Thanks Eric for the time you put into this email.
It gives me things to think about.
Thank you Mike & Brian also!
Bob
--------------------------------------------
On Mon, 3/10/14, ewdysar@yahoo.com <ewdysar@yahoo.com> wrote:
Subject: [Electric Boats] RE: 5KW or 10 KW
To: electricboats@yahoogroups.com
Date: Monday, March 10, 2014, 4:27 PM
Hi Bob,
Good questions. Short answer, a larger
motor will not use less amps to drive your boat at a given
speed. It will probably not run cooler at the same
given amperage (different motor, different design, who
knows?).
My 30' 10,200 lb Bermuda 30 ketch uses 5.1kW
(measured as draw from the batteries) to motor at 6 kts at
full throttle. At 5kts (2500W), the motor windings of
my Propulsion Marine 5.5kW drive never get to 80C, Motenergy
says the max temp for the ME913 motor is 150C. At full
throttle, the 6.8hp of power going into the electric drive
powers my boat 1/2kt faster than the old 12hp diesel could
push the boat at full throttle. At 28'and 8000lbs,
do you think you need more power than that?
Coincidentally, I believe that the Motenergy
ME913 is the motor in Electric Yacht's "10kW"
drive. In my opinion, that is a little over-stated.
The motor is rated for 125A continuous, so a typical
48V system maxes at 6.2kW continuous. Moving up to 72V
will only get one to a 9kW continuous rating, so I read 10kW
with a grain of salt (yes, I know that they state 10kW as
"maximum current" but what does that really mean?)
. Even so, the Propulsion Marine drive in my boat that
uses the same major components as the EY system was
conservatively rated at 5.5kW on purpose to allow for less
than optimal cooling in the bilge of an auxiliary sailboat.
With
all that said, I believe that using peak current ratings (1
minute rating or 5 minute rating) is misleading in the
marine market. In a car or motorcycle, peak ratings
make more sense (when was the last time that you drove your
car for 5 minutes at full throttle?). But we "set
and forget" boat throttles, running for long periods at
90% or more (gas and diesels) or 40-50% (electrics).
Nobody wants to hit the throttle to combat a strong
current and have the motor do a thermal shutdown 4 or 5
minutes later. Since we operate our drives in a
continuous mode, we should only be looking at continuous
ratings. Please note that the most important single
specification for sizing an electric marine drive has proven
to be displacement. Length of the boat (LOA) does not
appear to have much of a direct correlation to how much
power you need because LOA doesn't say much about the
boat's design.
Back to
the group's recommended ratings, 1kW per ton of
displacement will power a typical displacement boat to
90-95% of theoretical hull speed with a properly matched
driveline. You can put in more motor, but it really
doesn't do much besides raise your cost. There are
some in this group that believe that a larger motor is
actually less efficient being driven at a lower percentage
of it's design, though I believe that may be more
theoretical than practical.
Going
back to my boat, the ME913 (remember that Motenergy says
125A continuous) will push the boat to 95% of the
theoretical hull speed at 106A as long as I've got power
and it will never overheat. With 8kWh of Lithium, that
would be 69 minutes from a full charge to 80% depth of
discharge including losses to Peukert's Effect. If
I doubled my battery back to 16kWh (320Ah @ 50V), I could
run at full throttle for 140 minutes and still not overheat.
I've posted my detailed performance observations
many times here and my measured "watts to knots"
efficiency is better than any verifiable performance numbers
that have been posted here for a mid-size sailing auxiliary.
However, beating the performance of my boat should be
doable. She's a full keel, prop in an aperture,
rudder that is a 3" thick slab of teak, and her hull is
far from fair so a modern design should do
better.
We've got a rare collection
of practical information about electric boats in this group.
We've discovered that D. Gerr's calculations
don't work well for what we're doing here (his
predictions are more than 300% off for my boat, and D. Gerr
himself admitted that his work is more applicable to faster
boats). We've got electric boaters with hundreds
of hours (or more) of electric motoring experience. We
know how much power it really takes to push boats like ours
to practical speeds. We've got a number of vendors
that are a reasonably reliable lot (as vendors, they
probably have at least a subtle bias). We've got a
number of experimenters that want to build a better, or at
least a different, mousetrap. There is more unbiased
practical experience in the field of electric marine drives
here than anywhere else on the planet. There are also
a lot of unsubstantiated opinions too. But if you go
slow and read carefully, you can collect enough real
information to build or buy an effective electric drive for
your boat. So welcome and feel free to ask
anything.
Fair winds and smooth
seas,Eric1964 Bermuda 30 ketch, 5.5kW
Propulsion marine drive, 8kW LiFePO4 battery
packMarina del Rey, CA
---In electricboats@yahoogroups.com, <sabre281@...>
wrote :
Maybe this is a
stupid question but I've wondered about the
advantages/disadvantages to having a 10 kw motor vs. 5 kw.
My boat is 28', 8000#'s of displacement so let's
assume hull speed is 6 knots at full throttle. From what
I've read I know the 5kw will do the job. If you wanted
to motor lets say at 3.5-4 knots for a long period of time
would a 10KW unit run cooler & use less amps from the
battery bank when compared to a 5kw?
Thanks,
Bob
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