Eric,
Hi, Eric, I think you have great points here. Typical ICE setups are not
a good guide to real needs.
As a lurking, thinking rather than doing member, I have recently come to
some conclusions, remembering that my own boat is a cheapie fixxer-upper
sailboat, so budget is limited:
1 - Good tried and tested 'purchasable as a turn-key package' electric
setups are the way to go if you have the cash to throw at the project,
as with cars, a new car is up to date, more reliable, and convenient to
use than most other options.
2 - The comparison to ICE is often futile, as ICE setups were probably
never optimised, other than by sales people at boat manufacturers.
3 - The 'green' aspect might not be as high as figured, once the carbon
footprint of making (consumable) batteries is factored in.
4 - It is probably not possible in a practical sense to recharge to any
large degree from wind or solar if propulsion is used a fair bit.
5 - The needs for propelling a (sail)boat other than with sails can
probably be broken down into two main parts - frequent low power short
use for docking, and longer term running at higher speeds (hull speed?)
for convenience and/or safety, such as to beat nightfall, or make port,
or beat the absence of wind.
The above would seem to indicate that high-ish power is needed to be
available for long time frames, that is, reasonably long range
capability at higher speeds. Seeing as the lower power 'docking' setup
is easy to justify, this means that to get long range and high power, a
large battery investment is needed every few years, and a larger type
electric motor also. The logic would seem to be to go with a large
enough electric drive, but limit the total battery size, and add a
generator as has been advocated in the past. Good thinking. The snag for
me is that a large electric motor and controller costs a lot, whereas I
can nickel and dime smaller ones.
Putting ethical considerations aside, and using electric if it is your
own preference for practical reasons (mine is erring towards not wanting
an Atomic 4 gas engine in the 'living room') then the ideal setup for
me, as a starting out learner weekend boat guy, would seem to be a
couple of HP electric as far as motor(s) goes, maybe just 1 or 2 'floor
cleaner' specification batteries, 12 volt system, and have a gas
outboard available but hopefully seldom used. The fact remains that
fossil fuel is very compact, and does not pollute if never used!
It has been interesting to read about the power and thrust needs. Seems
that a couple of trolling motors, at 30 lbs or so thrust, would go a
long way as a starter setup if I wanted a turn-key type deal.
I can possibly charge for all my needs with wind, maybe a solar panel as
well (ugly darn oversized things).
Or, I can haul the batteries out to recharge at home, or even run a
small generator once in a while.
I finally got to test out my automotive DC generator idea (actually a
generator off the Atomic 4 ) and it seems to have no low end torque, I
can hold it back by hand. It does rev up quite well though. I am going
to retest with a fully charged battery, but if I get the same poor
results, then I am thinking multiple electric scooter or robotic motors
now.
John
2.2. Re: Thrust to speed numbers
Posted by: "Eric" ewdysar@yahoo.com ewdysar
Date: Thu Mar 10, 2011 9:35 am ((PST))
GNHBus,
For my conversion I went with a very aggressive 4 blade prop to maximize
slow speed prop efficiency. Because my prop is in an aperture I
couldn't increase the diameter significantly. I know that I can
mitigate the extra drag while sailing by "power-feathering" the prop
with a little throttle.
But the comment that you made that mentioned hull speed got my
attention. I don't think that any of the existing conversions spend any
time at hull speed, the power requirements just aren't worth it. So, my
drive system can't push my boat to hull speed (about 6.5kts) because I
would rather optimize the system for the speeds that I actually power
the boat at, the 3 to 5 knot range.
The recommendations of repowering a boat at 2.5-3hp ICE to 1hp electric
take this into consideration. Speed is a hp thing and reducing the
total available hp to 1/3 will probably reduce your top speed. There is
an assumption in that conversion guideline that the ICE is actually
over-spec'd and the boat would be just as usable with less hp. With
ICE, the smaller engine might not have enough torque at slower speeds
which justifies the larger installation. Electric doesn't have that
problem, even motors with less hp have enough torque for operation at
the lower end. So going with only enough hp to get near hull speed is
generally good enough. The funny thing is that my old Yanmar SVE-12
diesel couldn't drive the boat to hull speed either.
So it is my opinion that hull speed isn't really important to electric
conversion because the energy cost is way too high. I can't think of
any emergency situation that 80-85% of hull speed wouldn't be
sufficient. And my normal operation is closer is 40-75% of hull speed
to preserve any semblence of range.
Fair winds,
Eric
Marina del Rey, CA
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