerrs predictions I think the data base was
for higher speeds. The relatively low speeds used for most EP setups are in
a different ball park.
Are you comparing the real power output of the ICE at 3 knots to the real
output of the EP?
Measuring the actual HP from an ICE at the shaft is another possible cause
of difference. Not very easy to do. (especially on the water). You can get
an idea with a diesel by metering fuel.
To do the sums, and get the answers you require, could go back to first
principles. Calculate hull resistance at the speed required. Gives you the
power required to move the boat at that speed. The your theoretical power
train, prop pitch, diameter and propeller slip.
Then we need more empirical testing with EP to confirm the theoretical data.
The more numbers users of EP can give the closer we will get to an exact
comparison.
Some kind person, with time on their hands, could then compile a table, like
Gerr's, for EP.
-----Original Message-----
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
On Behalf Of Eric
Sent: Tuesday, 23 August 2011 12:22 p.m.
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] power requirements - predicted vs observed
Hi Kerry,
So the tug with the wrong prop uses 6 times the hp to go the same speed as
the tug with the correct prop at some speeds?
If this is true, then if I hadn't changed the 13x10 2-blade propeller on my
boat when I installed my electric drive, then my boat would take 3000W to
motor at 3 kts. I know that some people here have converted their boats to
electric without changing their props and they don't use 6 times the power
to go 3kts.
Does any electric boat here use more than 1500W to go 3kts? If so, what
kind of boat and what kind of propeller are you running?
While I believe that my 13x15.5 4 blade prop is more efficient than the old
prop, I have a hard time believing that is is 6 times more efficient (1/6
the energy to provide the same thrust). With the old prop and the diesel
engine with 2:1 reduction, engine rpm was about 1200 (shaft speed of 600) at
3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor
with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at
3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39%
slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts
and 37% slip at 6kts. Better, but not 6 times better.
Given that info, I would believe ratios around 2:1 comparing ICE to
electric. However 6:1 still seems like too much.
But though all of this, it is easier to see that the Gerr power predictions
don't hold true for our boats at our speeds.
Back to the real question, how can we predict how much power our boats will
actually need to motor at a given speed?
Fair winds,
Eric
Marina del Rey, CA
PS. I hadn't done the prop slip calcs on my boat yet. I'm glad that I know
those numbers now.
--- In electricboats@yahoogroups.com, "Kerry Thomas" <kjthomas@...> wrote:
>
> 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.
>
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